Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
  • G01N24/008—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using resonance effects in zero field, e.g. in microwave, submillimetric region

Definitions

• MRR molecular rotational resonance
• microwave spectroscopy identifies and characterizes molecules, including compounds, by exciting the molecules with electromagnetic radiation based on the molecules’ characteristic rotational angular momentum transitions and measuring coherent radiation emitted by the molecules via free induction decay (FID) as a result of the excitation.
• the emitted radiation is detected as a time-varying signal. Fourier transforming the signal yields a spectrum that can be used to identify the characteristic rotational angular momentum of the molecules in a sample, which in turn may be used for identifying and/or quantifying the analyte and its components.
• the rotational spectrum of a molecule is described by a Hamiltonian that depends on its moments of inertia in the three spatial axes. By using rotational spectroscopy, molecules can be unambiguously distinguished by their differences in structure.
• MRR spectroscopy is typically conducted in a vacuum chamber, which provides a low- pressure environment (e.g., less than 100 mTorr), using electromagnetic radiation in the microwave-to-millimeter wave spectrum for excitation (e.g., 1–40 GHz for the microwave region, and 30–3000 GHz for the millimeter region).
• the extremely fine resolution of MRR spectroscopy means that patterns (spectra) of different molecules can be resolved directly in a mixture without separation.
• the structure of the pattern depends only on the three- dimensional structure (mass distribution and electronic charge distribution) of the molecule, Attorney Docket No. BRSP-013WO01 which can be calculated accurately and efficiently by commercially available quantum chemistry software.
• MRR spectroscopy molecules can be identified directly in a complex mixture without the need for pure reference standards, which can be very expensive and difficult to produce.
• FIG. 1 shows MRR spectra of 13 CH3CN and CH3 13 CN, isotopomers of identical mass, in which clearly resolved spectral patterns for each can be observed.
• the MRR spectrum of each molecule can be calculated with high precision.
• SUMMARY MRR spectroscopy is typically performed in one of two ways – a broadband measurement or a targeted measurement.
• Broadband measurements are conducted by exciting a sample of analyte with one or multiple broadband pulses of electromagnetic radiation (e.g., a chirped pulse of microwave or millimeter-wave radiation).
• broadband measurements can obtain spectra of samples with a relatively larger bandwidth (e.g., greater than or equal to 5 MHz) at the expense of lower sensitivity and/or lower spectral resolution.
• Broadband measurements are often used to characterize most or, in some instances, all the components in a sample (e.g., a mixture) including molecules that are unknown or unexpected.
• Targeted measurements are conducted by exciting a sample with a narrowband pulse of electromagnetic radiation (e.g., a pulse of microwave or millimeter-wave radiation resonant with the cavity containing the sample).
• a narrowband pulse of electromagnetic radiation e.g., a pulse of microwave or millimeter-wave radiation resonant with the cavity containing the sample.
• the spectra of a sample acquired in a targeted measurement have a relatively smaller bandwidth (e.g., less than or equal to 1 MHz), which can provide greater sensitivity (e.g., by a factor of 10 to 100) due to the excitation power being concentrated within a smaller frequency range for the same measurement time and/or greater spectral resolution.
• Targeted measurements are often used to quantify and/or detect known components (e.g., species) in a sample.
• MRR spectroscopy can provide sufficient sensitivity to differentiate individual components in a sample (e.g., an isotopomer) and/or to quantify trace amounts of a particular component.
• multiple MRR measurements can thus be used to differentiate and compare the relative abundance of different chemical species within a sample or between different samples, particularly for chemical species that are difficult to distinguish and/or to quantify using other analytical chemistry techniques.
• a broadband MRR Attorney Docket No. BRSP-013WO01 measurement and a targeted MRR measurement can provide complementary information about a sample.
• the broadband MRR measurement can detect and monitor most or, in some instances, all the known and unknown components in a sample while a targeted MRR measurement can selectively detect and quantify known components with greater sensitivity and lower detection limits.
• a first targeted MRR measurement can detect and quantify a first component in a sample and a second targeted MRR measurement can detect and quantify a second component in the same sample to compare the relative amount of the first and second components.
• a conventional MRR spectrometer or MRR spectroscopy system can generally perform only one measurement on a sample at a time. In other words, a conventional MRR spectrometer cannot perform multiple broadband and/or targeted measurements simultaneously on the same sample.
• various factors affecting the repeatability of the measurements such as sample-to-sample variation or variations in environmental conditions over time, can make any quantitative analysis that combines the data obtained from multiple MRR measurements subject to greater measurement error and loss of precision.
• sample pulses are controlled amounts of a sample (also referred to herein as “sample pulses”) into the vacuum chamber.
• sample pulses also referred to herein as “sample pulses”
• the composition of the sample pulses can vary significantly.
• the total number density of molecules can vary by at least 10% between different sample pulses.
• MRR spectrometers are typically susceptible to long-term drift due to thermal effects in the spectrometer and/or changes in the sample, the chamber's vacuum level, and/or the pulsed nozzle (e.g., due to changes in the properties of the solenoid, or wear in the poppet) over time.
• the present disclosure is thus directed to various inventive MRR spectrometers capable of performing multiple, synchronized measurements on the same sample simultaneously.
• the synchronized measurements can include, for example, a combination of at least one broadband measurement and at least one targeted measurement, or a combination of two or more targeted measurements.
• the MRR spectrometer includes a vacuum chamber to hold the sample during measurement.
• the vacuum chamber provides a low vacuum environment generated, in part, by a vacuum pump and measured by a pressure gauge operably coupled to the vacuum chamber.
• the MRR spectrometer can include a broadband spectrometer and a targeted spectrometer to perform broadband measurements and targeted measurements, respectively, on a sample.
• the broadband spectrometer may include several components disposed in the vacuum chamber, such as a source horn antenna to transmit broadband pulses of electromagnetic radiation (e.g., a chirped pulse of microwave or millimeter-wave radiation) and a receiver horn antenna to receive the electromagnetic radiation emitted by the components in the sample in response to the broadband pulses.
• the targeted spectrometer may include several components disposed in the vacuum chamber, such as a fixed mirror and a movable mirror with a translation stage that together form a tunable Fabry-Pérot resonator, a source antenna to transmit narrowband pulses of electromagnetic radiation, and a receiver antenna to receive the electromagnetic radiation emitted by the components in the sample in response to the narrowband pulses.
• the MRR spectrometer may further include one or more signal generators to generate the broadband and/or narrowband pulses and an analog- to-digital converter (ADC) or digitizer to receive and convert the emitted radiation received by the receiver antennas of the broadband and targeted spectrometers from an analog signal to a digital signal for subsequent processing and analysis.
• ADC analog- to-digital converter
• inventive MRR spectrometers can include different combinations of spectrometers, e.g., two targeted spectrometers.
• An inventive MRR spectrometer may further include at least one sampling inlet to inject the sample of analyte into the vacuum chamber.
• the sampling inlet may operate in several ways.
• the sampling inlet may be coupled to a pulsed valve driver to controllably inject sample pulses into the vacuum chamber at desired time intervals.
• the sample pulses may propagate through the vacuum chamber before being removed by the vacuum pump.
• the sampling inlet may inject the sample as a continuous stream into the vacuum chamber and the vacuum pump may continuously remove the sample so that the sample does not accumulate within the vacuum chamber.
• sampling inlets may be used to selectively inject a sample and a reference into the chamber for measurement.
• the sampling inlet may be a pinhole nozzle that injects the sample together with a carrier gas as a fine spray where the sample is maintained in the gas phase.
• the sampling inlet may also be coupled to a sample source.
• the sample source may include a reservoir that receives a sample of analyte from, for example, a flow reactor or a syringe and a carrier gas source to provide a flow of carrier gas to facilitate injection of the sample through the nozzle and/or to purge the various fluid lines after each measurement.
• BRSP-013WO01 sample source may further include a gas chromatograph or a liquid chromatograph to separate individual components of a sample before injection into the vacuum chamber.
• the manner in which multiple MRR measurements are considered simultaneous or concurrent may differ depending on the way the sample is injected into the vacuum chamber.
• the sampling inlet injects the sample into the vacuum chamber as a series of pulses (also referred to herein as a “sample pulse”), multiple MRR measurements may be considered simultaneous if the respective measurements are performed on the same sample pulse.
• the respective source antennas of a broadband spectrometer and a targeted spectrometer may emit broadband and narrowband pulses that illuminate the same sample pulse and the respective receiver antennas of the broadband and targeted spectrometers may detect the FID emissions from that same sample pulse in response to the broadband and narrowband pulses.
• the sampling inlet injects the sample into the vacuum chamber as a continuous stream
• multiple MRR measurements are considered simultaneous if the respective measurements are performed on a similar or the same portion of the sample stream within a sufficiently short period of time such that the composition of the portion(s) of the sample stream being characterized remain substantially the same.
• the source antenna of a broadband spectrometer may emit a broadband pulse to illuminate a first portion of the sample stream and the source antenna of a targeted spectrometer may emit a broadband pulse to illuminate a second portion of the sample stream.
• the second portion of the sample stream should substantially overlap (e.g., they can overlap by 50%, 60%, 70%, 80%, 90%, or more) with the first portion of the sample stream.
• the broadband and targeted MRR measurements should both be performed within a predetermined period of time (e.g., less than or equal to 1 millisecond).
• the period of time a sample pulse or a portion of a sample stream may be exposed to broadband and narrowband pulses may be appreciably longer than the dephasing time for FID (e.g., the sample pulses may have durations of 1–4 ⁇ s, whereas the FID dephasing time may range from 10–40 ⁇ s).
• the MRR spectrometer may probe a particular sample pulse or portion of the sample stream with multiple broadband and/or narrowband pulses and detect corresponding FID emissions in response to the broadband and/or narrowband pulses.
• the MRR spectrometers disclosed herein can perform multiple MRR measurements simultaneously, it should be appreciated that individual MRR Attorney Docket No.
• BRSP-013WO01 measurements may be performed separately on a sample as desired.
• the MRR spectrometer further includes a timing controller to synchronize operation of at least the multiple spectrometers and the sampling inlet (e.g., via the pulsed valve driver).
• the timing controller may control when a targeted MRR spectrometer and a broadband MRR spectrometer generate respective excitation pulses for measurement, when the detector(s) (e.g., the receiver antenna and the ADC) receive and record signals generated by the excitation pulses interacting with the sample (e.g., an FID emission signal), and when the sampling inlet injects a sample pulse into the system (e.g., relative to the generation of the excitation pulse and/or the receipt of a signal by the detector(s)).
• the timing controller including a frequency standard (e.g., a rubidium atomic clock), which provides a reference clock signal for synchronizing the operation of the components in the MRR spectrometer.
• multiple pulses e.g., broadband pulses, narrowband pulses
• FID emissions may be present in the vacuum chamber at the same time, which may create additional noise for the measurements.
• the components of the spectrometers may be arranged to reduce the amount of electromagnetic radiation associated with one spectrometer that arrives at the detector of another spectrometer.
• the vacuum chamber may be shaped as a six-way cross with a first axis for a broadband spectrometer, a second axis for a targeted spectrometer, and a third axis for various other hardware in the MRR spectrometer (e.g., a port for connection to a vacuum pump, a pressure gauge, and/other instrumentation to facilitate operation of the MRR spectrometer).
• the first, second, and third axes may be orthogonal to one another.
• FID emissions occur in a similar direction to the excitation pulses that produce them.
• the FID emissions associated with a broadband pulse are less likely to propagate or travel along the axis for the targeted spectrometer particularly and the FID emissions associated with a narrowband pulse are less likely to propagate or travel along the axis for the broadband spectrometer.
• RF absorbing materials may be disposed on the interior walls of the chamber to reduce the effect of cavity ringing particularly at frequencies probed by the broadband MRR spectrometer.
• FIG. 1 shows molecular rotational resonance (MRR) spectra of two isotopomers of acetonitrile illustrating the selectivity to isobaric compounds as well as the exceedingly high resolution of MRR spectral lines.
• FIG.2 shows a block diagram of an inventive MRR measurement apparatus configured to perform broadband and targeted measurements simultaneously.
• FIG. 3 shows a three-port sampling interface suitable for use with an inventive MRR spectrometer.
• FIG.4A shows a front end for an inventive MRR spectrometer with separate channels and analog-to-digital converters (ADCs) for the broadband and targeted signal channels.
• ADCs analog-to-digital converters
• FIG. 4B shows a front end for an inventive MRR spectrometer with frequency- multiplexed channels and a single ADC for the broadband and targeted signal channels.
• FIG.5A shows a schematic of an inventive MRR spectrometer configured to perform broadband and targeted measurements of an analyte gas sample simultaneously.
• FIG.5B illustrates an MRR ratiometer instrument with two Balle-Flygare type targeted spectrometers arranged with orthogonal cavities and a single nozzle.
• FIG.5C illustrates an MRR ratiometer instrument with two Balle-Flygare type targeted spectrometers arranged with orthogonal cavities and a pair of symmetric nozzles.
• FIG.5D illustrates a dual-polarization MRR ratiometer with two movable flat reflectors (a metal mirror and a wire grid polarizer) and two pulsed nozzle sources for sample injection into the same volume of the MRR ratiometer’s vacuum cavity.
• FIG. 5E shows a folded-cavity MRR ratiometer with a wire-grid polarizing beam splitter fixed within a confocal microwave cavity and side-mounted nozzles.
• FIG. 5F shows a folded-cavity MRR ratiometer with a wire-grid polarizing beam splitter fixed within a confocal microwave cavity and coaxially mounted nozzles.
• FIG.5G shows an MRR ratiometer with dual-polarization, cavity-enhanced detection systems (targeted spectrometers) with a fixed flat mirror and movable spherical mirrors arranged in a semi-confocal mirror geometry.
• FIG. 5H shows an MRR ratiometer with dual-polarization cavity-enhanced detection systems (targeted spectrometers) with a fixed spherical mirror and movable flat mirrors arranged in a semi-confocal mirror geometry.
• FIG. 6A shows a flow chart for an example method of performing broadband and targeted measurements simultaneously.
• FIG.6B shows an example timing diagram for a sample pulse of analyte, a broadband excitation pulse from a broadband spectrometer, and a targeted excitation pulse from a targeted spectrometer.
• FIG. 7A illustrates chiral tagging, which is the gas phase formation of different diastereomers with distinct spectra by combining a chiral tag and enantiomers.
• FIG. 7B illustrates enantiomeric excess analysis from MRR spectroscopy. Measurements were made with racemic or even pure enantiomer of chiral tag.
• FIG. 8 is a calibration curve for determination of the enantiomeric excess of 1,1,1- trifluoro propan-2-ol (TFIP) using chiral tag rotational spectroscopy.
• TFIP 1,1,1- trifluoro propan-2-ol
• FIG. 9A shows the signal variation for individual MRR spectroscopy transitions observed in a measurement of TFIP.
• FIG. 9B shows the variation, or fluctuation, of the signals in back-to-back measurements of transitions of the TFIP dimer.
• FIG. 9C shows the variation, or fluctuation, of the signals for transitions of the TFIP monomer.
• FIG. 10 shows fluctuations in the ratio of the transition intensity for 13 C isotopomers and the 12 C “normal species” of TFIP in the monomer spectrum.
• FIG. 11 shows MRR ratiometer measurements for the 13 C: 12 C isotopic signal ratio in TFIP.
• FIG. 12 shows a series of 60 sequential measurements of the 13 C: 12 C ratio in a TFIP sample made with the MRR ratiometer of FIG.5A.
• FIG.13 is a plot of the differences between successive measurements in the series of 60 sequential measurements from FIG.12.
• FIG.14 is a plot of EE determinations of a high enantiopurity sample of (R)-TFIP made using the MRR ratiometer of FIG.5A.
• DETAILED DESCRIPTION Following below are more detailed descriptions of various concepts related to, and implementations of, an MRR spectrometer that is able to perform multiple MRR measurements simultaneously.
• the multiple MRR measurements include, for example, a combination of at least one broadband MRR measurement and at least one targeted MRR measurement, or a combination of two or more targeted MRR measurements.
• inventive MRR spectrometers are provided, wherein a given example or set of examples showcases one or more particular features of a vacuum chamber, a broadband spectrometer, a targeted spectrometer, a sampling inlet, timing circuitry, and various hardware to facilitate operation of the foregoing components, such as hardware to operate the sampling inlet or move a movable mirror in the targeted spectrometer.
• Features discussed in connection with a given example of an MRR spectrometer may be employed in other examples of MRR spectrometers according to the present disclosure, such that the various features disclosed herein may be readily combined in a given MRR spectrometer according to the present disclosure (provided that respective features are not incompatible with each other).
• An inventive MRR ratiometer can perform two Fourier transform microwave (FTMW) spectroscopy measurements on the same sample simultaneously. These measurements may be performed on different molecular components, or species, of the sample, which may be mixture of different compounds or components, and can be used to determine a relative indication of the components, such as the components’ number density ratio or mass ratio. Both measurements can be targeted (narrowband) measurements, or one measurement can be targeted and the other measurement can be broadband. In either case, the resonance frequencies for the two species being measured should be different, allowing both species in the same sample volume to be measured at the same time. The simultaneous measurements of the two chemically distinct species removes the signal fluctuations caused by variations in the number of molecules injected in each measurement cycle by the pulsed nozzle source. Attorney Docket No.
• At least one of the FTMW spectroscopy measurements can be performed using cavity-enhanced FTMW spectroscopy.
• the enhanced sensitivity of cavity-enhanced FTMW spectroscopy reduces the time to acquire the signal of the lower abundance molecular species to the designated detection limit.
• the cavity can be either confocal or semi-confocal.
• An inventive MRR ratiometer may also have the ability to inject, at different times, two different samples—the analyte and a reference—into the same spatial region of the spectrometer’s vacuum cavity. The reference provides a comparison to the analyte to calibrate the measurement.
• the analyte sample and reference sample can each be introduced separately into the vacuum cavity by pulsed jet expansion of a dilute sample mixture in an inert gas (typically 0.1% of the molecular sample diluted in neon). Furthermore, switching between the reference and analyte samples (e.g., at a rate of once per second or faster) over the course of the measurement can reduce or remove long-term drift in the ratio determination. For instance, an inventive MRR measurement apparatus can switch between reference and analyte samples on a pulse-to-pulse basis at a pulse rate of 10 Hz (5 Hz for the reference pulses and 5 Hz for the analyte pulses).
• the reference sample may be a sample with a known ratio for a given pair of species, in which case a difference in the ratio of that pair of species in the analyte could indicate counterfeiting.
• the reference sample may be a sample with a known ratio for a given pair of species, in which case a difference in the ratio of that pair of species in the analyte could indicate counterfeiting.
• the reference sample may be a mixture of the chiral sample being studied with a racemic sample of the tag and the analyte may be a mixture of the sample being studied with a high enantiopurity sample of the tag.
• the chemical species being monitored may be the homochiral and heterochiral complexes formed between the chiral sample and the tag in the pulsed jet expansion.
• An inventive MRR ratiometer may include a pulse generation system that can produce two microwave pulses at different frequencies in substantially simultaneous fashion. These pulses are applied to the different sample components being measured; each pulse is resonant with a selected rotational transition of a different molecular species.
• the broadband pulse may be resonant with two or more species and the target pulse may be resonant with only one of those species or with a different species.
• the pulse generation system uses microwave amplifiers to achieve sufficient pulse power to optimally excite the sample—the so-called ⁇ /2-pulse conditions in coherent excitation measurements. These amplifiers may be temperature stabilized to reduce long-term drift in the amplifier gain that could cause fluctuations in the pulse power and the signals from the reference and analyte.
• An inventive MRR ratiometer can include a coherent detection system that measures the FID signals from both the reference and analyte samples.
• the amplitude of the oscillating electric field in the FID which can be obtained by Fourier transform of the FID, is proportional to the number density of the molecular species being monitored.
• the electric field amplitudes of the FID signals are used to determine a relative indication, such as the number density ratio or mass ratio, of the two distinct molecular species being measured.
• the detection system can use separate receiver and digitizer systems to record the FIDs from the two distinct molecular species.
• the receiver design may employ frequency down-conversion using a mixer and a local oscillator (LO) signal to translate the signal to lower frequency at the digitizer.
• the receiver typically uses low noise microwave amplifiers in the signal processing. These may be temperature stabilized to reduce fluctuations in the amplifier gain.
• the signals from the two molecular species being measured may be combined using a microwave power divider/combiner circuit element (like a Wilkinson power divider) before any subsequent signal processing (signal amplification and/or frequency conversion).
• a microwave power divider/combiner circuit element like a Wilkinson power divider
• Signal processing signal amplification and/or frequency conversion
• Combining the signals in the microwave domain can reduce the fluctuations in the receiver performance for detection of the two, separate molecular signals at the different resonance frequencies by giving an identical signal path through the receiver (amplifiers, frequency conversion components, and digitizer) for both measured frequencies.
• An inventive MRR ratiometer’s ability to perform multiple MRR measurements simultaneously can provide considerable benefit for various applications.
• a broadband measurement can identify new compounds in a sample and the targeted measurement can then quantify the abundance of each new compound.
• multiple MRR measurements can be performed to compare the relative abundance of different compounds in the sample. For instance, the high sensitivity of a targeted measurement can be leveraged to quantify the amount of a first trace component in the sample.
• Another targeted Attorney Docket No. BRSP-013WO01 measurement can be performed (e.g., via a second targeted spectrometer) to quantify the amount of a second trace component in the sample.
• a broadband measurement may also be used to quantify the amount of the second component in the sample.
• an inventive MRR ratiometer may be used to characterize isotopically labeled compounds.
• broadband measurements may be used to identify the isotopic variants of the compound in the sample and targeted measurements may be used to quantify the amount of each isotopic variant, thus providing a distribution of the isotopic variants present in the sample. Additional details on the use of inventive MRR ratiometers can also be found below.
• an inventive MRR ratiometer may be used for high-throughput screening applications. For instance, a collection of samples obtained by the same chemical reaction, but under different experimental conditions may be characterized to determine which sample and, hence, the experimental conditions that provide the highest yield and/or greater conversion efficiency.
• Broadband measurements may be used to screen unexpected impurities in the respective samples and targeted measurements may be used to quantify the components of the sample to determine product yield.
• an inventive MRR ratiometer may be used for reaction monitoring.
• the sampling inlet of an inventive MRR ratiometer may receive a sample of a reaction mixture from a flow reactor. As the reaction occurs, the inventive MRR ratiometer can monitor and quantify various components of the reaction mixture to assess reaction completion, reaction kinetics, and/or generation of impurities.
• Broadband measurements may be used to screen unexpected impurities in the respective samples and targeted measurements may be used to quantify the components of the sample to determine product yield.
• an inventive MRR ratiometer can be used to evaluate raw material purity in a sample.
• Broadband measurements may be used to monitor the primary species of the sample (e.g., the species that remain if the sample is pure) and targeted measurements may be used to identify and quantify known impurities in the sample.
• an inventive MRR ratiometer may be used to analyze complex mixtures. Given batch samples of a mixture, the inventive MRR ratiometer may identify and/or quantify the components of the samples to assess batch-to-batch consistency of the mixture.
• Attorney Docket No. BRSP-013WO01 Broadband measurements may be used to determine a new, unknown compound is present in a sample and targeted measurements may be used to quantify the amount of the compound present. 2.
• FIG.2 shows a block diagram of an example MRR spectrometer 200 that can perform broadband and targeted measurements simultaneously.
• the MRR spectrometer 200 includes a vacuum chamber 202 to hold a sample (e.g., a sample pulse of analyte, a continuous stream of the sample), a pump system 204 operably coupled to the vacuum chamber 202 to generate and maintain a low vacuum environment within the vacuum chamber 202, and a sampling inlet 206 (e.g., a nozzle) coupled to a sample source (not shown) to inject the sample into the vacuum chamber 202.
• a sample e.g., a sample pulse of analyte, a continuous stream of the sample
• a sampling inlet 206 e.g., a nozzle
• the pump system 204 may include a vacuum pump, a pressure gauge, and/or a controller to manage operation of the vacuum pump and the pressure gauge.
• the sampling inlet 206 may be coupled to a pulsed valve driver 208, which may be used to controllably inject sample pulses into the vacuum chamber 202 at desired time intervals (see, for example, FIG.6).
• the sampling inlet 206 may also inject a continuous flow of the sample into the vacuum chamber.
• the MRR spectrometer 200 includes both a targeted spectrometer 220 and a broadband spectrometer 230 operably coupled to the vacuum chamber 202 to perform targeted and broadband measurements, respectively, on the same sample at (substantially) the same time.
• the broadband spectrometer 230 may include two horn antennae 232 in the vacuum chamber 202: a source antenna to emit a broadband pulse (e.g., a chirped pulse) for excitation and a receiver antenna to receive the FID emission from the sample in response to the broadband pulse (see, for example, the source horn antenna and the receiver horn antenna in FIG.3).
• the targeted spectrometer 220 may include also include two antennae in the vacuum chamber 202: a source antenna to emit a narrowband pulse (e.g., a resonant pulse) for excitation and a receiver antenna to receive the FID emission from the sample in response to the narrowband pulse (see, for example, the source antenna and the receiver antenna in FIG.3).
• the targeted spectrometer 220 may also include a pair of mirrors forming a Fabry-Pérot cavity 222 to amplify the excitation of the sample by the resonant pulse and the resultant FID emissions.
• One of the mirrors may further be movable to facilitate adjustment of the resonant frequency of the Fabry- Pérot cavity 222 as desired, e.g., to correspond with the desired frequency for the targeted measurement.
• the movable mirror may be coupled to a translation stage as Attorney Docket No. BRSP-013WO01 described below.
• Other inventive MRR spectrometers can include two targeted MRR spectrometers.
• the MRR spectrometer 200 includes hardware control circuitry and/or electronics 210 to control, for example, the operation of the sampling inlet 206, targeted spectrometer 220, and/or broadband spectrometer 230 in the MRR spectrometer 200.
• the hardware control circuitry 210 may set and maintain a desired rate of sample injected into the vacuum chamber 202 via the sampling inlet 206 and/or the timing intervals between sample pulses if pulsed operation is used.
• the hardware control circuitry 210 may adjust the position of the movable mirror in the targeted spectrometer 220 via the translation stage described above.
• the MRR spectrometer 200 also includes timing control circuitry and/or electronics 212 to synchronize the operation of at least the sampling inlet 206, targeted spectrometer 220, and broadband spectrometer 230.
• the timing control circuitry 212 may ensure broadband and targeted measurements are performed simultaneously on the same sample. Further aspects of timing control are discussed in more detail below.
• the MRR spectrometer 200 also includes a processor 216, such as a field programmable gate array (FPGA), operably coupled to the hardware control circuitry 210, timing control circuitry 216, targeted spectrometer 220, broadband spectrometer 230, and pump system 204.
• the processor 216 may also receive the FID signals detected by the receiver antennas in the targeted and broadband spectrometers 220, 230.
• FID signals may be converted from analog signals to digital signals by one or more digitizers 214 before transmission to the processor 216.
• the processor 216 may include or be operably coupled to a display device (e.g., a computer monitor) to render and display a graphical user interface and at least one user input device (e.g., a mouse and/or a keyboard) for users to interact with the graphical user interface.
• the graphical user interface may provide controls for the MRR spectrometer 200 (e.g., selection of the desired range of frequencies for broadband and targeted measurements, the timing intervals for sample injection).
• the processor 216 may Fourier transform the detected FID emissions to generate corresponding MRR spectra and display the MRR spectra to the user via the graphical user interface.
• the MRR spectrometer 200 generally measures the MRR spectra of a sample by illuminating the sample with one or more excitation pulses of microwave and/or millimeter- Attorney Docket No. BRSP-013WO01 wave radiation.
• the excitation pulses may be generated, for example, by one or more signal generators, such as an arbitrary waveform generator, direct digital synthesizer, or pulse pattern generator, and may be filtered, frequency-multiplied, and/or up-converted with appropriate circuitry.
• the MRR spectrometer 200 may include a single signal generator shared by the broadband spectrometer 230 and the targeted spectrometer 220 or one signal generator for the broadband spectrometer 230 and another signal generator for the targeted spectrometer 220. If the MRR spectrometer 200 include multiple signal generators, these signal generators may be locked to a common timebase/frequency standard to ensure that they are synchronized and phase-coherent.
• the excitation pulses generated by the signal generator(s) may include both broadband pulses and narrowband pulses generated concurrently. Respective source antennas in the broadband spectrometer and the targeted spectrometer emit the broadband and narrowband excitation pulses, respectively, to probe the sample in the vacuum chamber.
• the sample’s components may be excited by one or more excitation pulses if the excitation pulses include a frequency component that matches the characteristic rotational frequencies of the analyte. Once a sample component is excited, the sample component may then emit FID signals for several microseconds.
• the receiver antennas in the broadband spectrometer and the targeted spectrometer detect the FID signals (e.g., using heterodyne detection) emitted by the sample components in response to the broadband pulses and narrowband pulses, respectively. Each FID signal may then be digitized by the digitizer 214.
• the FID signals may be mixed down to an intermediate frequency range so they can be digitized with a lower-bandwidth digitizer as discussed in more detail below with respect to FIGS.3B and 3C.
• the processor 216 receives the time-domain digitized FID signals and Fourier-transforms the them to produce the MRR spectra.
• the processor 216 may then use an MRR spectral library containing MRR spectra for previously characterized molecules to identify and/or quantify the individual components of the analyte based on the MRR spectra. For example, data obtained from broadband and targeted measurements may be used to determine a relative indication, such as the number density ratio or mass ratio, of one chemical species to another chemical species in the sample.
• multiple MRR measurements may be used to determine various ratios of the components in the sample including, but not limited to, an enantiomeric ratio, a ratio of isomers, a ratio of isotopologues, and a ratio of isotopomers. Unassigned peaks in the MRR spectra can be further analyzed for characterization using theoretical predictions of relevant species.
• Attorney Docket No. BRSP-013WO01 The processor 216 may record the FID signals with fine time resolution (e.g., at sampling rates of 5–10 Hz) since the digitizer can achieve the desired throughput rate and the processor can perform data handling operations without dead time.
• the processor 216 may continuously measure and record the MRR spectra of the sample as the sample is injected into the vacuum chamber.
• the processor records and processes all of the MRR spectra. In other cases, the processor 216 records all of the time-domain data and Fourier- transforms only those segments corresponding to “interesting” outputs from the sampling inlet in order to conserve processing resources and reduce total processing time. The processor may discard unprocessed or unexamined time-domain and/or Fourier-domain data.
• the broadband spectrometer 230 of the MRR spectrometer 200 performs broadband measurements by generating a broadband excitation pulse, illuminating the sample with the excitation pulse with a source antenna (e.g., a horn antenna), and receiving the FID emission by the sample in response to the excitation pulse with a receiver antenna (e.g., another horn antenna).
• the broadband spectrometer 230 may include a dedicated signal generator to generate the broadband excitation pulse.
• a single signal generator may be shared between the broadband spectrometer and the targeted spectrometer (see, for example, the MRR spectrometer 400 in FIG.4A, described below).
• the signal generator may generate both broadband and narrowband pulses simultaneously and transmit the respective pulses to the source antennas in the targeted and broadband spectrometers accordingly.
• the broadband spectrometer 230 may perform broadband measurements based on the chirped-pulse Fourier transform technique, which involves illuminating a sample with one or more pulses of chirped microwave or millimeter-wave radiation and detecting and Fourier-transforming the FID signals emitted by the sample in response to the chirped pulses.
• FHSS Frequency Hopping Spread Spectrum
• the targeted spectrometer 220 of the MRR spectrometer 200 performs targeted measurements by generating a narrowband excitation pulse, illuminating the sample with the excitation pulse via a source antenna, and receiving the FID emission by the sample in response to the excitation pulse via a receiver antenna.
• Targeted measurements may be facilitated, in part, by a Fabry-Pérot cavity 222, which can be used to amplify the excitation and/or FID emission at a desired frequency and/or suppress unwanted excitations and/or FID emissions at undesirable frequencies.
• the targeted spectrometer 220 may include a dedicated signal generator to generate the narrowband excitation pulse.
• a single signal generator may be shared between the broadband spectrometer and the targeted spectrometer as described above and below.
• the targeted spectrometer 220 can acquire, process, and store targeted measurement data relatively quickly, e.g., at a rate of 2 Hz, 3 Hz, 5, Hz, 10 Hz, or faster. This measurement rate can be increased by performing Fourier transforms directly on a field- programmable gate array (FPGA) that acquires the data.
• FPGA field- programmable gate array
• a targeted spectrometer 220 can operate without a “species recognition” capability because the excitation frequency or frequencies and targeted species are known ahead of time.
• a targeted MRR measurement typically involves looking at single line time versus intensity because the measured intensity is proportional to species concentration.
• the targeted spectrometer 220 may be programmed ahead of time to target different spectral lines or bands for different species.
• the broadband and targeted spectrometers may generally carry out broadband and targeted measurements in one or more frequency bands in the microwave spectrum (e.g., over a band from 4–18 GHz, including all values and sub-ranges in between) and/or the millimeter- wave spectrum (e.g., 75–110 GHz, 260–290 GHz, or 520–580 GHz, including all values and Attorney Docket No. BRSP-013WO01 sub-ranges in between).
• the bandwidth of the targeted measurements performed by the targeted spectrometer may generally be less than the bandwidth of the broadband measurements performed by the broadband spectrometer.
• the bandwidth of the targeted measurements may be less than or equal to 1 MHz, including all values and sub-ranges in between.
• the bandwidth of the broadband measurements may be greater than 5 MHz.
• the MRR spectrometer 200 may perform broadband and targeted measurements with non-overlapping frequencies.
• the broadband spectrometer 230 may perform a measurement between 8–18 GHz while the targeted spectrometer 220 may perform a measurement at 6 GHz.
• the vacuum chamber 202 may generally be designed so that the multiple spectrometers (e.g., the broadband spectrometer and the targeted spectrometer in the MRR spectrometer 200) may interact with the same volume of sample injected into the chamber.
• multiple MRR measurements e.g., broadband and targeted measurements
• multiple pulses e.g., broadband pulses and narrowband pulses
• FID emissions in response to the pulses may be present in the vacuum chamber 202 at the same time, which may distort the measurements.
• the components of the broadband spectrometer 230 and the narrowband spectrometer 220 may be arranged to reduce the amplitude of electromagnetic radiation associated with the broadband spectrometer 230 that arrives at the detector of the targeted spectrometer 220 and vice-versa.
• cavity ringing modes e.g., resonant modes determined by the shape and dimensions of the vacuum chamber cavity
• RF absorbing materials may be added to the interior walls of the vacuum chamber 202 to absorb at least a portion of the cavity ringing modes.
• the quality factor of the vacuum chamber 202 may also be designed to be sufficiently low such that any cavity ringing modes that are excited dissipate within about 5 microseconds or less.
• the FID signal generated in response to the broadband pulse may also be time gated to reduce detection of the cavity ringing modes since the cavity ringing modes typically dissipate more quickly than the FID emissions.
• Any artifacts Attorney Docket No. BRSP-013WO01 in the detected FID signals caused by cavity ringing may also be digitally removed from the MRR spectrum by measuring a background spectrum when there is no sample present.
• the sampling inlet 206 may generally be fluidically coupled to a sample source (not shown) that provides the sample of analyte analyzed by the MRR spectrometer 200.
• the sampling inlet 206 may inject the sample into the vacuum chamber in several ways including, but not limited to, a continuous flow, pulsed injection at predetermined time intervals, and/or pulsed injection based on a trigger.
• the sample source of the MRR spectrometer 200 may include various components to prepare the sample for measurement including, but not limited to, a gas chromatograph, a liquid chromatograph, a programmable temperature vaporizer, and/or the like.
• the MRR spectrometers disclosed herein may generally include one or more sampling inlets 206 fluidically coupled to the same or different sources.
• some of the MRR spectrometers described below include two sampling inlets: one to inject an analyte sample into the vacuum chamber 202 and another to inject a reference sample into the vacuum chamber 202.
• FIG.3 shows a sampling inlet 306 that can be used in the MRR spectrometer of FIG.2 to inject both an analyte sample and a reference sample through a shared pulsed solenoid valve 368.
• This sampling inlet 306 has a three-port sample feed design: it includes an analyte sample port 362 for receiving the analyte, a reference sample port 364 for receiving the reference sample, and a purge port for purging the shared sample volume when switching between the reference and analyte samples.
• This design allows a high degree of sample introduction matching for reference and analyte samples since the same pulsed nozzle source 368 is used to inject both the reference and the analyte samples into the vacuum chamber 202.
• the sampling inlet 306 is especially useful in applications where high measurement precision is desired, like stable isotope analysis, because it provides identical sample injection characteristics for reference and analyte.
• An alternative sampling interface may include different solenoid injectors for the reference and analyte to eliminate sample cross-over contamination.
• the two sample injection systems can be placed in symmetric positions in or about the vacuum chamber 202 so that the pulsed jet expansion occurs mainly in the same active volume of the vacuum chamber 202.
• two separate sample injection systems coupled to gas sources using flexible tubes can be placed on a moveable mount that can translate the nozzle for each sample injection Attorney Docket No. BRSP-013WO01 system into an identical injection position. It is also possible to use two separate nozzles embedded in one of the cavity mirrors using the coaxially oriented beam-resonator design.
• the sample source of the MRR spectrometer 200 can include or be coupled to a gas chromatograph (GC; not shown) with a temperature-regulated flow interface.
• the GC has a carrier gas source that flows a carrier gas, such as helium, hydrogen, neon, or argon, through a column.
• the carrier gas pushes an analyte, which may have many different chemical constituents, including isomers, isotopes, isotopomers, and isotopologues, through the column and into the vacuum chamber via the flow interface.
• This analyte may be (periodically) siphoned off a continuous stream or flow of gas or liquid, accumulated and volatilized, if appropriate, and injected into the column such that the gas chromatograph effectively samples the continuous flow, much like an analog-to-digital converter (ADC) samples an analog signal.
• ADC analog-to-digital converter
• Some or all of the analyte’s constituents propagate through the column at different rates and so may appear at the end of the column at different points in time. If these points in time are separated widely enough, the constituents can be resolved at the output of the column.
• the carrier gas pushes the (at least partially separated) components through the interface and into the vacuum chamber so that the MRR spectrometer 200 can measure the components’ MRR spectra.
• This interface makes it possible to inject samples either through the GC column or directly into the vacuum chamber (e.g., for pure compounds or simple mixtures where GC separation is not necessary). In other words, some samples may require GC separation whereas others may not. Samples that do not require GC separation can be injected samples directly into the MRR spectrometer (not through the GC) while other samples could be injected through the GC.
• the sample source of MRR spectrometer 200 includes or is coupled to a liquid chromatograph (LC).
• the sample source may include a volatilization interface to volatilize at least a portion of the analyte for injection into the vacuum chamber.
• volatilization interfaces please see U.S. Pre-Grant Publication No.2021/0302340 A1, which is incorporated herein by reference in its entirety.
• the sampling interface 206 can also be coupled to (in fluid communication with) a second carrier gas source.
• the second carrier gas source flows a second carrier gas to the Attorney Docket No. BRSP-013WO01 interface for pushing or propelling the analyte components into the vacuum chamber.
• the first and second carrier gases can be different—for example, the first carrier gas may be helium or hydrogen, and the second carrier gas may be neon or argon as described in greater detail below.
• the sampling interface 206 can be coupled to (in fluid communication with) and a (chiral) tag source. Mixing chiral tags from the chiral tag source with the analyte components in a reservoir in the interface causes the chiral tags to attach themselves to the different components. The chiral tags change the moments of inertia of different enantiomers among the analyte components, making it possible to resolve and quantify the enantiomers from their MRR spectra as described below.
• the tag source may store and supply other types of tags, including polar molecules for tagging a nonpolar molecule, which has no MRR spectrum, to produce a complex that has a dipole moment and therefore can be detected by MRR.
• the sample source may include an auxiliary (universal) detector, such as a thermal conductivity detector (TCD), to trigger an MRR measurement based on the output of the GC column.
• TCD thermal conductivity detector
• This auxiliary detector may either be in-line (sample the same gas stream) or split (such as an FID or MS detector where the analytes are destroyed).
• the MRR spectrometers disclosed herein can also be coupled to a sample source that does not include a GC or a LC.
• the sample source can include a programmable temperature vaporizer to vaporize an analyte and thereafter inject the analyte into the vacuum chamber of the MRR spectrometer 200.
• the processor 216 of the MRR spectrometer 200 may also record all of the MRR data, as explained above, and discard MRR data that does not map to a chromatographic peak sensed by the auxiliary detector.
• the auxiliary detector data may be combined with the MRR data to provide a more complete analysis of the analyte, e.g., the auxiliary detector may sense components without a dipole moment while isotopic information from the MRR spectrometer 200 may complete characterization of other components.
• triggering MRR spectroscopy measurements see, e.g., U.S. Patent No.
• the MRR spectrometer 200 may also be configured to analyze samples with high molecular weight analytes.
• the upper limit on molecular weight may be about 150 amu, and the sensitivity at molecular weights of 100–150 amu may be limited because MRR spectra of room-temperature molecules tend to become extremely weak above 150 amu.
• the sampling inlet 206 of the MRR spectrometer 200 may include a pulsed- jet expansion source, continuous-wave jet, or buffer gas cooling cell to analyze molecules with higher molecular weights (e.g., up to 400 amu or higher) via this rotational cooling.
• a pulsed-jet supersonic nozzle can be used to inject analyte into the measurement chamber of the MRR spectrometer 200.
• the nozzle can receive one or more gas flows including, but not limited to, a flow inlet for analyte components, a vent valve inlet for quickly getting rid of solvents or other volatile matrix components, and an optional inlet for (additional) carrier gas.
• the inlets can be made of 1/16” tubing and can be heated to at least 300 °C to receive a sample at an elevated temperature (e.g., a direct inlet of the GC column). These tubes can be formed of PEEK plastic, but the connections could instead be made of metal(s) with better thermal properties.
• a gas connection e.g., a 1/4” gas connection
• the carrier gas is optionally provided and can be used for a purge gas to clean samples quickly.
• the multiple gas flows can combine in a reservoir, which can have a volume of about 500 mL or less.
• the combined gas flows can be co-expanded with a rare carrier gas through a pinhole nozzle with approximately 1 mm diameter.
• the pinhole nozzle is opened and closed rapidly by a solenoid valve sealed with a Teflon poppet.
• analyte component(s) travel through the pinhole and into the MRR spectrometer measurement chamber, which is maintained at high vacuum (about 10 –6 Torr).
• the molecules undergo many collisions with the carrier gas. These collisions reduce the analyte components’ rotational temperatures to approximately 1–2 K.
• analyte components separated by a GC column can be injected into the vacuum chamber with a pulsed-jet supersonic nozzle.
• the carrier gas e.g., neon
• the nozzle pulses at 10 Hz.
• the nozzle injects about 1 nmol of analyte component(s) and 1 ⁇ mol of neon into the vacuum chamber. This corresponds to approximately Attorney Docket No. BRSP-013WO01 10–15 mL/min (STP) of carrier gas going through the MRR spectrometer 200. This is comparable to typical flow rates of a wide-bore GC column.
• Injecting neon carrier gas at the nozzle may enhance pulsed valve operation. If neon carrier gas is injected at the nozzle, then the GC column can operate with a different carrier gas, such as hydrogen or helium, since the neon would dominate the rotational cooling caused by the supersonic pulsed injection of the analyte component(s) into the vacuum chamber. Both carrier gases can be injected into the vacuum chamber.
• the pulsed-jet supersonic nozzle can be used with a GC column or with other sample source components including, but not limited to, a LC and a programmable temperature vaporizer.
• the MRR spectrometers disclosed herein may include a timing controller (e.g., timing circuitry and/or electronics) to synchronize the operation of the multiple spectrometers (e.g., the broadband spectrometer and the targeted spectrometer) and/or the sampling inlet (e.g., via the pulsed valve driver).
• the timing controller may synchronize the emission of the broadband excitation pulse and the emission of the resonant excitation generated by the signal generator(s) associated with the broadband and targeted spectrometers.
• the timing controller may further synchronize the receivers to receive respective FID signals emitted in response to the broadband and resonant excitation pulses and/or the operation of the processor (e.g., to process the digitized FID signals and determine a ratio of the analyte).
• the timing controller may include a frequency standard (e.g., a rubidium atomic clock), which provides a reference clock signal (e.g., a 10 MHz clock signal) for synchronizing the operation of various components in the MRR spectrometer.
• the broadband spectrometer and the targeted spectrometer may each include a dedicated signal generator to generate the broadband and narrowband pulses and a detector (e.g., a receiver antenna) to detect the emissions from the sample.
• the signal generators, processor, and electronics for triggering the pulsed valve driver may be operably coupled to the frequency standard and locked to the same reference clock signal.
• the timing controller may include a pulse generator and/or a pulse pattern generator to generate timing signals for the respective spectrometers and/or sampling inlets of the system.
• the system includes a single signal generator that generates both the broadband and targeted excitation pulses
• that signal generator can also be used to trigger the pulsed valve driver or to emit the broadband and targeted excitation pulses in response to triggering pulses Attorney Docket No. BRSP-013WO01 from another circuit or component that controls the pulsed valve driver.
• This signal generator can also emit one or more local oscillator signals for heterodyne detection of the FID pulses emitted by the sample in response to the broadband and targeted excitation pulses. If the broadband and targeted excitation pulses and local oscillator signals are all generated by the same signal generator, then they should be phase coherent with each other, regardless of whether or not the signal generator is locked to a separate frequency reference.
• the system may include separate signal generators for the targeted and broadband measurements.
• both signal generators can be locked to the same frequency reference, with one signal generator triggering the other signal generator and other electronics or both signal generators responding to trigger pulses from another circuit or component locked to the same frequency reference.
• each signal generator emits a local oscillator (LO) for heterodyne detection of the corresponding FID pulse.
• the system may include a separate generator that generates respective LO signals in parallel with the signal generators that generate the excitation pulses.
• the separate generator may be, for example, circuitry incorporated into the broadband or targeted spectrometers.
• FIGS.4A and 4B illustrate analog front ends 440a and 440b, respectively, for detecting and digitizing FID pulses emitted by a sample in response to the broadband and targeted excitation pulses.
• the analog front end 440a in FIG.4B has separate channels for the broadband and targeted FID pulses.
• Each channel includes a mixer 442a, 442b for mixing the corresponding FID pulses with a corresponding local oscillator (LO) signal to produce an intermediate frequency (IF) output at a lower center frequency (e.g., 100 MHz versus 2–18 GHz for the FID pulses).
• LO local oscillator
• Each channel also includes its own digitizer (ADC) 444a, 444b for digitizing the corresponding IF output and provided the resulting digitized FID pulse to a corresponding input channel of the processor 216, which Fourier-transforms the digitized FID pulses to produce the broadband and targeted MRR spectra of the sample.
• ADC analog front end 440b in FIG.4B also includes separate mixers 442a, 442b for the FID pulses emitted by the sample in response to the broadband and targeted excitation pulses.
• the FID signals are mixed with LO signals at different frequencies f1 Attorney Docket No. BRSP-013WO01 and f2 to different, non-overlapping IF bands (e.g., 75–125 MHz and 150–200 MHz).
• the IF may range between about 0 GHz and 3 GHz, including all values and sub- ranges in between.
• the LO frequencies may range from about 2 GHz to about 18 GHz, depending on the FID center frequency and the target IF bands.
• the bandwidth of each frequency channel may be about 30 MHz.
• the spacing between the different IF ranges may range from about 500 MHz to about 1500 MHz.
• These frequency-downconverted FID signals emitted by the mixers 442a, 442b are digitized by the same digitizer 446, which provides the digitized output to the processor 216 for spectrum generation and other additional processing.
• the mixers 442a, 442b and LOs operate to frequency-multiplex the broadband and targeted FID signals onto separate frequency-multiplexed channels within the band of the same ADC 446, allowing the ADC 446 to digitize both signals simultaneously.
• the processor 216 can separate or demultiplex the signals by filtering them in the frequency domain. Spurs in the signals can also be filtered during post-processing using a background measurement.
• FIGS. 4A and 4B are non-limiting examples.
• the broadband and targeted FID pulses may be combined before mixing and/or amplification.
• the analog front end includes a single mixer and a single ADC.
• the analog front ends in FIGS.4A and 4B may also be used in MRR spectrometers that include two targeted spectrometers where the FID pulses generated for each targeted spectrometer is mixed and/or digitized in separate channels. 3.
• MRR Spectrometer Cavities FIGS. 5A–5H illustrate different cavity designs for MRR ratiometers. Each cavity design features two spectrometers: one targeted spectrometer and either another targeted spectrometer or a broadband spectrometer.
• FIG.5A shows an example MRR ratiometer 500a with a vacuum chamber 502 shaped as a six-way cross with a first axis 521 for a targeted spectrometer 520, a second axis 531 for a broadband spectrometer 530, and a third axis (not shown) for other hardware (e.g., a port for connection to a vacuum pump and/or a pressure gauge).
• the first, second, and third axes may be orthogonal to one another.
• the targeted spectrometer 520 is a Ball-Flygare spectrometer with a Fabry-Perot cavity formed by a fixed mirror 522a and a movable mirror 522b at opposite ends of the vacuum cavity 502 along the first axis 521.
• a motorized translation stage 524 can move the movable mirror 522b back and forth over a travel range L along the first axis 521 to change the targeted spectrometer’s resonance frequency.
• the targeted spectrometer 520 also includes a source antenna 526a and a receiver antenna 526b mounted coaxially with the fixed mirror 522a.
• the broadband spectrometer 530 includes a source horn antenna 532a and a receiver horn antenna 532b in the vacuum chamber 502 at opposite ends of the second axis 531.
• the MRR ratiometer 500a also includes hardware control circuitry 510, timing control circuitry 512, an analog front end and digitizer 514, a processor 516, and one or more signal generators 518. These electronics function as described above, with the signal generator(s) 518 generating the targeted and broadband excitation pulses; the analog front end and digitizer 514 amplifying, filtering, and digitizing the FID signals; and the processor 516 computes the Fourier transforms of the FID signals and the ratios of the species of interest.
• the hardware control circuitry 510 and the timing control circuitry 512 synchronizes the emission of the excitation pulses with the injection of the analyte or reference sample into the vacuum chamber 502 with a a sampling interface or nozzle 506 fluidically coupled to a sample source 508.
• the source antenna 526a emits a targeted excitation pulse and the receiver antenna 526b receives the FID signal from the sample in response to the targeted excitation pulse.
• the source horn antenna 532a emits a broadband excitation pulse and the receiver horn antenna 532b detects an FID signal from the sample in response to the broadband excitation pulse.
• FID emissions travel in the same direction as the corresponding excitation pulse.
• FID emissions stimulated by a broadband pulse are likely to propagate along the second axis 531 and less likely to propagate along the first axis 521.
• FID emissions associated with a narrowband pulse are unlikely to enter the axis 531 for the broadband spectrometer 530.
• the frequency range of detection for each spectrometer 520, 530 is typically different. Thus, even if a portion of the FID emission associated with the broadband (narrowband) pulse travels along the axis associated with the targeted (broadband) spectrometer 520 (530), the detector for that spectrometer shouldn’t detect that FID emission.
• FIG. 5B and 5C illustrate MRR ratiometers 500b and 500c, respectively, each of which includes two targeted, cavity-enhanced FTMW (Balle-Flygare) spectrometers 520a, 520b with orthogonal cavity axis orientations.
• the reference and analyte samples are Attorney Docket No. BRSP-013WO01 introduced into a region 501 of the spectrometer where the cavity volumes for the two FTMW spectrometers 520a, 520b overlap. This can be achieved by placing the pulsed nozzle source(s) 506b for the analyte and reference samples above (and/or below) the plane of the FTMW spectrometers 520a, 520b as shown in FIG. 5B.
• FIG. 5D shows an MRR ratiometer 500d with a pair of coaxial, overlapping, polarization-multiplexed microwave cavities in the vacuum chamber.
• the first cavity is defined by a fixed, curved mirror 582 and a movable flat mirror 584 on a first translation stage 585.
• the second cavity is defined by the fixed, curved mirror 582 and a movable wire grid polarizer 586 between the fixed, curved mirror 582 and the movable flat mirror 584.
• the movable wire grid polarizer 586 is on a second translation stage 587 that can be actuated independently of the first translation stage 585 and may be mounted on the first translation stage 585 as shown in FIG.5D.
• Two nozzles 560, 562 inject the analyte and reference samples, respectively, into a common sample region between the fixed, curved mirror 582 and the movable wire grid polarizer 586.
• Source and receiver antennas (not shown) can be mounted with or through the fixed, curved mirror 582 or the movable flat mirror 584, just like the source and receiver antennas 526a, 526b in FIG.5A, though mounting them in the fixed, curved mirror 582 tends to simplify the design and construction of the MRR ratiometer 500d.
• the wire grid polarizer 526 transmits radiation polarized orthogonal to the grid wires and reflects radiation polarized parallel to the grid wires. As a result, radiation polarized orthogonal to the grid wires resonates in the first cavity, and radiation polarized parallel to the grid wires resonates in the second cavity.
• the first cavity has a resonance frequency defined by the distance between the fixed, curved mirror 582 and the movable flat mirror 584
• the second cavity has a resonance frequency defined by the distance between the fixed, curved mirror 582 and the movable wire grid polarizer 526. Both cavities are semi-confocal because they have movable flat reflective surfaces and a (shared) curved mirror.
• the MRR spectrometer 500d in FIG.5D has at least two advantages over the crossed cavity designs shown in FIGS.5A–5C: (1) it has a smaller footprint, making it better suited to analytical chemistry lab spaces; and (2) the cylindrical symmetry of the cavity axes for both molecular species makes it easier to position dual pulsed-jet sample sources such that they inject the sample into the same active volume of the spectrometer.
• Wire grid polarizers with exceptional performance are available commercially. For example, Millitech, Inc. makes a wire grid polarizer for microwave and THz applications that can be fabricated up to 9” diameter. The microwave optic uses tungsten wire with a diameter of 0.001”.
• a wire grid polarizer with 200 wires per inch can have > 99% transmission for radiation at 200 GHz when the electric field is polarized orthogonal to the wires and less than 1% transmission when the electric field is parallel to the wire winding. Performance at the lower frequencies used in MRR spectroscopy (e.g., below 20 GHz) can be even better. Wire grid polarizers are also excellent reflectors at THz frequencies. Future manufacturing advances may make it possible to fabricate a high-performance wire grid polarizer with a spherical shape for use as a confocal cavity mirror.
• FIGS. 5E–5H show MRR ratiometers 500e–500h, respectively, with folded cavities using a wire grid polarizer 540 as a polarizing beam splitter.
• the wire grid polarizer 540 is a fixed at a 45° angle with respect to the axis of a microwave cavity for a Balle-Flygare (targeted) spectrometer.
• source and receiver antennas (not shown) can be mounted with or through any of the mirrors in these ratiometers, though mounting them in the fixed mirror tends to simplify design and construction.
• the folded cavity is a confocal cavity defined by a fixed, curved mirror 542, 542’ on one side of the wire grid polarizer 540 and a pair of movable curved mirrors 544 on the other side of the wire grid polarizer 540.
• the folded cavity is a semi-confocal cavity defined by either a fixed, flat mirror 546 and movable curved mirrors 544 or by a fixed, curved mirror 542 and movable flat mirrors 544’.
• an analyte nozzle 560 injects the analyte sample into the vacuum chamber from the top
• a reference nozzle 562 injects the reference sample into the vacuum chamber from the side.
• the analyte and reference nozzles 560, 562 inject the samples into the vacuum chamber through the fixed, curved mirror 542’.
• the nozzles inject the analyte and reference samples into substantially identical or overlapping volumes in the vacuum chamber for more consistent FID stimulation and detection.
• Both MRR ratiometers 500e and 500f include source and receiver antennas (not shown) mounted with or through the fixed, curved mirror 542, 542’ just like the source and receiver antennas 526a, 526b in FIG.5A.
• the wire grid polarizer 540 acts a polarizing beam splitter, transmitting microwave radiation polarized orthogonal to its wires and reflecting Attorney Docket No. BRSP-013WO01 microwave radiation polarized parallel to its wires.
• the transmitted radiation reflects off one movable mirror 544, 544’ back through the wire grid polarizer 540 to the receiver antenna and fixed, curved mirror 542, 542’.
• the reflected radiation reflects off the other movable mirror 544, 544’ back to the wire grid polarizer 540, which reflects the radiation back to the receiver antenna and fixed mirror 542, 542’, 546.
• the moveable mirrors 544, 544’ can be moved independently of each other to tune the cavity resonances for the orthogonal polarizations to different molecular resonance frequencies.
• Having separate moveable mirrors to define the cavity lengths for the two orthogonal electric field polarizations provides additional space to place the antennas for coupling the microwave pulses into and out of the cavity for the FTMW measurement.
• Some of the MRR ratiometer designs described herein be extended to measure two different ratios simultaneously. For instance, an MRR ratiometer as in FIG. 5A with a dual- polarization horn antenna can monitor two different “strong” signals at the same time using two orthogonal polarizations. Incorporating a wire grid polarizer in the spectrometer cavity makes it possible to measure two separate “weak” signals (from the sample impurity) at the same time.
• FIGS.6A and 6B illustrate an example method for performing broadband and targeted measurements simultaneously to determine a ratio of one component of the sample to another component of the sample (e.g., a ratio of isomers, isotopologues, and/or isotopomers, an enantiomeric ratio).
• an MRR ratiometry measurement involves injecting the sample (either an analyte or a reference sample) into the vacuum chamber of the MRR ratiometer Attorney Docket No. BRSP-013WO01 (602). Once the sample is in the chamber, the MRR ratiometer measures a first MRR spectrum of the sample with a first excitation pulse (604) and, at the same time, a second MRR spectrum of the sample with a second excitation pulse (606).
• Both excitation pulses can be resonant pulses with different resonance frequencies, or one excitation pulse can be a broadband (e.g., chirped) pulse and the other pulse can be a resonant pulse.
• the MRR ratiometer quantifies first and second components (species) of the sample based on the first and second MRR spectra, respectively (608, 610), compares quantities of the first and second components (612), and determines a ratio of the first and second components (e.g., a ratio of isomers, isotopologues, and/or isotopomers, an enantiomeric ratio) (614).
• the period of time a sample pulse or a portion of a sample stream may be exposed to broadband and narrowband pulses may be appreciably longer than the dephasing time for FID.
• a sample pulse may be present in the vacuum chamber for a time period on the order of milliseconds while FID emissions may occur on the order of microseconds as shown in FIG. 6B.
• the MRR ratiometer may probe a particular sample pulse or portion of the sample stream with multiple broadband and/or narrowband pulses and detect corresponding FID emissions in response to the broadband and/or narrowband pulses (see, for example, the timing diagram of FIG.6B).
• the durations of the excitation pulses on the spectrometers are analyte dependent, and may be the same of different. Ideally, the detections are exactly simultaneous (though this is not be a strict requirement; the pulses could be alternated to reduce or avoid cross talk).
• the MRR spectrometer may illuminate the sample pulse with the same broadband pulse and/or narrowband pulse multiple times to improve the signal-to-noise ratio of the detected FID signals (e.g., by summing the spectra obtained from the multiple broadband pulses or narrowband pulses).
• the MRR spectrometer may perform a frequency sweep on the same sample pulse where multiple narrowband pulses covering a set of frequencies are generated one after the other and recording the FID signals in response to the narrowband pulses at the different frequencies.
• the MRR ratiometers disclosed herein can perform broadband and targeted measurements on the same sample simultaneously, they can also perform broadband measurements and targeted measurements separately, on separate samples, as desired.
• MRR Molecular rotational resonance
• MRR spectroscopy that used pulsed-jet expansions of the analyte seeded in a noble gas (commonly neon) into vacuum have unmatched spectral resolution in the field of analytical chemistry.
• This feature of the instruments for MRR spectroscopy makes it possible to analyze individual chemical species within a complex sample matrix without prior chemical separation by chromatography methods.
• MRR spectroscopy is especially useful for making measurements that are difficult or impossible to make with established analytical chemistry techniques like nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (often coupled with gas chromatography), infrared spectroscopy, and electronic spectroscopy.
• NMR nuclear magnetic resonance
• mass spectrometry often coupled with gas chromatography
• infrared spectroscopy and electronic spectroscopy.
• An inventive MRR ratiometer can perform a spectroscopic chiral analysis with high accuracy and speed. Chiral analysis using an inventive MRR ratiometer builds on chiral tag rotational spectroscopy. Chiral tag measurements provide accurate determinations of the enantiomeric excess (EE) of the analyte. EE is a measure of the relative abundance of the two non- superimposable mirror image stereoisomers. This measurement approach uses chiral derivatization to generate spectroscopically distinguishable chemical species from the enantiomers (where the enantiomers have identical rotational spectra absent derivatization).
• chiral tag rotational spectroscopy derivatization is accomplished using noncovalent attachment of a small chiral molecule, or tag, via cluster formation in the pulsed-jet expansion of a mixture of the analyte and tag molecules. No additional chemical synthesis steps are performed unlike chiral derivatization approaches in NMR spectroscopy. Additionally, with this approach, the derivatization does not affect the stereochemistry of the chiral center (a process known as racemization that compromises the accuracy of the analysis). A small number of small, volatile, chiral molecules have been shown to efficiently complex with a wide range of chemical analytes and allow for precise enantiomeric excess determinations of these compounds within mixtures.
• FIGS.7A and 7B show the idea behind chiral tagging.
• chiral tagging involves “tagging” analytes with a small, chiral molecule of known stereochemistry, such as the Attorney Docket No. BRSP-013WO01 verbenone/butynol system in FIG. 7A.
• FIG. 7A shows that enantiomers of verbenone (an example chiral molecule) have identical MRR spectra as they have the same moments of inertia.
• FIG. 7B shows two spectral lines, one of each of the heterochiral and homochiral complexes in FIG.7A.
• an inventive MRR ratiometer may include an additional valve coupled to a source supplying the gas phase chiral tag at the interface of a gas chromatography outlet before the sampling inlet (e.g., the nozzle).
• the chiral tag may be mixed in with the eluting sample post-column (e.g., after the sample exits the outlet of the gas chromatography).
• a spectral library containing known compounds and their respective MRR spectra may include compounds with chiral tags.
• broadband and targeted measurements may be used to determine the enantiomeric ratio between two enantiomers in the sample, the enantiomeric excess (e.g., the purity of chiral substances), the absolute configurations of chiral components in a sample, and/or the achiral purity of the sample.
• Heterochiral complexes are formed by clusters of opposite chirality.
• the geometries of the homochiral and heterochiral complexes have different mass distributions (and, therefore, different rotational constants), producing distinct MRR spectra.
• the derivation of the formula used to determine the EE has one assumption.
• the number densities of the homochiral and heterochiral complexes are linearly proportional to the number densities of the tag and analyte in the pulsed jet expansion.
• the intensity of a transition in the homochiral and heterochiral MRR spectra can be written: (1) (2)
• the constants C Homo and C Hetero include all of the instrument response and spectroscopic terms that relate a measured signal to the number density (or concentration) of the species in the sample.
• the chiral tag measurement uses transition intensities from two different samples. In one case, a racemic sample of the tag is used (this is the reference sample). The second measurement uses a high enantiopurity tag sample (where the EE is known from separate analysis). The EE determination uses the normalized transition intensities defined using the intensities in the spectrum using racemic and enantiopure tag samples as: and similarly for the heterochiral spectrum intensities.
• the analyte serves as both the tag and the analyte and the analysis uses the homochiral and heterochiral dimers of TFIP formed in the pulsed jet expansion.
• the EE is determined from: FIG.8 shows a measured calibration curve, with at least three replicate measurements for each sample.
• the EE for each reference sample was measured using the “auto tag” variant of chiral tag rotational spectroscopy in a broadband chirped-pulse FTMW spectrometer where dimers of TFIP are used in the analysis.
• the EE determination for each reference sample is performed in triplicate. Chiral tag rotational spectroscopy gives the EE directly from the measurement so that analyte samples of known EE are not required.
• the characteristics of this measurement can be summarized as follows: (1) the measured EE is exactly the reference sample EE (that is, the measurement does not need to be calibrated using a sample with a known EE from a different chiral analysis technique); (2) the method is accurate over the full EE range; and (3) the measurement has high precision as indicated by the small spread in the replicate measurements for each reference sample.
• the chiral tag method can be implemented in a MRR ratiometer. As seen in Eq. (5), the EE determination is based on an accurate determination of the quantity, R, defined using Eqs. (3) and (4): The second expression indicates that an MRR ratiometer should simultaneously measure two different transitions – one from the spectrum of the homochiral complex and one from the heterochiral complex.
• an inventive MRR ratiometer can: (1) use a cavity- enhanced FTMW spectrometer to reduce the measurement time to measure the weaker signal in the enantiopure tag measurement; (2) simultaneously measure both the homochiral and heterochiral signals on each injection of the sample; and (3) inject two different samples into the instrument – one where enantiopure tag is used and the other where the reference measurement uses a racemic tag sample.
• the accuracy and precision of chiral tag rotational spectroscopy exceeds the “gold standard” method of chiral gas chromatography.
• chiral tag rotational spectroscopy has the potential for high-speed operation.
• the MRR ratiometers described here make it possible to realize the potential of the technique.
• the MRR ratiometer instrument described here has two important design additions that combat signal drift: the simultaneous measurement of the two signals used to determine the EE (coming from rotational transitions of the chemically distinct homochiral and Attorney Docket No. BRSP-013WO01 heterochiral tag complexes) and the ability to rapidly switch between the sample being measured (the analyte) and reference sample that is used in measurement calibration 5.4 Stable Isotope Analysis
• a second analytical chemistry application enabled by an inventive MRR ratiometer is site-specific stable isotope analysis. Stable isotope analysis is an established field of analytical chemistry that measures small variations of the relative abundance is isotopes relative to a reference sample.
• a common analysis determines the variation in the 13 C: 12 C isotope ratio of chemical substance relative to a reference sample. These variations are a small fraction of the average natural abundance of the stable isotope.
• the variation is reported in the parts-per-thousand level: Variations in the 13 C: 12 C ratio are caused by small kinetic isotope effects in the synthetic pathway that generated the sample.
• stable isotope analysis can differentiate molecular samples that are created by different natural or human production processes. This measurement capability can be used, for example, to validate the origin of a chemical sample. This measurement can be used to validate the origin of expensive chemicals and combat counterfeiting of chemical products.
• the isotopomers, isotopic isomers with the same number of 13 C nuclei but with different structural positions for the isotopic substitutions are not easily distinguished by mass spectrometry.
• To unlock the full chemical information in a 13 C stable isotope measurement involves a technique that gives different, fully-resolved signatures for each isotopomer and can detect the signal at 100,000:1 signal-to-noise ratio (or better) so that the variations from natural abundance can be measured with sufficient precision.
• very small Attorney Docket No. BRSP-013WO01 molecules where infrared spectroscopy is applicable only two analytical chemistry techniques have the possibility of meeting these measurement criteria: NMR spectroscopy and MRR spectroscopy.
• the high spectral resolution of MRR spectrometers ensures that these different spectral signatures can be measured without spectral overlap.
• Rewriting of Eq. (9) shows that the stable- isotope measurement can be performed in an MRR ratiometer where the two signals monitored are for the 12 C and one of the singly substituted 13 C isotopomers:
• the measurement should perform the measurement on the two different samples (the analyte and reference).
• the measurement should preferably remove the effects of number density fluctuations in the amount of sample injected in each measurement cycle so that the measurement precision is limited primarily or, in some instances, only by the signal-to-noise ratio of the weaker 13 C isotopomer MRR transition.
• FIGS. 9A–9C illustrate signal characterization measurements using TFIP. This sample is also used to validate ratiometer measurements for EE determinations and stable isotope measurements as described below.
• FIG. 9A shows the signal variation for individual MRR spectroscopy transitions observed in a measurement of TFIP.
• the variation, or fluctuation, of the signals in back-to- back measurements are analyzed in FIG.9B for transitions of the TFIP dimer and FIG.9C for transitions of the TFIP monomer. In both cases, the intensity fluctuations, measured as the standard deviation in the ten separate measurements, are proportional to the intensity of the transition.
• the proportionality constant is twice as large for dimer transitions compared to monomer transitions.
• the number density of the dimer is proportional Attorney Docket No. BRSP-013WO01 to the square of the number density of the monomer, the percentage fluctuation should be twice as large for dimer signals as for monomer signals.
• the expectations of signal fluctuations caused by variation in the number density for sample infection are met as shown in FIGS.9A–9C.
• FIG.9A when the signal fluctuations are caused by pulse-to- pulse variations in the number density from the pulsed valve used to inject the gas mixture into the vacuum chamber, there are correlations in the signal levels for all transitions. This behavior enables the ratiometer design. Performing simultaneous measurements of the signal intensity for two transitions and determining the ratio for each measurement removes the effects of number density variation.
• the effectiveness of a performing ratio measurement on each signal acquisition cycle can be assessed by considering the fluctuation in the ratio.
• the signal intensities, I1 and I2 measured used to measure a ratio R, under the conditions where the fluctuations of the two signals are uncorrelated, have a normal distribution and non-zero mean (with standard deviation much smaller than the intensity).
• the fluctuation in the ratio is related to the fluctuations in the two signals as, Consider the example of measuring the fluctuation in the ratio of the 13 C: 12 C signals of TFIP.
• FIG. 10 shows 13 C: 12 C isotope ratio measurement results from the broadband spectrometer.
• TFIP has three carbon atoms, so for each of these 12 C transitions, there are three 13 C: 12 C ratios that can be measured and these are shown as the individual data points.
• the fluctuation in the ratio is given on the y-axis.
• the isotope ratios are measured for three different numbers of signal averages.
• the solid lines in FIG. 10 are calculated from Eq. (15) and the measured noise properties of the chirped-pulse FTMW spectrometer used to make the measurement.
• the isotope ratio fluctuation depends on both the number of signal averages and intensity of the 13 C transition used in the ratio.
• the MRR ratiometer design in FIG. 5A is an extension of a Balle-Flygare cavity-enhanced FTMW spectrometer where two horn antennas are used to make broadband MRR measurements across the cavity of the FTMW spectrometer.
• the horn antennas do not require any mechanical motion control to tune the measurement system to resonance.
• the horn antennas can monitor multiple transition intensities in each measurement cycle since there is no resonance frequency restriction. This capability adds flexibility in the analytical chemistry measurement methodology.
• the MRR ratiometer of FIG.5A is especially useful in cases where the impurity species is present in low abundance—a key measurement challenge in analytical chemistry. Since one species is present in much higher abundance, the signals can be detected with good sensitivity without the enhancement offered by a cavity resonator FTMW spectrometer.
• the pulsed jet source for injecting the sample into the vacuum chamber can be either perpendicular to the cavity and horn-antenna axis (used for the 13 C measurements) or placed in the cavity mirror in the coaxially oriented beam-resonator arrangement.
• FIGS.11–14 show the performance of this MRR ratiometer for both EE determinations and 13 C stable isotope ratio measurements. TFIP was used as the test sample.
• the 13 C stable isotope ratio tests were performed on the monomer spectrum. EE determinations of high enantiopurity sample of (R)-TFIP used ratios of transitions for the TFIP dimer employing the auto tag method. Two different prototypes were constructed. A ratiometer where the sample was injected into the shared active sample volume using a nozzle placed above the plane of the instrument, and centered on the common active volume, was used for the 13 C stable isotope Attorney Docket No. BRSP-013WO01 test measurements. An instrument where the sample is injected coaxially with the cavity- enhanced measurement system, the COBRA arrangement, was used for EE determinations. FIGS. 11–13 show the results for the 13 C stable isotope measurements. FIG.
• FIG. 11 demonstrates that the ratiometer design of FIG.5A removes the effects of back-to-back number density variation in the pulsed-jet sample introduction.
• the black data points show the measured values for the 12 C (normal species) and 13 C isotopomer transition intensities for ten sequential 10kavg measurements. Individually, both the 12 C and 13 C transition intensity measurements have a measurement fluctuation, measured by the standard deviation of the ten different intensity measurements of about 10%. This result, a constant percentage fluctuation of the signal, is consistent with number density fluctuations being the cause of the signal variation as discussed above. If the measurements were uncorrelated, these fluctuations would produce a 14% percent variation in the 13 C: 12 C stable isotope signal ratio.
• the data in FIG.11 illustrate that 12 C and 13 C transition intensities measured simultaneously in the same pulsed jet expansion are highly correlated (i.e., the data points fall on a line not in the circular pattern that would be observed for uncorrelated transition intensities).
• the ratio measurement is shown by the data points with lighter shading.
• the fluctuation in the ratio is 0.8%—limited by the signal-to-noise ratio on the 13 C transition monitored in the instrument.
• This performance shows that the effects of number density fluctuation, which would have produced a 14% variation in the signal ratio, are mitigated with the MRR ratiometer design.
• a successful ratiometer should decrease or, more preferably, eliminate long-term drifts in the signal ratio.
• FIG. 12 shows the 13 C: 12 C stable isotope ratio in sixty back-to-back ratiometer measurement on TFIP (with 1 kavg in each measurement, about 1.5 minutes per observation). A slow drift in the ratio is clearly observed. The effect of switching between reference and analyte samples—and measuring the difference in the ratio as in Eq.
• the MRR ratiometer can remove the effects of sample number density fluctuations by simultaneous measurement of the signals of homochiral (strong) and heterochiral (weak) dimer transitions.
• three TFIP test measurements five back-to-back measurements were performed.
• the individual homochiral and heterochiral measurements show a constant percent fluctuation: 7% for both the cavity- enhanced heterochiral dimer complex transition intensity and the homochiral transition intensity. If these signals were uncorrelated, a 10% fluctuation in the term in Eq. (8) would be observed. However, the ability to measure both signals simultaneously gives a reduced fluctuation of the ratio of 2%.
• FIG.14 !
• a molecular rotational resonance (MRR) measurement apparatus comprising: a vacuum chamber to hold a sample; at least one signal generator to emit a first excitation pulse and a second excitation pulse; a first transmitter, operably coupled to the at least one signal generator, to illuminate the sample with the first excitation pulse; a first receiver to receive a first free-induction decay (FID) signal emitted by the sample in response to the first excitation pulse; a second transmitter, operably coupled to the at least one signal generator, to illuminate the sample with the second excitation pulse while the first transmitter illuminates the sample with the first excitation pulse; a second receiver to receive a second FID signal emitted by the sample in response to the second excitation pulse; and a processor, operably coupled to the first receiver and the second receiver, to determine a relative indication
• FID free-induction decay
• the MRR measurement apparatus of clause 1 wherein the first transmitter is configured to illuminate the sample with the first excitation pulse along a first axis and the second transmitter is configured to illuminate the sample with the second excitation pulse along a second axis different than the first axis.
• Clause 3. The MRR measurement apparatus of clause 1, wherein the first excitation pulse is a first resonant excitation pulse and the second excitation pulse is a second resonant excitation pulse.
• the MRR measurement apparatus of clause 1, wherein the first excitation pulse comprises a broadband excitation pulse and the second excitation pulse comprises a resonant excitation pulse encompassing a bandwidth narrower than a bandwidth of the broadband excitation pulse. Clause 5.
• Clause 6 The MRR measurement apparatus of clause 5, wherein the resonant excitation pulse and the broadband excitation pulse include frequency components ranging from 2 GHz to 18 GHz.
• Attorney Docket No. BRSP-013WO01 Clause 7.
• the MRR measurement apparatus of clause 4, wherein the broadband excitation pulse is a chirped excitation pulse.
• a timing controller operably coupled to the sampling interface and the at least one signal generator, to trigger emission of the first excitation pulse and the second excitation pulse in coordination with injection of the sample.
• the MRR measurement apparatus of clause 12 wherein the timing controller includes a frequency standard configured to generate a reference frequency clock signal to synchronize the emission of the first excitation pulse with the emission of the second excitation pulse. Clause 14. The MRR measurement apparatus of clause 12, wherein the timing controller is further configured to trigger the first receiver to receive the first FID signal, the second receiver is further configured to receive the second FID signal, and the relative indication is a ratio of the first component to the second component. Clause 15. The MRR measurement apparatus of clause 1, further comprising: a sampling interface, in fluid communication with the vacuum chamber, to inject the sample into the vacuum chamber. Attorney Docket No. BRSP-013WO01 Clause 16.
• the MRR measurement apparatus of clause 1 further comprising: at least one analog-to-digital converter, operably coupling the first receiver and the second receiver to the processor, to convert analog outputs of the first receiver and the second receiver into digital signals for the processor.
• at least one analog-to-digital converter has a bandwidth ranging from 0 GHz to 3 GHz.
• Clause 18 The MRR measurement apparatus of clause 1, wherein the first excitation pulse is in a first polarization state and the second excitation pulse is in a second polarization state different than the first polarization state and further comprising: a wire grid polarizer, disposed in the vacuum chamber, to transmit the first excitation pulse and to reflect the second excitation pulse.
• the MRR measurement apparatus of clause 1, wherein the relative indication of the at least two components is one of an enantiomeric ratio, a ratio of two isotopic species, or a ratio of two different chemical species.
• a method of measuring a ratio of a first component of a sample to a second component of the sample comprising: injecting the sample into a vacuum chamber; obtaining a first molecular rotational resonance (MRR) spectrum of the sample; obtaining a second MRR spectrum of the sample with a second excitation pulse contemporaneously with obtaining the first MRR spectrum of the sample; and determining the ratio of the first component to the second component based on the first MRR spectrum and the second MRR spectrum.
• MRR molecular rotational resonance
• BRSP-013WO01 Clause 24.
• the sample comprises about 10 to about 100 compounds; measuring the first MRR spectrum includes measuring free induction decay (FID) signals from about 10 to about 100 compounds in the sample; and measuring the second MRR spectrum includes measuring at least one FID signal from a single compound in the sample.
• FID free induction decay
• the first excitation pulse is a first resonant excitation pulse and the second excitation pulse is a second resonant excitation pulse emitted at a frequency different from the first resonant excitation pulse.
• any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.
• Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions and arrangement of respective elements of the exemplary implementations without departing from the scope of the present disclosure.
• the use of a numerical range does not preclude equivalents that fall outside the range that fulfill the same function, in the same way, to produce the same result.
• various inventive concepts may be embodied as one or more methods, of which at least one example has been provided. The acts performed as part of the method may in some instances be ordered in different ways.
• respective acts of a given method may be performed in an order different than specifically illustrated, which may include performing some acts simultaneously (even if such acts are shown as sequential acts in illustrative embodiments).
• BRSP-013WO01 identified by the “and/or” clause, whether related or unrelated to those elements specifically identified.
• a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.
• “or” should be understood to have the same meaning as “and/or” as defined above.
• At least one of A and B can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

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