JPH0361982B2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to improvements in quadrupole mass spectrometers.

(Prior art and its problems) As shown in FIG. 2, a quadrupole mass spectrometer includes quadrupole 1, 2, 3,
4, one opposing electrode pair 1 and 3 has +U+
The voltage Vcosωt is applied to the other opposing electrode pair 2,
By applying a voltage −U−Vcosωt to 4, among the ions ionized by an ion source (not shown) and incident on this quadrupole part, the angle between the DC voltage U and the high-frequency voltage peak value V is determined. Only ions with a mass-to-charge ratio m/z determined by the value of the frequency ω are allowed to draw a stable trajectory and pass through the quadrupole analysis section, and then are transferred to an ion collector at the exit of the quadrupole analysis section. This is how it is detected.

Here, the mass scanning of the ions to be detected is performed while keeping the ratio U/V of the DC voltage U and the high-frequency voltage peak value V in a constant relationship. ) or by changing the angular frequency ω.

FIG. 2 shows the mass scan described above in U/V and ω
This is an electric circuit of a conventional example of a quadrupole electric field power source in a quadrupole mass spectrometer which is operated by changing U (and therefore V) while holding constant.

To explain FIG. 2, when a reference voltage Vref (generally a sweep voltage) corresponding to the scanning mass number is outputted from a reference signal generator 5, a high frequency generator 7 with an angular frequency ω is generated via an error amplifier 6 (described later). is suppressed,
The output high frequency voltage is input to the primary winding of the high frequency transformer 8. By the inductance of the secondary winding of the high frequency transformer 8 and the capacitors 8 and 10 which are the combined equivalent capacitance of the tuning capacitor, the balance capacitor, and the capacitance between the quadrupole 1, 2, 3, 4, etc. A resonant circuit is formed, and a high-frequency voltage +Vcosωt (the negative side is V-cosωt, hereinafter the negative side is shown in parentheses) is induced in the secondary winding of the high-frequency transformer 8, and the DC blocking capacitor C ₄ (C ₄ ') to the quadrupole electrodes 1, 3 (2, 4). On the other hand, the high frequency divided voltage divided from the voltage dividing point P (P') of the secondary winding of the high frequency transformer 8 is passed through the rectifying element D ₂
(D ₂ ′), a rectifier circuit consisting of a resistor R ₃ (R ₃ ′) and a capacitor C ₅ (C ₅ ′), the DC voltage +U (-U)
This DC voltage is used as a high-frequency blocking circuit.
Quadrupole electrodes _{1 ,} 3 (2,
4) and the above high frequency voltage +Vcosωt(-
Vcosωt) and quadrupole electrodes 1, 3 (2,
4) is applied with a superimposed voltage of +U+Vcosωt (-U−Vcosωt). Here, the error amplifier 6 has
The DC voltage component U is input and compared with the reference voltage Vref to control the high frequency generator 7, thereby maintaining the above-mentioned U/V in a constant relationship.

However, even when attempting to accurately detect the high-frequency voltage Vcosωt using the power supply circuit as described above, diodes generally have a fairly large curved portion in their forward characteristics, and these characteristics are greatly affected by temperature. Therefore, the linearity of detection is poor and unstable. Even if a DC detector for general communications with relatively good linearity is used, there is also a curved portion of the forward characteristic according to the 3/2 law, and the linearity of detection is still poor.

Therefore, in the circuit shown in Fig. 2, it is not possible to keep the U/V constant and keep the resolution and transmittance constant over the entire wide scanning range, and the resolution and transmittance are particularly high in the low voltage parts of U and V. Analysis accuracy cannot be expected.

In order to eliminate this drawback, various methods have been devised and tried over the past ten years, including those in the following publications.

Special Publication No. 45-26679 (quadrupole mass spectrometer) uses a synchronous rectification method.

Special Publication Showa 47-4588 (power supply for quadrupole mass spectrometer)
Here is a method that keeps the operating point of the rectifier constant.

Special Publication Showa 47-18191 (power supply for quadrupole mass spectrometer)
Here's the DC feedback method.

Special Publication No. 45-32783 (quadrupole mass spectrometry) uses a separate rectifier for correction.

However, none of the systems disclosed in the above-mentioned publications are currently commercially available, and the circuit shown in FIG. 3 is currently commonly used.

To explain FIG. 3, the reference voltage Vref is output from the reference signal generator 5, the high frequency generator 7 is controlled via the error amplifier 6, and the high frequency voltage generated here is applied to the primary winding of the high frequency transformer 8'. input to the line. A high-frequency voltage is generated by a resonant circuit including capacitors 9 and 10, which are the combined equivalent capacitance of the inductance of the secondary winding of the high-frequency transformer 8' and various capacitances.
Vcosωt (−Vcosωt) is induced. On the other hand, voltage dividing capacitors C ₁ and C ₂ (C ₁ ′, C ₂ ′) and DC detector D ₃
The peak value Vdet of the high frequency voltage detected by the detection circuit consisting of (D ₃ ′) and resistor R ₄ (R ₄ ′) is
On the one hand, it is input to the error amplifier 6 and compared with the reference voltage Vref to control the high frequency generator 7, and on the other hand, it is input to the DC amplifier 11 (12) to generate the DC voltage U.
(-U) and high frequency blocking CH ₂
(CH ₂ ′) is superimposed on the high-frequency voltage, and the voltage +U + Vcosωt (-U
−Vcosωt) is applied. However, even with this circuit, the detection characteristics of the rectifier need to be corrected, and when trying to accurately measure a wide area including low mass areas, it is necessary to add a nonlinear circuit somewhere in the system to correct it. There is.

FIG. 4, FIG. 5, and FIG. 6 are all examples of currently used correction circuits.

FIG. 4 shows a correction circuit inserted into the P1 portion of FIG. 3, which utilizes the reference voltage Vcosωt. VR1
is a variable resistor for high mass range correction. variable resistance
VR2 and transistor Q1 form a low mass region correction circuit.

FIG. 5 shows a correction circuit inserted into the P2 portion of FIG. 3, which also uses the reference voltage Vref. VR
1 is a variable resistor for high mass range correction. A1, A
A low mass region correction circuit is carefully constructed using operational amplifiers No. 2, A3, and A4.

In FIG. 6, a low mass range correction circuit is inserted into the output section P3 of the DC voltage ±U generator.

As is clear from the above examples and descriptions, a quadrupole mass spectrometer requires a well-adjusted low value superimposed voltage, especially when mass spectrometering ions with low mass numbers. However, the nonlinearity of high-frequency voltage detection that inevitably exists in the low voltage region causes a major obstacle when analyzing this low mass number.
Furthermore, the fact that this nonlinear characteristic changes unstably depending on the ambient temperature and filament temperature makes the analysis error extremely large. Therefore, a solution to this problem has been strongly desired.

(Objective of the Invention) The present invention is a long-life semiconductor device that is stable against temperature, has good detection characteristics even for small high-frequency signals, and can detect high-frequency waves with good linearity over a wide operating range. The objective is to provide a quadrupole mass spectrometer equipped with a quadrupole electric field power source with high frequency detection capability. (Structure of the Invention) The present invention provides a quadrupole mass spectrometer that performs mass spectrometry by applying a superimposed voltage of a high frequency voltage and a direct current voltage to a quadrupole, and causing ions to enter an electric field constituted by the quadrupole. In the meter, the DC voltage is generated by detecting a divided voltage of the high frequency voltage,
In addition, the generation device includes amplifying the voltage difference between the divided voltage and its detection output voltage, and passing the amplified voltage through a diode to a parallel connection circuit of a capacitor and a resistor that holds the detection output voltage. The above object has been achieved by using a quadrupole mass spectrometer equipped with a high frequency detection means for applying a high frequency wave.

(Example) An example of the present invention will be described below based on the drawings.

FIG. 1a is an example of the quadrupole mass spectrometer of the present invention, showing only the high frequency detection circuit that constitutes the main part, and shows the high frequency detection circuit of 1 to 4 MHz applied to the input terminal Po. The voltage is divided by voltage dividing capacitors C ₁ and C ₂ . R ₁ is the resistance. The high frequency voltage obtained by voltage division is input to the non-inverting input terminal of comparator A, that is, the (+) terminal in the figure.
The output of comparator A is connected to a peak value hold capacitor _C3 via a diode _D1 ,
The capacitor _C3 is first charged with the peak value of the divided voltage and output as the detected output voltage Vdet.
However, the detected output voltage Vdet is input to the inverting input terminal of the comparator A, that is, the (-) terminal in the figure, so that the difference from the above-mentioned divided voltage value is amplified by the comparator A. Care should be taken to ensure that the charge charged in capacitor _C3 is discharged with a discharge time constant that is sufficiently long for the period of the input high-frequency voltage, but sufficiently short compared to the ion mass number sweep speed. A discharging resistor _R2 is connected in parallel to the capacitor _C3 . When the high frequency detection circuit of the quadrupole mass spectrometer is formed as described above, detection with good linearity can be performed even for a small input high frequency voltage. By experiment, the linearity of amplifier A
It has become clear that the higher the amplification factor and the better the high frequency characteristics, the better. This is also theoretically acceptable.

FIG. 1b is a circuit diagram of a quadrupole electric field power source of a quadrupole mass spectrometer of the present invention, which is configured to include the high frequency detection circuit Q of FIG. 1a described above. The configuration of each part other than the high frequency detection circuit section is the same as the conventional circuit shown in FIG. 3. The U/V ratio retention characteristic of this quadrupole electric field power source shown in Fig. 1b is extremely excellent, and it is comparable to the conventional
The correction circuits shown in Figures 4, 5, and 6, which were used in the power supply circuit shown in the figure, are all no longer needed, and the circuit can be used from low mass range to high mass range, which was previously impossible, without any correction. It has become possible to perform mass scanning over the entire area with high accuracy.

To explain by giving a specific example for comparison, if C ₁ and C ₂ are selected appropriately in Fig. 3 and a diode detector 6AL5 is used, the plate reverse voltage of this detector is 330V, so Vdet is can detect and use up to 165V. In order to improve linearity, we will detect this maximum voltage, divide it, and use it in the next stage circuit.If we calculate the ratio of the nonlinear part in the maximum voltage, the nonlinear part of the detector tube will be Since it is about 0.7V, a nonlinear portion occurs in a portion of 0.7/165=0.0042, that is, 0.42% on the small side of the detection voltage, and accurate detection becomes impossible in this portion.

On the other hand, according to the circuit of FIG. 1b of the present invention, C ₁ ,
Select C ₂ appropriately and set the maximum value of the detection output to a fairly low value.
Even if you select 2V, the differential input discrimination sensitivity is 1m.
If comparator A with maximum V is used, the ratio of the nonlinear part will be 1 x 10 ^-3 /2 = 0.0005, that is, only the 0.05% part on the small side of the detected voltage will be nonlinear.
An improvement in accuracy of approximately one order of magnitude is obtained.

In this example, a peak value hold circuit is formed using a comparator, but this does not have any limiting meaning; the difference between the input high-frequency voltage and the detected output voltage is amplified, and the detected output is Many other variations of the circuit for charging the capacitor are possible.

(Effects of the Invention) According to the quadrupole mass spectrometer of the present invention, mass scanning over the entire range of mass numbers is possible with highly accurate linearity.

[Brief explanation of drawings]

FIG. 1a is a diagram of a high frequency detection circuit of an embodiment of the quadrupole mass spectrometer of the present invention. FIG. 1b is a circuit diagram of the quadrupole electric field power source. FIG. 2 is a similar diagram of a conventional quadrupole mass spectrometer. FIG. 3 is a similar diagram of another conventional quadrupole mass spectrometer. 4, 5, and 6 are diagrams of nonlinearity correction circuits combined with the circuit of FIG. 3. 1, 2, 3, 4... Quadrupole electrode, 5... Reference signal generator, 6... Error amplifier, 7... High frequency generator, 8, 8'... High frequency transformer, 9, 10...
Capacitor, 11, 12...DC amplifier, A...
Comparator, A ₁ , A ₂ , A ₃ , A ₄ ... operational amplifier, C ₁ ,
C ₁ ′, C ₂ , C ₂ ′……Voltage dividing capacitor, C ₃ ……Peak hold capacitor, C ₄ , C ₄ ′…DC blocking capacitor, CH ₁ , CH ₁ ′, CH ₂ , CH ₂ ′ ... High frequency blocking choke, D ₁ , D ₂ , D ₂ ′ ... Diode, D ₃ , D ₃ ′ ... DC detector, R ₁ , R ₂ , R ₃ , R ₃ ′,
R ₄ , R ₄ ′...Resistance, VR ₁ , VR ₂ ...Variable resistance, Q ₁
...Transistor.

Claims (1)

[Claims] 1 In a quadrupole mass spectrometer that performs mass spectrometry by applying a superimposed voltage of a high frequency voltage and a direct current voltage to a quadrupole and causing ions to enter an electric field constituted by the quadrupole, the direct current voltage is It is generated by detecting a divided voltage of a high frequency voltage, and the generation device includes a diode that amplifies the voltage difference between the divided voltage and the detected output voltage, and A quadrupole mass spectrometer characterized by comprising: high frequency detection means for applying the detected output voltage to a parallel connected circuit of a capacitor and a resistor holding the detected output voltage.