JPH08201281A - Photodiode array detector - Google Patents

Abstract

(57) [Summary] [Structure] Light source 1, flow cell 2, prism 3 for splitting transmitted light, photodiode array circuit 4 for detecting split light, sample / hold circuit 5 for the signal, and LOG for absorbance calculation. The DA converter circuit 10 for the R side input signal of the converter circuit 6 and the LOG converter circuit and the input signal are selected.
It is composed of an input signal changeover switch 7, an AD conversion circuit 8 for taking the signal into the control unit 9, and a personal computer 11 including data processing. [Effect] Since the noise of the LOG conversion circuit can be minimized, the S / N ratio of the signal can be improved especially for the wavelength where the light amount is small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the S / N ratio in single beam photometry in a photodiode array detector which is one of detectors for liquid chromatographs.

[0002]

2. Description of the Related Art In a conventional device, a constant voltage is input to the reference side of a LOG conversion circuit.

[0003]

In the above prior art, the output of the photodiode array element is output corresponding to the difference in the light amount with respect to the wavelength of the light source, and this signal is input to the sample side of the LOG conversion circuit. The smaller the wavelength, the larger the difference from the signal on the reference side, and the larger the noise of the output of the LOG conversion circuit at that wavelength.
Expressed by a mathematical expression, when the sample side signal is S and the reference side signal is R, the output of the LOG conversion circuit is log (S / R).

If the noise ratios are the same 1/100,

[0005]

[Equation 1]

Therefore, when R is 10 times larger, most of the noise components are noise components on the R side.

[0007]

In order to achieve the above object, the present invention provides a signal close to the sample side signal in order to reduce the difference between the sample side input signal level and the reference side input signal level of the LOG conversion circuit. A DA conversion circuit is provided for inputting the reference.

[0008]

The sample signal is recorded as a digital value before measuring the sample, and this digital value is input to the DA conversion circuit. The output of this DA conversion circuit becomes the reference signal of the LOG conversion circuit.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

The light from the light source 1 passes through the flow cell 2, is dispersed by the prism 3, and is applied to a photodiode (hereinafter abbreviated as PDA) circuit 4. In the PDA circuit 4, when the start signal 12 is input from the control unit 9 at a constant cycle, a bit signal corresponding to the wavelength is output in time series, and the signal is the optical signal 13. Since the optical signal 13 has a pulse shape, it becomes an S / H circuit signal 14 by the sample and hold (hereinafter abbreviated as S / H) circuit 5. Before the sample measurement, when the background of the eluent is corrected, the input signal changeover switch 7 is controlled by the control unit 9 so as to be on the E side, the S / H circuit signal 14 is input to the AD conversion circuit 8, and each bit thereof is changed. The digital value is stored in the control unit 9. Based on this digital value for each bit, it becomes the input signal of the DA conversion circuit 10,
The signal waveform is the DA conversion circuit signal 15. Next, the input signal changeover switch 7 is controlled by the control unit 9 so as to be on the A side, and the signal of the LOG conversion circuit 6 is input to the AD conversion circuit 8. After that, the output of the DA conversion circuit 10 repeats the same waveform until the next background correction is performed. The S / H circuit signal 14 is input to the S terminal of the LOG conversion circuit 6, the DA conversion circuit signal 15 is input to the R terminal, and the output thereof becomes the LOG conversion circuit signal 16. L
It is desirable that the OG conversion circuit signal 16 be zero for all the bits, but due to the difference in bit variations between the AD conversion circuit 8 and the DA conversion circuit 10, and the R terminal from the S terminal input level of the LOG conversion circuit 6. This is because the level is set slightly higher. In addition, the personal computer 1
Reference numeral 1 is for processing the measured data and displaying the result and for inputting the measurement conditions, and is connected to the control unit 9 by communication.

[0011]

According to the present invention, the noise of the LOG conversion circuit itself can be minimized.
It is possible to improve the A / N ratio, particularly the S / N ratio of a signal for a wavelength with a small amount of light.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a signal waveform diagram of the main circuit of FIG.

[Explanation of symbols]

1 ... Light source, 2 ... Flow cell, 3 ... Prism, 4 ... Photodiode array circuit, 5 ... Sample and hold circuit,
6 ... LOG conversion circuit, 7 ... Input signal changeover switch, 8 ...
AD conversion circuit, 9 ... Control unit, 10 ... DA conversion circuit, 11
…Personal computer.

Claims (1)

[Claims] 1. A configuration comprising a light source, a flow cell for a sample, a photodiode array circuit for detecting transmitted light of the flow cell, a LOG conversion circuit for calculating the absorbance of the sample, and an AD conversion circuit for digitally converting the signal of the LOG conversion circuit. A photodiode array detector characterized in that a DA conversion circuit is provided on the reference side of the LOG conversion circuit for inputting an approximate value of a signal indicating the amount of light in a state where absorption by the sample does not occur.