JPS58117424A - Spectrophotometer - Google Patents

Abstract

PURPOSE:To make it possible to measure the absorbance of a sample with a constant noise level veing kept at all times, by measuring luminous intensity at a time which is inversely proportional to light transmittance, regardless of the absorbance. CONSTITUTION:Light from a light source 1 is alternately sent to a preamplifier 18 and a preamplifier 17 through a spectroscope 2, a sample chamber 3, a photelectric multiplier 4, and an anaolog switch 6. A signal outputted by the preamplifier 17 from the side of a sample cell is inputted into a first counter 9 through a V-F converter 7 and a gate circuit 8. Meanwhile clock pulses from a CPU and the like are inputted into a second counter 13 through a gate circuit 15. The counted value of the first counter 9 inputted through an input port 10 is divided by the counted value of the counter 13 inputted through an input port 14 by a controller 12, and a quotient is displayed by a display part 16 as the light transmittance. The logarithm of the inverse number of said quotient is computed and displayed by the display part 16 as the absorbance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digital spectrophotometer that converts the output of a photodetector from analog to digital.

Generally, the output of a photodetector using a photomultiplier tube etc. is A-
In a spectrophotometer that measures the absorbance of a sample by converting it into a digital signal proportional to the voltage using a D converter, if the measurement time is kept constant, the amount of information about the digitized light intensity can be It decreases proportionally as the luminous intensity decreases, thus increasing the noise level of the measurements. Therefore, in conventional spectrophotometers using A/D converters, it is difficult to maintain a constant noise level.

An object of the present invention is to provide a spectrophotometer that eliminates the above-mentioned drawbacks of conventional spectrophotometers and measures the absorbance of a sample while always maintaining a constant noise level regardless of the measured light intensity.

Photodetectors such as photomultiplier tubes should be placed in a
There is a noise current Δi due to noise, and its root mean square is:
It is expressed by the following formula with K as a constant: Δi”=KIa
...Slap...Slap...
(1) Therefore, when the light transmitted through the sample is incident on this photodetector, the output voltage of the photodetector is 1 T=R(Ia+Δi) ・・・・・・・・・・・・・・・・・・
・Scream...(2). Here R is the load resistance of the photodetector.

(Equation (2) should include the equivalent noise voltage of the load resistance, but this is generally very small compared to RΔi and can be ignored.) On the other hand, absorbance (the reciprocal of optical transmittance) logarithm) is =-kgR(I
a+Δi) Δi! −kgR−1ogIa −−・・・・・・・・・・・・・
・It is determined as (3)a. The third term on the right side of equation (3) is the noise Δ that appears in the absorbance due to the shit noise of the photodetector.
In A, using equation (1) and ignoring the sign, Δ<k Δ”” = (k=V'T: constant Wy)Ia
It is expressed as &. Here, from equation (2), T'=RI&
Taking into account, it becomes l, and the noise appearing in the absorbance is inversely proportional to the square root of the light transmittance. On the other hand, static noise is random noise, and it is known that the time average <ΔA> of ΔA decreases in inverse proportion to time t. Therefore, by measuring the light intensity at a time that is inversely proportional to T, that is, inversely proportional to the light transmittance, it is possible to always maintain a constant noise level regardless of the absorbance.

Embodiments of the present invention will be described below with reference to the drawings.

Light from a light source 1 is separated into spectra by a spectrometer 2 and guided to a sample chamber 3, where it is time-divided by a rotating sector or the like and is alternately irradiated onto a standard cell and a sample cell. The light transmitted through the standard cell and the sample cell enters a photomultiplier tube 4, and the output of the photomultiplier tube 4 is alternately sent to a preamplifier 18 by an analog switch 6 synchronized with the time division period of the light incident on the sample chamber. and is sent to the preamplifier 17. The standard cell side signal output from the preamplifier 18 is fed back to the photomultiplier tube 4 through the DC-DC converter 5, and is used as a control signal to keep its sensitivity constant. The signal from the sample cell side output from the preamplifier 17 is converted by the V-F converter 7 into a pulse signal with a frequency proportional to the signal voltage, and is input to the first counter 9 through the gate circuit 8. A clock pulse from an OPU or the like is input to the second counter 13 through a gate circuit 15. When the first counter 9 reaches a full count, it issues a signal to close the gate circuit 8 and gate circuit 15, and at the same time, the controller 1
2 to start the calculation. According to this command, the controller 12 converts the count value of the first counter 9 inputted through the first input port 10 (always constant at full count) into the count value of the second counter 16 inputted through the second input port 14. The quotient divided by the count value and the logarithm of the reciprocal of the quotient are calculated, and the former is displayed on the display section 16 as the light transmittance of the sample and the latter as the absorbance. After the calculation is completed, the controller 12 issues a signal to advance the spectrometer 2 to the next measurement wavelength region, and then outputs the output port 11 to send a clear signal to the first counter 9 and the second counter 1.
Send it to 6 and clear both counters at the same time.

The present invention does not use a V-F converter as in the above embodiment, but instead provides an AD converter and a timer that defines the output interval of the A-D converter, and sets the timer to the spectrometer output. It can also be implemented by varying the reciprocal value.

As described above, according to the present invention, a constant number of pulses with a frequency proportional to the intensity of the sample transmitted light is always counted, and the transmittance is determined as the ratio of the counted value to the time required for counting. The measurement time is inversely proportional to the transmittance of the sample, and the absorbance as the logarithm of the reciprocal of the transmittance is always measured while maintaining a constant noise level, as explained in the above theoretical statement.

[Brief explanation of the drawing]

The drawing is a block diagram showing an embodiment of the present invention. 1...Light source 2... Spectrometer 6...Sample room 4...Photomultiplier tube 5...DC-DC converter 6...Analog switch 7...V-F converter 8...Gate circuit 9...First counter 10...First input boat 11... Output boat 12... Controller 13...Second counter 14...Second input boat 15...Gate circuit 16...Display section 17...Preamplifier 18... Preamplifier patent applicant

Claims (1)

[Claims] (1) Measure the spectrophotometric intensity of the sample from the time-divided reference signal and sample-side signal obtained by time-dividing the multiple beams of light from the reference cell and sample cell and alternately entering a single photodetector. means for converting the output of the photodetector into a digital signal proportional to the voltage thereof;
means for controlling the conversion time in accordance with the output of the photodetector, and controlling the conversion time of the A-D conversion means in inverse proportion to the output of the photodetector, and controlling the conversion time of the A-D conversion means in inverse proportion to the output of the photodetector, and controlling the conversion time of the A-D conversion means in accordance with the output of the photodetector, and controlling the conversion time of the A-D conversion means in inverse proportion to the output of the photodetector. Spectrophotometer hole (2) for measuring the light transmittance and absorbance of a sample while keeping the product with time constant; means for converting the output of the photodetector into a digital signal proportional to its voltage; means for counting a predetermined number;
Means for measuring the time required to count the predetermined number of pulses, and means for calculating the quotient obtained by dividing the predetermined pulse count value by the time required for counting, and the logarithm of the reciprocal of the quotient. 2. The spectrophotometer according to claim 1, characterized in that the light transmittance and absorbance of the sample are characterized by keeping constant the product of the measured light intensity and the time required for the light intensity measurement.