JPS591972B2 - Spectrometer with calculation compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a spectrometer that measures the second derivative of the absorption spectrum of a substance to be measured in order to perform qualitative or quantitative analysis of the substance to be measured, such as gas or liquid. This invention relates to a spectrometer with an arithmetic compensation circuit, which is useful for measuring low-concentration pollutants contained in the atmosphere, by compensating for the effects of the spectrum of the gas and the absorption spectrum of the medium, and extending the linear relationship between the electrical output and the concentration of the substance to be measured to low concentrations. It is something.

There is a high-order derivative method (higher-order differential method) as a spectroscopic method for performing qualitative and quantitative analyzes of gases or liquids with high sensitivity based on their absorption spectra.

The principle of this higher-order derivative method is - ``The first-order differential spectrum obtained by differentiating the absorption spectrum of a gas or liquid with respect to wavelength largely captures slight changes in the original absorption spectrum before differentiation, and the spectral profile is steep ( The second-order differential spectrum obtained by differentiating the first-order differential spectrum is even sharper, so qualitative or quantitative analysis of the substance is performed using the first-order or second-order differential spectrum. It allows for highly sensitive measurements." Conventional spectrometers based on this higher-order derivative method can be broadly classified into the following two types.
That is, a) a wavelength scanning spectrometer is equipped to measure the absorption spectrum, and this absorption spectrum is differentiated using an attached electronic computer, thereby producing a first-order differential spectrum or a second-order differential spectrum on recording paper. The spectrometer and the (mouth) spectrometer's entrance slit or exit slit, or the light flux inside the spectrometer, are swung and temporally sinusoidally modulated in the vicinity of the wavelength at the center wavelength of the exit slit. There is a spectrometer that measures the differential value of the spectrum near the wavelength. Considering these two types of conventional spectrometers, the spectrometer (a) requires an attached computer and is expensive, so it is not popular. Spectrometers belonging to this category are simple devices, and various spectrometers have been proposed so far. here (mouth)
Let me explain in some detail the principle of the spectrometer. That is,
It will be explained that by temporally applying sinusoidal wavelength modulation, the differential value of the spectrum in the vicinity of the wavelength can be obtained. The exit slit of the spectrometer is waved sinusoidally in time to apply sinusoidal modulation to the wavelength of the light passing through the exit slit, and the modulation amplitude at that time is α, the modulation angular frequency is ω, and the exit slit is The wavelength of the light passing through the exit slit when stopped is λ.

, time is t, the wavelength λ of the light passing through the exit slit can be expressed as (C).On the other hand, the spectrum I(λ) is λ.

It can be expressed as Taylor expansion in the vicinity of .

Substituting equation (1) into equation (2) yields. (In the 4-component, the first term is a DC signal, the second term is in S1 order, c
o is an AC signal containing ωt. be. m=1 in equation (4)
Then, paying attention to the si order and the cos term, equation (5) shows that the amplitude of the optical intensity modulation signal with an angular frequency ω corresponds to 10) (λo), and from equation (6), the amplitude of the modulation signal with an angular frequency 2ω corresponds to It can be seen that the amplitude of I(2)(λo) corresponds to I(2)(λo).

Similarly, the amplitude of the modulation signal whose angular frequency is nω is n
It is understood from equation (4) that this corresponds to the second derivative I(n)(λ.). In other words, the center wavelength of the exit slit is λ
. When modulated in the vicinity of “'(λo) PYl(λo) PO( *There was a problem when quantitatively measuring low concentration substances.

The present invention compensates for the influence of the light source spectrum, which is a problem when measuring low-concentration substances, in an electric circuit, extends the linearity of the sensitivity characteristics to low concentrations, and improves the sensitivity of substances contained in a medium at low concentrations. The object of the present invention is to provide a spectrometer with an arithmetic compensation circuit that can perform accurate quantitative measurements. Before describing the configuration of the electric circuit according to the present invention, we will consider how the influence of the light source spectrum appears on the output, and based on that consideration, we will determine what kind of compensation should be applied in the electric circuit to achieve correct compensation. I will explain in detail whether it is possible.

When considering the absorption of a substance contained at a low concentration in a transparent medium, the Lampert-Beer law holds, so the absorption optical path length is 2, the absorption cross section of gas molecules is σ (λ), and the molecular density is n1 light source intensity is P.

(λ), the spectrum 1 (λ) caused by gas absorption is...
・・・・・・(7) In the second-order differential, * is common,
In this case, the first derivative of the absorption spectrum of the substance to be measured (the low concentration substance) is zero, that is, σ(1)(λo)−0, so equation (8) is expressed as (2)...l −n −l・σ(
λ. ．． ) , 1, * Since the target is measurement, n-2
・σ(λ)<1, and in the end Exp(−n−j・σ(λ
o)) can be regarded as 9.

In other words, equation (9) is expressed as
......The second term of ClI is a term corresponding to the second derivative of a pure absorption spectrum that is proportional to the concentration of the substance to be measured.

According to the formula AO), when the concentration of the substance to be measured becomes low and the first term can no longer be ignored compared to the second term, the second derivative value 1 (
2) It is understood that (λo) is no longer proportional to the concentration of the substance to be measured. In other words, the light passing through the exit slit of the spectrometer is subjected to wavelength modulation with an angular frequency ω, resulting in an angular frequency of 2.
A spectrometer that determines the concentration of a substance to be measured by measuring the second derivative value (2) (λo) of the absorption spectrum from the amplitude of the modulation signal ω has an absorption spectrum that is purely proportional to the concentration of the substance to be measured. The second derivative of the light source spectrum and the second derivative of the light source spectrum
It is understood from the above consideration that the sum with the second-order differential value appears as an output, and if the concentration of the substance to be measured is low, the second-order differential value of the light source vector must be removed.

Here, if the substance to be measured is determined, the wavelength to be measured (wavelength indicating the absorption maximum or minimum value of the absorption spectrum of the substance to be measured) is determined, so P(2) (λ.) is also fixed. Therefore, in the present invention, a reference signal corresponding to p(2)(λo) is provided in advance and the measurement output 1(2)
The influence of the light source spectrum was removed by subtracting the reference signal from )(λo). This will be explained below with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the present invention.
In the same figure, light emitted from a deuterium discharge tube 1 is collimated by a collimator lens 5, and then inside an absorption cell 10 in which the atmosphere (measuring medium) is sampled and introduced to generate an absorption spectrum. is incident through the entrance window 11 of the absorption cell. A medium to be measured is introduced into the absorption cell 10 through, for example, intake and exhaust ports 13 and 14 . This incident parallel light flux is reflected multiple times by the mirrors (21, 22, etc.) of the repeating reflection optical device 20 provided in the absorption cell, and then exits from the exit port 12 of the absorption cell, and then exits from the collector lens 30. The light is focused on a fixed entrance slit 41 of the spectrometer 40 and enters the interior of the spectrometer 40. The light incident on the spectrometer 40 is wavelength-dispersed by the dispersion mechanism 42 of the spectrometer, and monochromatic light passes through the exit slit 55.The light-receiving surface of the head window photomultiplier tube 80 has a substantially uniform light-receiving surface. incident on . The exit slit 55 is formed in a slit plate 50, and the slit plate 50 is attached to a slit vibration device 60. By vibrating the slit vibrating device 60 by the drive circuit 70, the exit slit 55 is made to vibrate sinusoidally in the lateral direction (X direction in FIG. 1) over time, and the light passing through the exit slit is The wavelength of the signal is temporally modulated sinusoidally. Here, S.O.
Taking the measurement of two gases as an example, the wavelength that shows the maximum absorption of SO2 gas (for example, 300 nm) is the center, and the wavelength is about 2 nm.
The exit slit is vibrated so that wavelength modulation is applied with an amplitude of This occurs in the processing unit 90 within the system 85. At this time, the output of the processing unit 90 includes the second-order differential value of the light source spectrum, as described above. Since p(X)(λo) is superimposed, the reference signal generator 92 generates a reference value corresponding to this p(X)(λo), and the subtraction circuit 93 generates a reference value from the output signal of the processing unit 90. subtracting the reference value;
The output of the subtraction circuit 93, that is, the output not affected by the light source spectrum, is displayed on an output display 96. Next, the slit vibrating device 60 used in the first embodiment of the present invention will be explained in detail. In Fig. 2, a U-shaped tuning fork 61 with a high natural frequency and stable amplitude is inserted into the exit slit 5.
5. Used as a vibration source. A slit plate 50 with an outlet slit 55 formed at one free end of this U-shaped tuning fork 61
is attached via a holder 63, and a balancer 62 is attached to the other free end to maintain the balance of the tuning fork, thereby constructing an extremely high Q vibrator. A pick-up electromagnetic head 64 for picking up the natural vibration of the oscillator, a drive circuit 70 for amplifying the picked-up AC current and generating a drive AC signal with a constant output, and a drive circuit 70 for exciting the oscillator. Electromagnetic head 6 for driving
5 and the vibrator constitute an electro-mechanical oscillation device, and the exit slit 55 is vibrated by self-excited oscillation of this oscillation device. In FIG. 2, the electromagnetic heads 64, 65 each consist of a core 68, 68' and a coil 69, 69' wound around the core.
The drive circuit 70 includes amplifiers 73 and 75 and a constant current generation circuit 7.
1, a detection circuit 76, a low-pass filter 77, and an output voltage adjustment circuit 74. According to the experimental results using the slit vibration device explained above, the amplitude was approximately 1m71L.
Very stable and accurate sinusoidal vibrations with a frequency of approximately 400 Hz were obtained.

Next, the electrical processing system 85 will be explained in detail with reference to FIG.
In the figure, the electrical output signal of the head window type photomultiplier tube 80 is amplified by a preamplifier 120 built in a processing section 90 in an electrical processing type 85, and after the DC component is removed by a capacitor 121. An alternating current signal with a frequency 2ω, which is twice the frequency ω of the exit slit, is extracted by a bandpass filter 122, further amplified by an amplifier 123, and input to a synchronous switch circuit 124. This synchronous switch circuit 124 is turned on when the modulation signal of frequency 2ω is a positive half-wave, and turned off when it is a negative half-wave, thereby performing half-wave rectification of the signal. Furthermore, when the polarity of the output of the amplifier 123 is inverted by the polarity inverting circuit 125, the negative half wave of the output signal of the amplifier 123 becomes a positive half wave, so this positive half wave is extracted by the synchronous switch circuit 124'. By combining the output signal of the synchronous switch circuit 124 and the input of the low-pass filter 130, the output signal of the amplifier 123 is full-wave rectified. The synchronization signals of the synchronization switch circuits 124 and 124' are generated by passing the drive signal of the frequency ω of the drive circuit TO through a phase shifter 110 for phase adjustment, and a multiplier circuit 111.
to create a signal with a frequency of 4ω, and further create two rectangular waves with a frequency of 2ω with a phase shift of 180° using a frequency divider 112.
I used this. The aforementioned full-wave rectified signal is smoothed by a low-pass filter 130 and then enters the numerator of a divider 135. on the other hand,
The DC component of the output of the preamplifier 120 is transferred to the DC amplifier 1.
31 and then input into the denominator of the divider 135. As mentioned above, the numerator input to the divider 135 is I('')(λ.)
, and the denominator input is the DC signal of the first term in equation (4), which corresponds to Hopo I (λo), which is determined by Equation (7) to Hopo P
. Since it is equal to (λo), the operation in the divider 135 is as follows.

al) formula, the substance to be measured (in the first embodiment of the present invention, S
The output of the divider 135 is the sum of a term (first term) proportional to the concentration of 02 gas) and a term (second term) corresponding to the second-order differential value of the light source spectrum. Then, the term p(:)(n)/P corresponds to the second-order differential value of this light source spectrum. (λ.
) is the light source intensity P. Since this value is a constant value that does not depend on The term corresponding to the value is removed and an output that is purely proportional to the concentration of the substance to be measured is displayed on the output display 96. At this time, the value p(:)(λ.)/P corresponds to the second-order differential value of the light source spectrum. (
λ. ) was experimentally determined as follows. That is, high purity nitrogen (zero gas) is introduced into the absorption cell 10 to create a state in which the substance to be measured does not exist within the absorption cell. At this time, what appears in the output of the processing unit 90 is a value pA'')(λ.)/Po(
λo). In this way, the reference signal of the reference signal generator 92 is experimentally determined. In short, the present invention focuses on light source intensity P. After obtaining an output that does not depend on , an amount corresponding to the differential value of the light source spectrum is subtracted, and it is also possible to modify it as in the second embodiment described below. Second
An embodiment is shown in FIG. 4, and will be explained based on the same figure.

After the output of the head window type photomultiplier tube 80 is amplified by the preamplifier 120, only the DC component of the head window type photomultiplier tube 80 is further amplified by the DC amplifier 131'.

A comparison circuit 160 compares the reference voltage preset by the reference voltage circuit 155 with the output of the DC amplifier 131', and adjusts the head window photomultiplier tube 80 so that the difference between them becomes zero. Adjust the applied voltage. This adjustment is performed by applying the output voltage of the DC high voltage generation circuit 165 to the applied voltage regulator 17.
This is done by adjusting 0. By adjusting the applied voltage so that the DC output of the photomultiplier tube 80 is always constant, the same effect as keeping the light source intensity constant can be obtained, so the light source intensity is compensated. There is no need for a divider 135 for this purpose. Therefore, the low pass filter 1
By immediately subtracting the output of the reference signal generator 92 from the output of the reference signal generator 30 using the subtraction circuit 93, the influence of the light source spectrum can be removed. Next, a third embodiment will be described.

FIG. 5 is a block diagram showing the third embodiment, and the light source and absorption cell are omitted and not shown. In the figure, 45 is a wavelength switching device that changes the wavelength by changing the incident and output angle of a dispersion element (for example, a diffraction grating) in the dispersion mechanism 42 of the spectrometer 40, and a switch 95 is interlocked with the wavelength switching device 45. ing. In order to measure SO2 gas, NO gas, and NO2 gas in the atmosphere, the respective measurement target wavelengths λ are set by the wavelength switching device 45. , λ1, λ2 can be switched. First, when trying to measure SO2, the wavelength switching device 4
5, the center wavelength of the modulated light passing through the exit slit 55 is λ. At that time, the switch 95 switches the wavelength switching device 4.
5 and connected to the reference signal generator 92. The reference signal generator 92 generates the wavelength λ of the light source spectrum. It generates an output corresponding to the second-order differential value Pl2)(λo) in
Therefore, by subtracting the output of the reference signal generator 92 from the output of the processing section 90 by the subtraction circuit 93, the influence of the light source spectrum is removed. Similarly, when measuring NO gas, the wavelength switching device 45 adjusts the center wavelength of the modulated light to λ1, and in conjunction with this, the switch 95 operates the reference signal generator 92 which generates the reference signal Pl2) (λ1).
', and the subtraction circuit 93 calculates the influence of the light source spectrum on NO gas measurement, that is, Pl2)(λ1)
remove. The same goes for measuring NO2 gas; when the wavelength switching device 45 sets the wavelength to λ2, the switch 95 is linked and connects the reference signal generator 92''.
It generates an output corresponding to the order differential value p(2)(λ2), and the subtraction circuit 93 generates an output corresponding to the order differential value p(2)(λ2), which is affected by the light source spectrum P9)(λ
2) is subtracted and compensated. In the embodiments described above, there were three types of substances to be measured, but it goes without saying that the number of substances to be measured is not limited to three types.Next, a fourth embodiment will be described.

In the first, second and third embodiments, the absorption cell 10 was placed between the light source 1 and the input loss slit 41 of the spectrometer, but the absorption cell was placed between the output slit 55 of the spectrometer and the photomultiplier tube. It may be placed between 80 and 80. With this kind of arrangement, when the exit slit is swung, the light beam will repeatedly sway within the reflective optical device, and the light intensity will decrease due to slight dirt on the mirror that makes up the repetitive reflective optical device, or the periphery of the light beam coming off the mirror. Since a modulation signal (zero time signal) is generated, which is undesirable, it is better to swing the input loss slit without swinging the exit slit. Such an arrangement is shown in FIG. In the same figure, 4'' is an input loss slit of the spectrometer 40 formed on the slit plate 507, and 55/ is a fixed exit slit of the spectrometer 40.The slit plate 5'' is attached to a slit vibration device 60, By vibrating the input loss slit 41' with the slit vibrator 60, wavelength modulation is applied to the light passing through the fixed exit slit 55/.Here, the light source spectrum P described above.

Let us consider (λ) a little. Lenses 5 and 10, each having a wavelength characteristic, are provided on the path from which the light emitted from the light source 1 reaches the photomultiplier tube and is converted into an electrical signal.
Mirrors (21, 22, etc.), a spectroscope 40, a photomultiplier tube 80, etc. are present. That is, the photomultiplier tube 80
The electrical signal photoelectrically converted has a wavelength characteristic that is the product of all of these wavelength characteristics. Therefore, Pl2)(λo)/PO( which appears in the output of the processing unit 90)
λo) is a value obtained by second-order differentiation of the above-mentioned multiplied wavelength characteristics, and this value is obtained experimentally and used in the reference signal generator 9.
2 and compensated by the subtraction circuit 93, the present invention not only eliminates the influence of the pure light source spectrum, but also eliminates the influence of the pure light source spectrum on the optical path from the light source 1 to the photomultiplier tube 80. This means that all wavelength characteristics of existing optical elements and optical devices are also compensated for. As explained above, according to the present invention, the modulation degree output (output of the processing unit 90) with zero gas introduced into the absorption cell is experimentally determined, and a reference signal equal to this output is generated by the reference signal generator. Since the reference signal was generated in
It was also possible to eliminate the influence of the wavelength characteristics of each optical element and optical device, and it was possible to extend the linear relationship between the measurement output and the concentration of the substance to be measured down to low concentrations.

To be more specific, 30% of SO2 gas in the atmosphere
In order to measure concentrations below ~10 ppb, correct measurements cannot be made unless the influence of the light source spectrum is removed by the present invention. As such, it is especially effective for applications in pollution-related fields that involve low-concentration measurements.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a first embodiment of a spectrometer according to the present invention. FIG. 2 is a diagram illustrating the configuration and operation of the slit vibrating device used in the first embodiment of the present invention. FIG. 3 is a block diagram of the electrical processing system used in the first embodiment of the present invention, and is a diagram for explaining the operation of the electrical processing system. FIG. 4 is a configuration diagram showing a second embodiment of the present invention. FIG. 5 is a block diagram showing a third embodiment of the present invention. FIG. 6 is a block diagram showing a fourth embodiment of the present invention. 1...Light source, 10
... Absorption cell, 20 ... Repeated reflection optical device, 40 ... Spectrometer, 41 ... Fixed input loss slit, 4 "... ...Vibrating inlet slit, 55...Vibrating exit slit, 5
5'...Fixed exit slit, 60...
... Slit vibration device, 80 ... Photomultiplier tube, 85 ... Electric processing system, 90 ... Processing section that generates a display output value, 92 ... ...Reference signal generator, 93...Subtraction circuit.

Claims (1)

[Claims] 1. A spectrometer comprising a light source, an entrance slit through which the focused light beam from the light source passes, and an exit slit arranged at spectral imaging positions dispersed through a dispersion mechanism, and a center wavelength of the exit slit. a photoelectric converter that receives the output light beam from the exit slit; and a photoelectric converter that receives the output signal of the photoelectric converter and calculates the second derivative value of the absorption spectrum of the substance to be measured. an electric circuit adapted to display; the electric circuit generates a signal corresponding to the spectrum of the light source or a differential value of the spectrum of the light source and the absorption spectrum of a medium containing the substance to be measured; and an output of the reference signal generator from the display output value influenced by the spectrum of the light source obtained from the output signal of the photoelectric converter or the spectrum of the light source and the absorption spectrum of the medium. A spectrometer with an arithmetic compensation circuit, characterized in that it is equipped with a subtraction circuit for subtracting values.