JP2621004B2 - Concentration measurement method using light absorption - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention utilizes the fact that the absorption of light is changed by the concentration of a specific substance gas or liquid sample, measuring the absorption of light in the sample (or transmittance) The present invention relates to a concentration measuring device for measuring the concentration of a specific substance.

[0002]

BACKGROUND OF THE INVENTION concentration measuring apparatus utilizing absorption of light is usually not an absolute measurement only absorption rate of the measurement sample (or transmittance) the determination of the concentration, the concentration known transmission standard sample
Relative measurements are used to determine the concentration by comparison with the ratio . Although this method it is necessary to measure the transmittance of two samples of principle measurement sample and the standard sample, between the two measurements,
Various factors such as light source intensity, optical filter absorbance, detector / amplifier sensitivity, sample cell contamination, sample temperature, pressure, etc. may differ, resulting in zero drift, span drift, etc. Measurement accuracy.

[0003] Regarding the absorption of light while passing through a substance,
The following Lambert-Beer law is known. As shown in FIG. 5, the amount of transmitted light I after the light having the initial light amount I0 has passed through the sample containing the substance having the concentration C by the length L is: I = I0 · exp (−K · C · L) ... (10) Here, K is an absorption coefficient specific to a substance in a sample to be measured.

Although this law fits well with the case where monochromatic light is used, in industrial or mobile automatic analyzers, only monochromatic light characteristics are provided by a non-dispersive optical filter or the like. Therefore, generally, the relationship between the incident light amount I0 and the transmitted light amount I does not follow Lambert-Beer's law. In addition, in order to reduce the size of the entire apparatus, incident light is not completely collimated, and light is irregularly reflected on the inner wall of the sample cell so that the effective cell length L is often increased. In this case, since the cell length L is not constant, the above-mentioned Lambert-Beer's law cannot be simply applied, and the relationship between the density C and the amount of transmitted light I becomes complicated.

[0005]

For this reason, conventionally, several points within the measurement range (for example, 0% of full scale, 20%
%, 40%, 60%, 80%, 100%, etc.), and by actually measuring the light transmittance of these samples, the actual density value and transmittance are determined in advance. The calibration curve method is used in which a calibration curve representing the relationship with the value is prepared. However, in general, these measured points do not always fall on a straight line, and a linearization device for creating a calibration scale, approximating a broken line, and the like is required. In addition, since this calibration curve characteristic actually changes due to aging of each part of the device,
Calibration had to be repeated every 1 to 6 months (JIS
K0055 “General rules for calibration of gas analyzers”, JIS K
0115 “General rules for spectrophotometry”, JIS K015
1 "Infrared gas analyzer" etc.).

The present invention has been made to solve such a problem, and an object of the present invention is to eliminate the need to prepare a calibration curve from a large number of standard samples in advance.
In addition, it is an object of the present invention to provide a concentration measuring device that can easily and accurately obtain a measured value by correcting various fluctuation factors collectively.

[0007]

According to the present invention, which has been made to solve the above problems, the concentration of a specific component contained in a sample is determined by measuring the absorption or transmittance of light passing through the sample. In the method for measuring the concentration, the following procedure is provided. a) Measure the transmittance TC1 and TC2 of light passing through two samples having known concentrations C1 and C2, respectively. b) A correction coefficient K3 is calculated from the following three equations (1), (2) and (3). LC1 = log ((TC1-K3) / (1-K3)) (1) LC2 = log ((TC2-K3) / (1-K3)) (2) LC1 / LC2 = C1 / C2 (3) C) Measure the transmittance TM of light passing through the measurement sample having the unknown concentration CM. d) The unknown concentration CM is calculated by the following equation (4) or (5). CM = ((log ((TM−K3) / (1-K3))) / LC1) × C1 (4) CM = ((log ((TM−K3) / (1-K3))) / LC2) × C2… (5)

In the above description, the light transmittance T of each sample
It is desirable to obtain A = TC1, TC2, and TM in the following procedure. e) Measure the output D0 of the detector when the measurement light is not incident and the output D100 of the detector when the measurement light is incident after passing through a zero sample containing no component to be measured. f) from D0 and D100 calculates the next compensation coefficient K1. K1 = 1 / (100-D0) (6 ) g ) From the output DA of the detector when the measurement light passes through the sample containing the component to be measured, the light transmittance TA of the sample is determined by TA = (DA−D0) × K1 (8)

[0009]

SUMMARY OF] By modifying Equation (10) of the Lambert-Beer, transmissivity T = I / I0 T = I / I0 = exp (-K · C · L) ... (11) , and the large C · L 6 (that is, when the concentration C is increased or the cell length L is increased), FIG.
As shown in (a), T should approach 0 (zero).

However, due to the various factors described above, FIG.
As shown in (b), T is not 0 but approaches a certain non-zero value K3. Therefore, in such a case, the transmission of the measurement sample
When calculating the concentration CM from the ratio TM, the value of TM is not used as it is, but as shown in FIG. 6B, (TM−K3) / (1−K3) and the value corrected for K3 Is used. That is, when the transmissivity TM is measured, an unknown concentration CM L
log- (IM / I0) = log (TM) =-K.CM.
Instead of calculating L ... (12), it is calculated by the equation log ((TM-K3) / (1-K3)) =-K · CM · L (13) including the correction value K3.

Here, the correction value K3 is determined by two known densities C
1, C2 respectively, for sample transmittance TC1 having,
TC2 is measured and substituted into the above equation. Log ((TC1-K3) / (1-K3)) = LC1 (14) log ((TC2-K3) / (1-K3)) = LC2 (15 LC1 / LC2 = (− K · C1 · L) / (− K · C2 · L) = C1 / C2 (16) By solving simultaneous equations from three equations (14) to (16) obtained as Desired. Expressions (14) to (1)
6) is the equation (1) to (3) itself, and the equation (1)
By substituting CM into C1 or C2 of 4) to (16), Expression (4) or (5) is derived. Formula (1)-
It is difficult to solve the simultaneous equations according to (3) analytically, but K3 can be easily obtained by numerical calculation using a computer.

Since the detector output causes zero drift and span drift (fluctuation) due to the various factors described above, the light transmittance T of each sample used in the above equations (1) to (5) is used.
It is desirable that A = TC1, TC2, and TM also use values corrected for these drifts. Correction coefficient K1 is spa
And zero drift.

[0013]

FIG. 1 shows a concentration measuring apparatus according to one embodiment of the present invention.
This will be described with reference to FIG. As shown in FIG.
Light from 1 is blocked / passed by the shutter 12. Here, the shutter intermittent speed (repetition frequency) varies depending on the application, but is usually 1 to 3,600 cpm.
(0.017 to 60 Hz). The light source 1
In the case where a light source for lighting synchronous pulses is used, the shutter 12 is unnecessary. The light that has passed through the shutter 12 passes through the sample cell 14. By the sample fluid switching devices 15 and 16, the zero concentration fluid (zero sample) and the measurement sample fluid or the standard sample fluid are alternately flown into the sample cell 14.
You. In the case of automatic analysis concentration measurement, the switching cycle is set to be at least twice the intermittent cycle of the shutter 12 (1/2 or less at the repetition frequency). Depending on the application, the frequency of fluid switching is 0.1 to 1,200 cpm (0.0017 to
20 Hz) is common. Also relatively stable
In the case of a suitable device, the inflow of zero sample is set to once a day, etc.
The interval can be extended.

The light that has passed through the sample in the sample cell 14 has its interference component removed by an optical filter (FIL) 17 and is measured by a detector 18. The detected value of the amount of transmitted light output from the detector 18 is amplified by an amplifier (AMP) 19,
The data is input to the control unit 21 via the D converter 20.

As shown in FIG. 2, the control unit 21
1, a microcomputer including a ROM 32, a RAM 33, and various I / O interfaces 34. The CPU 31 controls the operation of each unit of the concentration measuring apparatus according to a program stored in the ROM 32. The control unit 21 calculates the density value of the measurement sample by performing a calculation described later based on the transmitted light amount data input from the detector 18, and causes the digital display (DISP) 22 to display it. Various instructions from the operator are transmitted from the keyboard (KB) 23 to the control unit 21.
Is input to

In the concentration measuring apparatus of this embodiment, first, a calibration operation is performed using a zero concentration sample and a standard sample having two kinds of known concentrations. This calibration operation will be described with reference to the timing chart of FIG. (A1) The shutter 12 is closed, a zero sample is introduced into the sample cell 14, and the output (detection signal of 0% of transmitted light) of the detector 18 at a time t1 when a predetermined time has elapsed is set to D0 and the RAM 33 is set.
To memorize. Note that this predetermined time is a time for waiting for the output of the detector 18 to stabilize. (A2) Next, the shutter 12 is opened, and the output (detection signal of 100% of transmitted light) of the detector 18 is stored in the RAM 33 as D100 at a time t2 when a predetermined time has elapsed. (A3) If necessary, the above (A1) and (A2) are repeated, and the average values of D0 and D100 are stored in the RAM 33. (A4) a signal D0 and D100 from 0% transmission and 100% transmission detector 18, the value of each correct transmittance 0
And 100, the correction coefficient K1 is calculated as follows. K1 = 1 / (D100-D0) (21) The correction coefficient K1 thus calculated is stored in the RAM 33. (A5) The shutter is closed, and a standard sample having a known concentration C1 as close to full scale as possible is introduced into the sample cell 14. The shutter is opened at the timing when the sample cell 14 is filled with the standard sample, and after a predetermined time, the output of the detector 18 is stored in the RAM 33 as DC1 (time t3). This is also measured repeatedly as necessary, and the output D
The average value of C1 may be used. (A6) from the output DC1 detector 18, by using the correction coefficient K1 read from the RAM 33, calculates the transmittance TC1 of standard sample concentration C1, as follows. TC1 = (DC1-D0) × K1 (23) (A7) A standard sample having another known concentration C2 is introduced into the sample cell 14, and the transmittance TC2 is similarly calculated from the output DC2 of the detector 18 as follows. Is calculated as follows. TC2 = (DC2−D0) × K1 (24) (A8) Next, the coefficient K3 is obtained from the following equations (25) to (27). LC1 = log ((TC1-K3) / (1-K3)) (25) LC2 = log ((TC2-K3) / (1-K3)) (26) LC1 / LC2 = C1 / C2 (27) In order to obtain the coefficient K3 from the equations (25) to (27), a numerical calculation by a computer is used. For example, 0 for K3
Is substituted for the candidate value that gradually increases from
It can be obtained by repeating the calculation until LC2 approaches the value of C1 / C2. The coefficient K3 obtained as described above is also stored in the RAM 33. This completes the calibration operation.

Next, the procedure for measuring the concentration of a sample whose concentration is unknown (measurement sample) will be described. (B1) The shutter 12 is closed, and a zero sample is introduced into the sample cell 14. After a predetermined time, the output of the detector 18 is changed to the transmitted light 0
% As the detection signal D0 (time t
1). (B2) The zero sample in the sample cell 14 is left as it is,
The shutter 12 is opened, and a detection signal D100 of 100% of the transmitted light by the detector 18 is stored in the RAM 33 after a predetermined time (time t2). (B3) for converting to the same manner as when the calibration time of the (A4), 0% transmittance and 100% correct transmission of the detection signal D0 and D100 respectively transmittance values 0 and 100, the correction coefficient, as follows Perform the calculation. K1 = 1 / (D100-D0) (21) The correction coefficient K1 is stored in the RAM 33. (B4) The shutter 12 is closed and the measurement sample is transferred to the sample cell 14
To be introduced. After a predetermined time, the shutter 12 is opened, and after a further predetermined time, the transmitted light detection signal D
M is stored in the RAM 33 (time t3). (B5) from the detection signal DM and the coefficient K1, to calculate the transmission TM of the measurement sample as follows. TM = (DM−D0) × K1 (28) (B6) The correction coefficient K3 obtained at the time of calibration is read from the RAM 33, and the concentration M of the measurement sample is calculated as follows (time t4). LM = log ((TM-K3) / (1-K3)) (29) M = (LM / LC1) × C1 (30) (B7) The value M of the concentration of the measurement sample calculated in this manner.
Is displayed on the digital display 22.

In the above-mentioned calibration and measurement operation, the zero sample, the standard sample or the measurement sample is successively introduced into the sample cell 14 by the control unit 21 controlling the sample fluid switching units 15 and 16. The opening and closing are performed by the control unit 21 controlling the shutter drive motor (M) 13. Therefore, by instructing the control unit 21 to repeat the measurement operation in advance, the automatic continuous measurement of the sample can be performed.

When the measurement is performed manually instead of the automatic continuous measurement, the measurement is performed according to the procedure shown in FIG. First, in a standby state in which no sample is introduced into the sample cell 14, after the shutter 12 is closed, the output D0 of the detector 18 is changed to RA at a time t11 when a predetermined time has elapsed and the output of the detector 18 has stabilized.
It is stored in M33. Next, a zero fluid is introduced into the sample cell 14 by an operator's key operation or the like, and D100 is measured at a time point t12 when a predetermined time has elapsed after the shutter 12 was opened. Next, a measurement sample (or a standard sample) is introduced into the sample cell 14, and the output DM (or DC) of the detector 18 is stored in the RAM 33 at a time point t13 after a predetermined time has elapsed. And RA
These measured values D0, D100, DM, stored in M33
From DC, the concentration M of the measurement sample is calculated in the same manner as described above (time t14), and the concentration value is displayed on the digital display 22. After that, D0 and DM are detected alternately, the density calculation is performed successively, and the value is displayed on the display 22. It should be noted that the measurement accuracy can be improved by performing D100 detection with zero fluid immediately before reading the final value and performing correction. If such accuracy is not required, the value of K3 can be calculated at the stage of the preliminary test and set in advance by key input.

The output data of the measured concentration value of each sample calculated by the control unit 21 is transmitted to the digital display 2 as described above.
In addition to the output of the analog signal, the analog signal can be output to a recorder or the like by D / A conversion. In this case, by performing range switching in advance at the stage of outputting from the control unit 21 according to the size of output data,
A recorder or the like can always output a high-resolution analog value without performing an operation in hardware.

As shown in FIG. 1, a temperature detector 25 is provided at the outlet of the sample cell 14, and a barometer or a pressure gauge for measuring the pressure in the sample cell is provided. It is also possible to perform high-accuracy concentration measurement that compensates for. That is, the equation (30) for calculating the concentration M of the measurement sample is changed as follows. M = {(LM / LC1) × C1} × {(273 + ThM ) / (273 + ThC1 )} × {(PC1 / PM) × K4} (30b, where ThC1 is the value at the time of measurement of the standard sample of concentration C1) Temperature PC1: Atmospheric pressure or sample pressure ThM at the time of measuring the standard sample having the concentration C1 ThM : Temperature at the time of measuring the measuring sample PM: Atmospheric pressure or the sample pressure at the time of measuring the measuring sample K4: Set to 1 for atmospheric pressure compensation. When the pressure in 14 is greatly increased or decreased, the pressure is set to a value corresponding thereto.

[0022]

In the concentration measuring apparatus according to the present invention, there is no need to prepare a calibration curve from a large number of standard samples in advance.
In addition, by collectively correcting various fluctuation factors, a simple and accurate measurement value can be obtained.

It should be noted that the concentration measuring apparatus according to the present invention is not limited to the one exemplified in the above embodiment, but can be applied to the following various types. A. A multi-component densitometer that can simultaneously measure a plurality of components by using a detector having a plurality of filters having different transmission and absorption wavelengths, or by using a plurality of detectors each having its own wavelength characteristic. B. A densitometer wherein the multi-component meter is used as described above, and a correction operation is performed on a component which interferes with the wavelength of the measurement sample, thereby accurately calculating the measurement value of the measurement sample. C. A concentration meter with an alarm which compares the obtained measured value with a preset alarm critical value and issues a concentration alarm signal when the measured value exceeds the critical value. The alarm critical value can be set arbitrarily, such as an upper limit, a lower limit, and a range, and can be set for each component. In addition to issuing an alarm, it is also possible to make a pass / fail decision according to the concentration value, or in the case of a multi-component meter, to issue a leak alarm when the total value of each component is equal to or less than a predetermined value.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a concentration measuring device according to one embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a control unit of the concentration measuring device according to the embodiment.

FIG. 3 is a timing chart in the case of performing automatic continuous density measurement by the density measuring device of the embodiment.

FIG. 4 is a timing chart in a case where the density measurement is manually performed by the density measurement device of the embodiment.

FIG. 5 is an explanatory diagram of Lambert-Beer's law.

FIGS. 6A and 6B are a graph of a change in transmittance when obeying Lambert-Beer's law and a graph of an actual change in transmittance, respectively.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 ... Light source 12 ... Shutter 13 ... Shutter drive motor (M) 14 ... Sample cell 15, 16 ... Sample fluid switching device 17 ... Optical filter (FIL) 18 ... Transmitted light detector 19 ... Amplifier (AMP) 20 ... A / D converter 21 ... Control unit 22 ... Digital display (DISP) 23 ... Keyboard (KB) 25 ... Temperature / pressure compensation detector

Claims (2)

(57) [Claims] 1. A concentration measuring method for measuring the concentration of a specific component contained in a sample by measuring the absorptance or transmittance of light passing through the sample, comprising the following steps: Measuring method. a) Measure the transmittance TC1 and TC2 of light passing through two samples having known concentrations C1 and C2, respectively. b) A correction coefficient K3 is calculated from the following three equations (1), (2) and (3). LC1 = log ((TC1-K3) / (1-K3)) (1) LC2 = log ((TC2-K3) / (1-K3)) (2) LC1 / LC2 = C1 / C2 (3) C) Measure the transmittance TM of light passing through the measurement sample having the unknown concentration CM. d) The unknown concentration CM is calculated by the following equation (4) or (5). CM = ((log ((TM−K3) / (1-K3))) / LC1) × C1 (4) CM = ((log ((TM−K3) / (1-K3))) / LC2) × C2… (5) 2. The light transmittance of each sample TA = TC1, TC
2. The method according to claim 1, wherein TM is determined by the following procedure. e) The output D0 of the detector when the measurement light is not incident and the output D100 of the detector when the measurement light is incident after passing through a zero sample containing no measurement target component are measured. f) The following correction coefficient K1 is calculated from D0 and D100. K1 = 1 / (D100-D0) (6 ) g ) From the output DA of the detector when the measurement light passes through the sample containing the component to be measured, the light transmittance TA of the sample is determined by TA = (DA−D0) × K1 (8)