JPH04262239A - Quantitative analysis method using ftir - Google Patents

Abstract

PURPOSE:To analyze even a component to be measured having high concentration with the minimum detecting sensitivity being maintained at a constant level and to make it possible to perform continuous analysis even for a samples to be measured whose concentration change is large by automatically switching the groups of wave-number points used for the concentration computation of the component in response to the magnitude of the concentration of the component to be measured. CONSTITUTION:The group of wave-number points used for concentration computation corresponding to the concentration range based on the spectrum data obtained from a Fourier- transformation infrared spectrophotometer is used, and operation is performed. Whether the computed value can be outputted or not is compared with a threshold value. When the concentration range which is set by the computed value is judged as adequate, the computed value is outputted as the computed value of the concentration. When the range is not adequate, the concentration range is changed and set at the adequate value. The operation is performed again based on the absorbance of the group of the wave-number points for the corresponding concentration computation. These computation, judgment and change of range are all performed with computers automatically. Only the result of the concentration computation based on the group of the wave-number points for the most adequate concentration computation is always outputted in this way.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention irradiates a measurement sample with infrared light and determines the components contained in the measurement sample based on the absorbance at a plurality of designated wave number points in the absorption spectrum obtained at that time. This invention relates to a quantitative analysis method using FTIR (Fourier transform infrared spectrometer) to analyze the concentration of .

0002

2. Description of the Related Art The above-mentioned FTIR is configured as shown in FIG. 1, for example. That is, in this figure, 1 is an analysis section, and 2 is a data processing section that processes the interferogram output from this analysis section 1. The analysis section 1 consists of an infrared light source 3 configured to emit parallel infrared light, a beam splitter 4, a fixed mirror 5, and a movable mirror 6 that is moved in parallel in, for example, the X-Y direction by a drive mechanism not shown. It is composed of an interference mechanism 7, a cell 8 that accommodates a measurement sample and the like and is irradiated with infrared light from the infrared light source 3 via the interference mechanism 7, and a semiconductor detector 9. The data processing unit 2 is composed of, for example, a computer, and is configured to average the interferograms, perform Fourier transform on the average output at high speed, and perform spectral calculation regarding the component to be measured based on this Fourier transform output. It is configured.

[0003] With the FTIR configured as described above, quantitative analysis can be performed as follows. That is, a comparison sample or a measurement sample is accommodated in the cell 8, and the cell 8 is irradiated with infrared light from the infrared light source 3, and the interferogram of the comparison sample or measurement sample is measured. These interferograms are processed in the data processing unit 2,
After Fourier transforming each to obtain the power spectrum,
After obtaining the absorption spectrum by calculating the ratio of the power spectrum of the measurement sample to the power spectrum of the comparison sample and converting this to an absorbance scale, the ratio of the power spectrum of the measurement sample to the power spectrum of the comparison sample is obtained. The components (single component or multiple components) contained in the product are quantitatively analyzed.

As the above-mentioned quantitative analysis method, for example, a patent application filed on June 28, 1990 (Japanese Patent Application No.
-171038), the outline of which is to calculate the sum of relative absorbances, which are the differences between local peak values and local valley values, at multiple wavenumber points in the absorption spectrum, and then calculate the concentration of each component based on this sum. According to FTIR, a single component or multiple components in a measurement sample can be quantitatively analyzed by appropriately selecting a group of wavenumber points in an absorption spectrum.

[0005]

By the way, when quantitatively analyzing components in a measurement sample using FTIR, the concentration range in which the concentration can be calculated accurately from a certain group of wave number points for concentration calculation is relatively narrow. In other words, the wave number point group for concentration calculation that corresponds to low concentrations cannot obtain correct analysis values for high concentrations, and the wave number point group that corresponds to high concentrations has a lower minimum detection sensitivity and cannot detect low concentrations. That is, for example, if the concentration range is 0
~100 ppm and FS (full scale) 1% zero noise can detect up to about 2 ppm, but when the concentration range is 0 to 1000 ppm, even with FS 1% zero noise, there must be a concentration of about 20 ppm. cannot be detected. This is because the level of zero noise relative to full scale often does not change much depending on the concentration range, so the absolute value of noise becomes large for high concentrations.

[0006] Simultaneous multi-component analysis, for example, Fig. 4(A)
As shown in Figure 4(B), when two target components A and B are analyzed simultaneously, if the concentration of one target component A exceeds its measurement limit LA, as shown in Figure 4(B). Not only the calculated concentration of the component to be measured A, but also the calculated concentration of the other component to be measured B, which has a low concentration that does not exceed the measurement limit LB, may become inaccurate. Therefore, highly accurate analysis may not be possible in continuous measurements of measurement samples whose constituent components change rapidly in concentration, such as exhaust gas from automobiles.

The present invention has been made with the above-mentioned considerations in mind, and its purpose is to continuously analyze measurement samples with high accuracy even in cases where the concentration of the component to be measured changes rapidly. The objective is to provide a quantitative analysis method that can

[0008]

[Means for Solving the Problem] In order to achieve the above object, in the quantitative analysis method using FTIR according to the present invention, a plurality of wave numbers for concentration calculation corresponding to a plurality of concentration ranges of the component to be measured are provided. A point group is specified in advance, and at the time of analysis, concentration calculation is first performed using the wave number point group of the plurality of types for concentration calculation that corresponds to the set concentration range, and the calculation result is If the concentration range is determined to be inappropriate after comparing it with the threshold value, change the setting to a more appropriate concentration range and calculate the concentration of each component to be measured using the corresponding wave number point group for concentration calculation. It is recalculated and output.

[0009]

[Operation] According to the above method, it is possible to analyze a component to be measured at a high concentration while maintaining a constant minimum detection sensitivity, and it is possible to perform continuous analysis with high accuracy over a wide concentration range.

0010

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The major difference between the quantitative analysis method using FTIR according to the present invention and conventional methods is that for a given component to be measured, multiple types of wave number point groups for concentration calculation can be specified depending on the desired concentration range. (e.g. 0-100
ppm, 0-1000 ppm, 0-1%, etc.). Then, at the start of measurement, concentration is calculated using a specific one of these wave number point groups (this may be set in advance or may be set at that time). If the calculated value is not appropriate when compared with a preset threshold, recalculation is performed using a group of wavenumber points for concentration calculation that corresponds to a more appropriate concentration range according to that value. . Then, from the second time onwards, as a general rule, the concentration is calculated using the same wave number point group as the previous time, and depending on the calculated value, the wave number point group for concentration calculation is changed and recalculated in the same way as above. .

FIG. 2 is a flowchart showing this. Based on the spectrum data obtained from the FTIR interferometer (step S1), a group of wave number points for concentration calculation corresponding to the set concentration range is used. Calculation is performed (step S2). Then, it is compared with a threshold value to see whether the calculated value at this time can be output as is (step S
3), if it can be output as is, that is, if it is determined that the concentration range set based on the calculated value is appropriate (YES in step S3), output the calculated value as a calculated concentration value (step S4), If not (NO in step S3), the concentration range is changed to a more appropriate setting (step S5) and calculation is performed again from the absorbance of the corresponding wave number point group for concentration calculation (step S2). All of these calculations, judgments, and range changes are automatically performed within the computer 2. In this way, only the concentration calculation results from the most appropriate group of wave number points for concentration calculation are always output.

When comparing the calculated value with the threshold value, a difference is made between the threshold value for changing the range from high concentration to low concentration and the threshold value for changing the range from low concentration to high concentration. is preferable. By doing this, variations in switching can be suppressed.

[0014] To explain this in the case where two components A and B are simultaneously and continuously analyzed, it is assumed that there are two components A and B whose concentrations change as shown in FIG.
That is, assuming that the concentration range is automatically switched depending on either low concentration or high concentration, four types of wave number point groups corresponding to the combination of concentration ranges of components A and B are determined as shown in Table 1.

[0015]

[Table 1]

The concentrations of components A and B in the measurement sample change as shown in FIG. 3, and the threshold values for component A from low to high concentration and from high to low concentration are Ua and Da (however, Ua > Da), respectively, and
The threshold value from low concentration to high concentration and the threshold value from high concentration to low concentration of component B are respectively Ub and Db (however, U
b > Db), the switching takes time T1
, T2 and T3. Therefore, in this example, the time T0 to T1 is the wave number point group ll, and the time T1
~T2 is the wave number point group hl, time T2 ~T3
The density values calculated in the wave number point group hh are output, and the density values calculated in the wave number point group lh are output at times T3 to T3.

[0017] The method of the present invention is disclosed in the above-mentioned Japanese Patent Application No. 2-1
It goes without saying that this method is not only applicable to the method described in No. 70138, but may also be applied to other methods.

[0018]

[Effects of the Invention] As explained above, in the present invention, the wavenumber point group used to calculate component concentration based on spectral data is switched depending on the concentration of the component to be measured contained in the measurement sample. As a result, even high-concentration target components can be accurately measured while keeping the minimum detection sensitivity constant. Since the switching is performed automatically, continuous analysis is possible even for a measurement sample in which the concentration of the component to be measured changes significantly. Further, when the concentration change is small, recalculation is not performed much, so there is an advantage that there is little time loss.

[Brief explanation of the drawing]

FIG. 1 schematically shows an example of a device for carrying out the method of the invention.

FIG. 2 is a flowchart showing the operation flow of the method of the present invention.

FIG. 3 is a diagram showing an example of an output waveform.

4A and 4B are diagrams for explaining the prior art, in which FIG. 4A is a waveform diagram representing an actual concentration change, and FIG. 4B is a waveform diagram representing an output change of a calculated concentration value.

Claims (1)

[Claims] Claim 1: FTIR that irradiates a measurement sample with infrared light and analyzes the concentration of components contained in the measurement sample based on the absorbance at a plurality of designated wave number points in the absorption spectrum obtained at that time. In a quantitative analysis method using Concentration calculations are performed using points that correspond to the set concentration range, and the calculation results are compared with a certain threshold. If the concentration range is determined to be inappropriate, the F characterized by changing the setting to an appropriate concentration range and recalculating and outputting the concentration of each component to be measured using the corresponding wave number point group for concentration calculation.
Quantitative analysis method using TIR.