JPH05296923A - Multicomponent analyzing method for spectrochemical analysis - Google Patents

Abstract

PURPOSE:To obtain a multicomponent analyzing method for spectrochemical analysis by which multiple components can be accurately and efficiently determined at the same time. CONSTITUTION:The concentrations of multiple components contained in a sample are simultaneously determined by irradiating the sample with the light from a light source and performing arithmetic processing based of the absorption spectrum obtained from the irradiation. Then the multiple components are classified into a plurality of component groups based on the shapes and absorption regions of absorption peaks and each component group is determined by using the most suitable determinating arithmetic technique.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of irradiating a sample with light from a light source and analyzing the concentration of multi-components contained in the sample based on the absorption spectrum obtained at that time.

[0002]

2. Description of the Related Art At present, there is a method for analyzing the concentration of multi-components contained in a sample by performing a matrix operation based on an absorption spectrum obtained by using, for example, FTIR (Fourier transform infrared spectrometer). Although several types are known, conventionally, it is general to use any one type.

[0003]

[0007] Meanwhile, automobile exhaust gas (hereinafter, the self exhaust gas referred to) in the, CO, CO _2,
It contains NO, H ₂ O, HC (hydrocarbons) and the like as main components. Of these, CO, CO ₂ , NO, H ₂ O
Has a sharp and characteristic absorption, as is clear from the absorption spectra of FIGS. 8 to 11, because of its simple molecular structure. And, since these components also have a relatively high concentration in their own exhaust gas, they are strongly absorbed. Also, from 1800
Focusing on the wave number range of 2400 cm ^-1 , there is almost no interference component for these components.

On the other hand, HC contains CH ₄ (methane) and C ₂
It contains a large number of compounds such as H ₆ (ethane) and C ₃ H ₈ (propane). 16 to 19 are C
_{2 H 6, C 3 H 8} , n-C 4 H 10 ( butane),
Although the absorption spectrum of iso-C ₄ H ₁₀ (isobutane) is shown, it is clear from these figures that what is common to these hydrocarbons with a large number of bonds is that they have absorption in almost the same region. Moreover, the absorption itself is broad and similar to each other (however, CH ₄ , C ₂ H ₂ (acetylene) having a relatively simple molecular structure has a sharp peak).

As described above, in the main components contained in the self-exhaust gas, CO, CO ₂ , NO, H ₂ O, and HC have different absorption characteristics, and also in HC, C having a large number of bonds. _{2 H 6, C 3 H 8} , n-C 4 H 10, iso -C
Absorption characteristics are different between ₄ H _{10 and the} like and CH ₄ and C ₂ H ₂ having a simple molecular structure. Therefore, it is difficult to accurately quantify many components contained in the self-exhaust gas when one type of method is adopted as in the past.

The present invention has been made in consideration of the above matters, and an object thereof is to perform multi-component analysis in spectroscopic analysis capable of quantitatively analyzing multi-components simultaneously with high accuracy and efficiency. To provide a method.

[0007]

In order to achieve the above object, in the present invention, a sample is irradiated with light from a light source and arithmetic processing is performed based on an absorption spectrum obtained at that time In the method of quantitatively analyzing the concentrations of the contained multicomponents at the same time, the multicomponents are grouped into a plurality of component groups based on the shape of the absorption peak and the absorption region, and the most appropriate quantitative calculation method is used for each component group. I have to.

[0008]

For example, when quantitatively analyzing the components of the self-exhaust gas, for the components such as CO, CO ₂ , NO, and H ₂ O, the absorption peaks are sharp. Patent application with attachment (Japanese Patent Application No. 2-274204
The quantitative analysis method based on the concept of relative absorbance can be applied suitably, and by using this method, the amount of memory used in the computer and the time required for the calculation can be reduced as much as possible. In addition to the above, each component can be quantified with high accuracy.

Among the HC, CH ₄ , C
_As for HC except ₂ H ₂ , since the absorption itself is broad, PLS which has been generally used conventionally
A method called (Partial Least Square) method is used. According to this, although the amount of memory used and the time required for the calculation are longer than those of the relative absorbance method, the respective components can be quantified with high accuracy.

As described above, by adopting an appropriate quantification method for each of the components to be quantified, the accuracy of the analyzer as a whole can be improved, and multiple components can be quantified simultaneously and accurately. Then, the amount of memory used on the computer and the calculation speed can be minimized.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an FTI that is an example of an apparatus for performing a multi-component analysis method in spectroscopic analysis according to the present invention.
2 shows a schematic configuration of R1. This FTIR
Reference numeral 1 denotes an analysis unit 2 and an interferogram output from the analysis unit 2, and a movable mirror 7 to be described later.
And a computer 3 which issues a control command to the drive mechanism.

The analysis unit 2 comprises a light source 4 configured to emit parallel infrared light, a beam splitter 5, a fixed mirror 6, and a movable mirror 7 which moves in X and Y directions by a drive mechanism (not shown). It is composed of an interference mechanism 8, a cell 9 for accommodating a measurement sample and the like, which is irradiated with infrared light from the light source 4 via the interference mechanism 8, and a detector 10 such as a semiconductor detector.

In the computer 3, an averaging processor 11 for averaging the interferograms output from the detector 10, and a fast Fourier transform process for fast Fourier transforming the output data from the averaging processor 11 The control unit (not shown) is provided in addition to the unit 12, the operation unit 13 for performing the spectrum operation and the quantitative operation based on the output data from the fast Fourier transform processing unit 12, and the like.

In the FTIR 1 thus constructed, the comparative sample or the measuring sample is housed in the cell 9, and the interferograms of the comparative sample and the measuring sample are respectively measured. Each of these interferograms is Fourier transformed to obtain a power spectrum, that is, a spectrum of light transmitted through the cell 9. Next, the ratio of the power spectrum of the measurement sample to the power spectrum of the comparative sample is obtained, and the absorption spectrum is obtained by converting this value into the absorbance scale.

Next, details of the multi-component analysis method of the present invention will be described. Now, as an example, FTIR1 of 0.25 cm ^-1
Consider the case of quantifying the multicomponents in the self-exhaust gas discharged from the LPG (liquefied propane gas) vehicle using. The self-exhaust gas contains CO, CO ₂ , NO, H ₂ O, HC having 4 or less carbon atoms as main components. And
CO, CO ₂ , NO, H ₂ O, N as components to be measured
_{H 3, CH 4, C 2} H 4 ( _{ethylene), C 2 H 6, C} 3
H _{6 (propylene), C 3 H 8, n} -C 4 H 10, iso -
Considering the 12 components of C ₄ H ₁₀ , as already explained, these components are grouped into four component groups based on the shape of the absorption peak and the absorption region, for example, as shown in Table 1 below. it can.

[0016]

[Table 1]

The first component group in Table 1 above is a component group having absorption in the wave number range of 1800 to 2400 cm ^-1 , as shown in FIGS. The components belonging to this component group have a characteristic and sharp absorption peak due to their simple molecular structure, and also have strong absorption because the concentration of their own exhaust gas is relatively high. The second component group is 800-1400 as shown in FIGS.
It is a group of components that also have a sharp absorption peak in the wave number range of cm ^-1 . Also, the third component group, as shown in FIGS.
Second in the wave number range of 800-1100 cm ^-1 , which overlaps with the second component group
It is a component group that has a broader absorption than the component group. In addition to these components, CO ₂ and H ₂ O also have absorption in the wave number range of the second and third component groups. The fourth component group is
As shown in FIGS. 16 to 19, the components are broad and have similar absorption in the almost same wave number range of 2800 to 3100 cm ⁻¹ . In the wave number range of this fourth component group, H ₂ O,
CH ₄ , C ₂ H ₄ and C ₃ H ₆ also have absorption.

Since the first and second component groups have sharp absorption, the relative absorbance method according to Japanese Patent Application No. 2-274204 is applied.

First, the concept of the sum of relative absorbances introduced in this method will be briefly described.
The spectrum output from the spectrometer is generally
It contains noise from various sources and therefore
The equation (1) can be expressed as A (ν _i ) = Cα (ν _i ) + ε _i (2) Here, ε _i is noise included in the spectrum.

Focusing on the difference between any two points in the spectrum, the above equation (2) has the following formula: A (ν _p ) −A (ν _b ) = C [α (ν _p ) −α (ν _b )] + Ε _p −ε _b (3), and it is shown that the relative absorbance between such two points also follows Lambert-Beer's law. here,
p and q are subscripts indicating the wave number points corresponding to the peak and base of the spectrum, respectively.

In general, the spectrum of a substance, particularly the spectrum of a gas, includes a large number of peaks, and it is possible to select a large number of pairs of the wavenumber point ν _{p of the} peak and the wavenumber point ν _b of the base, which are specific to a certain substance.

FIG. 2 shows an example of a typical gas absorption spectrum and the corresponding relative absorbances L ₁ , L ₂ and L ₃ . Each relative absorbance L ₁ , L ₂ , L
₃ corresponds to the value of the expression (3).

When the sum of such values is taken, Σ _k [A (ν _p ) −A (ν _b )] _k = CΣ _k [α (ν _p ) −α (ν _b )] + Σ _k (ε _p −ε _b ) _k (4) and this equation (4) also maintains Lambert-Beer's law. Then, from the equation (4), it is understood that the second term on the right side of the equation averages the random noise latent in the spectrum and cancels the disturbance such as the drift of the baseline.

Here, the value calculated by the above equation (4) is
It is defined as a MAS (Multiple Absorption Sum) value. That is, MAS≡Σ _k [A (ν _p ) −A (ν _b )] _k (5) Then, in particular, the MAS value obtained for a reference spectrum of a known concentration with a single component is set to M
AS _i .

FIG. 3 shows an example of designation of wave number points for the three gas species contained in the generalized component group. In the figure, curves I, II, and III represent reference spectra of the gases X, Y, and Z, respectively. Then, a set of wave number points for gas _{X Φ X: (X p,} X b) k, the set of wave number points for gas _{Y Φ Y: (Y p,} Y b) k, the set of wave number points for gas Z [Phi _Z: (Z _p , Z _b ) _k are designated respectively. When designating these peak-base pairs, it is preferable to obtain a relative absorbance as large as possible and not to be influenced by the absorption of each other.

Here, by generalizing the story, the set of the above-mentioned wave point specification values for m components is Φ _i (i = 1 to m).
And the peak of the component i for one absorption spectrum
An operation for calculating the set of base pairs as in the above equation (4) is defined as in the following equation. Σ _k [A (ν _p ) −A (ν _b )] ≡A ◎ Φ _i (6) where ◎ is a new operator, and expression (6) gives the MAS value.

Therefore, MAS _i = A⊚Φ _i (7) and m MAS _i are calculated for m Φ _i . If MAS _i , which is a set of m numbers, is Ψ, this can be considered as a spectrum having m elements, on the other hand. Ψ = (MAS _i : i = 1 to m) (8)

In this new spectral region (component spectral region), the horizontal axis represents gas species i, and the vertical axis represents the sum M of relative absorbances.
It can be displayed graphically as AS _i . Therefore, the equation (7) can be considered to be the spectrum A converted into the component spectrum Ψ using the set Φ.
This will be described below by applying it to the component group generalized and shown above.

FIG. 4 shows the three kinds of gases X, Y and Z shown in FIG.
Represents the absorption spectrum of a mixture of each at any concentration, that is, the absorption spectrum of the above generalized component group as a whole. Applying the wave point specified values of the respective peak-base pairs of the three kinds of gases specified in FIG. 3 to the absorption spectrum of FIG. 4, X,
The sum of the relative absorbances represented by Y and Z is calculated.
Then, as shown in FIG. 5, the sum of these relative absorbances is plotted in a region in which the horizontal axis represents the gas species and the vertical axis represents the sum of relative absorbances.

In the component spectrum thus obtained, the influence of noise is largely eliminated as described above, and the influence of interference between spectra between gas species is suppressed. As shown in FIGS. 4 and 5, if conversion without interference between components can be performed, the sum of relative absorbances of each component in the component spectral region is proportional to each component concentration, and thus the concentration of each component can be directly calculated. Obtainable.

However, in the actual gas absorption spectrum, it is seldom possible to specify the wave number point without interference as shown in FIGS. 4 and 5, and some interference remains in the converted component spectrum region. Is normal.

Therefore, in the quantitative analysis using the component spectral region, it is necessary to correct the above-mentioned slightly remaining interference, and the following processing is performed in the calibration step of creating a matrix (calibration matrix) used for interference correction, that is, It is divided into a step of preparing data for interference correction and an estimation step of calculating unknown concentration using the data.

At the calibration stage, the calibration matrix is obtained as follows. Now, if there is a reference spectrum α _i (i = 1 to m) of unit concentration for m components, generalization is performed, and the set Φ _i of the wave point specification values described above is applied to these reference spectra, and by applying a transformation represented by 7), the component spectrum corresponding to α _i Ψ _{i (i}
= 1 to m) can be obtained. More specifically, it can be obtained by obtaining the sum of the relative absorbances when a plurality of gases are individually used as samples.

FIGS. 6A, 6B, and 6C show reference component spectra for the three gas species X, Y, and Z contained in the component group generalized above. The calibration matrix is

[0035]

[Equation 1]

It becomes

On the other hand, the component spectrum of the component group can be obtained as shown in FIG. 7 from the absorption spectrum obtained by spectrally analyzing the sample to be measured in the wave number range assigned to the component group. The component spectrum Ψ _{u in} this case is

[0038]

[Equation 2]

It becomes Generally, m gas components (i = 1
˜m), if the unknown concentrations of the respective gas components are C _i (i = 1 to m), Ψ _u is represented by a linear combination of Ψ _i . That is, Ψ _u = C ₁ Ψ ₁ + C ₂ Ψ ₂ + ... + C _m Ψ _m (9) always holds, and if this is rewritten using a matrix, Ψ _u = CΩ ...... (10) It can be expressed as. Here, C is a vector of unknown concentrations, and Ω is a matrix having Ψ _i as a row, which is called a calibration matrix in the component spectral region.

Therefore, assuming that the unknown concentrations of the three gas species X, Y, Z of the above-mentioned component group are C _X , C _Y , C _Z ,

[0041]

[Equation 3]

It becomes At the calibration stage, it is necessary to accurately determine the Ω. Also, as already mentioned, Ψ
_{Since i} is a vector with high linear independence, that is, a vector with little interference, Ω is a matrix that can obtain a stable inverse matrix.

Next, in the estimation step, the unknown concentration can be estimated by solving the equation (10) for C, that is, C = Ψ _u Ω ⁻¹ .

Also with respect to the equation (11),

[0045]

[Equation 4]

By the above, C _X , C _Y , and C _Z can be obtained respectively.

Since the inverse matrix calculated here has a high linear independence of Ω, a stable solution can be obtained, and therefore the error due to the numerical calculation of the estimated concentration is extremely small.

As described above, the relative absorbance method is particularly suitable for the quantification of a component which has a large relative absorbance by reducing the influence of the absorption of other components, that is, a component having a sharp absorption peak. .. In addition, the quantitative calculation process is relatively simple, the calculation speed is fast, and the memory usage is small. The relative absorbance method having such characteristics is applied to the first and second component groups. In this case, the wave number range used for quantification does not have to be the entire infrared region, and the first component the group, 1800～2400Cm ^-1, for the second component group may be a limited range of 800~1400cm ^-1.

Further, with respect to the first component group, since there is no other interference component, the four components (CO, CO ₂ , NO, H
Only in consideration of ₂ O), quantitative calculation may be performed in the wave number range. In addition, regarding the second component group, the quantitative calculation is performed in the wave number range while considering other interference components (H ₂ O, CO ₂ , C ₂ H ₄ , C ₃ H ₆ ). in this way,
The components quantified by the same relative absorbance method are also the first component group, the second component
By dividing into component groups, it is possible to reduce the amount of memory used on the computer and to perform quantitative calculation at higher speed. However, since the quantitative calculation methods of the first component group and the second component group are the same, it is needless to say that the quantitative determination may be carried out collectively in one component group.

Next, regarding the third and fourth component groups,
Since the absorption is broad and the wave number range in which the absorption is present and the shape thereof are similar to each other, when the relative absorbance method is applied, the accuracy tends to be insufficient. Therefore, a method called PLS method that has been generally used conventionally is used for these component groups. In this method, a matrix for concentration calculation is created in advance using the absorption spectrum of the compound of which the concentration is desired to be known as a reference spectrum. The PLS method is, for example, Prac
tical Fourier Transform Infrared Spectroscopy, Aca
demic Press, Lindberg, W., et al., Anal. Chem., 19
83, 55, 643, Haaland, DM, Thomas, EV, Anal.Che
m., 1988, 60, 1193, Geladi, P., Kowalsky, BR,
Anal. Chem. Acta., 1986, 185, Wold, S., et al., Mat
It is explained in detail in rix Pencil, Springer Verlag, etc.

The PLS method uses a larger amount of memory and has a slower calculation speed than the relative absorbance method, but it can quantify broad absorption with relatively high accuracy. Even when this PLS method is used, the wave number range used for quantification does not have to be the entire infrared region, and for example, the third component group is 800 to 1100 cm ⁻¹ , and the fourth component group is A limited range of 2800 to 3100 cm ^-1 is sufficient. Also,
As for the third component group, CO ₂ and
In consideration of H ₂ O and NH ₃ , quantitative calculation is performed within the wave number range. Similarly, for the fourth component group, H ₂ O, C
In consideration of H ₄ , C ₂ H ₄ , and C ₃ H ₆ , quantitative calculation is performed in the wave number range. In this way, by dividing the components to be quantified by the same PLS method into the third component group and the fourth component group, it is possible to reduce the memory usage on the computer and increase the quantitative calculation speed. However, since the quantitative calculation methods of the third component group and the fourth component group are the same, it goes without saying that they may be collectively quantified into one component group.

As can be understood from the above description, in the case of quantifying multiple components at the same time, the components to be measured are divided into a plurality of groups, and the quantifiable groups (for example, the absorption peak is sharp or characteristic, or the absorption For a group consisting of components such that a certain region does not overlap with other components or has strong absorption intensity, among the quantitative methods that provide sufficient accuracy, a method with a simple arithmetic process or a memory on a computer is used. There are less ways to apply. On the contrary, a group that is difficult to quantify (a group consisting of a component that has a broad absorption and few characteristics, and the absorption region overlaps with other components)
With regard to, the optimal method is applied in terms of accuracy even if some conditions (computation speed, memory usage, etc.) are sacrificed.

In these cases, the wave numbers used for the calculation may overlap as in the above example. Further, it is not necessary to be particular about the type and number of the quantification method, and for example, CLS (Classical Least Squa) which has been generally used from the past.
re) method may also be used. Furthermore, the present invention is not limited to gas analyzers, nor is it limited to analysis using FTIR.

[0054]

As described above, according to the present invention,
By adopting an appropriate quantification method for each component to be quantified, the accuracy of the analyzer as a whole can be increased, and multiple components can be quantified simultaneously and accurately. Then, the memory on the computer and the calculation speed can be minimized.

[Brief description of drawings]

1 is a diagram schematically showing an example of an apparatus for carrying out the method of the present invention.

FIG. 2 is a schematic diagram of a gas absorption spectrum and a relative absorbance corresponding to the gas absorption spectrum, which are shown for explaining a calculation method used in an embodiment of the method of the present invention.

FIG. 3 is a diagram showing an example of designating wave number points for gas species by generalizing the component group in the embodiment.

FIG. 4 is a diagram showing an absorption spectrum of a generalized component group in the example.

FIG. 5 is a diagram showing an example of a component spectrum calculated from an absorption spectrum of a generalized component group in the example.

FIG. 6 is a diagram showing reference component spectra corresponding to a generalized component group in the example.

FIG. 7 is a diagram showing a component spectrum calculated from an absorption spectrum of a measurement sample in the example.

FIG. 8 is a diagram showing an absorption spectrum of CO.

FIG. 9 is a diagram showing an absorption spectrum of CO ₂ .

FIG. 10 is a diagram showing an absorption spectrum of NO.

FIG. 11 is a diagram showing an absorption spectrum of H ₂ O.

FIG. 12 is a diagram showing an absorption spectrum of NH ₃ .

FIG. 13 is a diagram showing an absorption spectrum of CH ₄ .

FIG. 14 is a diagram showing an absorption spectrum of C ₂ H ₄ .

FIG. 15 is a diagram showing an absorption spectrum of C ₃ H ₆ .

FIG. 16 is a diagram showing an absorption spectrum of C ₂ H ₆ .

FIG. 17 is a diagram showing an absorption spectrum of C ₃ H ₈ .

FIG. 18 is a diagram showing an absorption spectrum of n-C ₄ H ₁₀ .

FIG. 19 is a diagram showing an absorption spectrum of iso-C ₄ H ₁₀ .

[Explanation of symbols]

 1 ... FTIR, 2 ... analysis part, 4 ... light source.

Claims (1)

[Claims] 1. A method for simultaneously quantitatively analyzing the concentrations of multi-components contained in a sample by irradiating the sample with light from a light source and performing arithmetic processing based on the absorption spectrum obtained at that time. A multi-component analysis method in spectroscopic analysis, characterized in that a component is divided into a plurality of component groups based on the shape of an absorption peak and an absorption region, and a quantitative calculation method most suitable for each component group is used.