Qualitative and quantitative analysis using Raman scattering

Claims

We claim as our invention:

1. A method for determining the composition of an unknown sample comprising:

(a) producing a beam of photons which is substantially monochromatic and impinges on the unknown sample;

(b) collecting photons scattered by the unknown sample into a stream of scattered photons;

(c) resolving the photon stream into its component frequencies to form a Raman spectrum of the unknown sample;

(d) providing said unknown sample Raman spectrum to a computer;

(e) providing to the computer reference spectra obtained in the same manner as said unknown spectrum, where the reference spectra are of reference samples whose composition is known; and,

(f) identifying substances present in the unknown sample by comparing said unknown spectrum to the reference spectra, said comparison being accomplished by utilizing the computer and comprising the following steps:

(i) inspecting the unknown spectrum and selecting a plurality of separate spectral analysis regions;

(ii) determining a size characteristic associated with each of said spectral analysis regions of the unknown spectrum;

(iii) searching the reference spectra and choosing those reference spectra having in the selected spectral analysis regions features corresponding to those of the selected spectral analysis regions of the unknown spectrum and size characteristics substantially identical to those of the selected spectral analysis regions of the unknown spectrum;

(iv) if more than one reference spectrum is chosen, repeating steps (f)(i), (f)(ii), and (f)(iii), selecting additional spectral analysis regions, until only one reference spectrum is chosen, the substances present in said one chosen reference spectrum being present in the unknown samples;

(v) if no reference spectrum is chosen, establishing a number of working hypotheses, each hypothesis being that the unknown sample consists of a different combination of the reference samples which exhibited the spectra chosen to have features corresponding to those of the selected spectral regions of the unknown spectrum;

(vi) testing each working hypothesis by combining the spectra of the hypothesis to produce a single hypothetical spectrum and comparing it to the unknown spectrum;

(vii) discarding each working hypothesis which is not substantially identical to the unknown spectrum; and,

(viii) if more than one working hypothesis has not been discarded, repeating steps (f)(v) through (f)(viii), selecting additonal spectral analysis regions, until only one working hypothesis has not been discarded, the unknown sample composition then being that of said one remaining working hypothesis.

2. The method of claim 1 further characterized in that said size characteristic is the height of the highest peak of the spectral analysis region.

3. The method of claim 1 further characterized in that said size characteristic is the area of the spectral analysis region.

4. The method of claim 1 further characterized with respect to step (f)(vi) in that said testing is accomplised by:

(a) for each working hypothesis, establishing a set of equations consisting of one equation associated with each selected region of the unknown spectrum, which equation describes the area of the region in terms of concentrations of the reference samples and areas of reference spectra regions; and,

(b) attempting to solve each set of equations and concluding that each working hypothesis whose set of equations cannot be completely solved or yields unreal numbers is not substantially identical to the unknown spectrum.

5. The method of claim 4 further characterized in that said equation associated with each region is of the form

X.sub.1 C.sub.1 +X.sub.2 C.sub.2 + . . . +X.sub.n C.sub.n =A,

where

C.sub.1, C.sub.2, . . . C.sub.n =the concentration of the substance of reference samples 1, 2, . . . n in the hypothetical combination of reference samples,

X.sub.1, X.sub.2, . . . X.sub.n =the areas of those regions in each of the reference spectra of the hypothetical combination, and

A=the area of the region of the unknown spectrum.

6. A method for performing a quantitative analysis for preselected substances of an unknown sample comprising:

(a) producing a beam of photons which is substantially monochromatic and impinges on the unknown sample;

(b) collecting photons scattered by the unknown sample into a stream of scattered photons;

(c) resolving the photon stream into its component frequencies to form a Raman spectrum of the unknown sample;

(d) providing said unknown sample Raman spectrum to a computer;

(e) providing to the computer reference spectra obtained in the same manner as said unknown spectrum, where the reference spectra are of reference samples whose quantitative composition is known and where each reference sample is comprised of at least one of said preselected substances; and,

(f) identifying substances present in the unknown sample by comparing said unknown spectrum to the reference spectra, said comparison being accomplished by utilizing the computer and comprising the following steps:

(i) inspecting the reference spectra and selecting a plurality of separate spectral analysis regions;

(ii) determining the areas of the selected regions for each reference spectrum and for the unknown spectrum;

(iii) establishing a relationship between said reference spectra region areas and concentrations of said preselected substances in said reference samples; and,

(iv) determining the concentrations of said preselected substances in said unknown sample by applying the relationship established in step (f)(iii) to the unknown spectrum region areas.

7. The method of claim 6 further characterized with respect to step (f) in that said relationship is established and said concentrations determined by:

(a) selecting a number of spectral analysis regions equal to the number of said preselected substances;

(b) determining the areas of the selected regions for each reference spectrum and for the unknown spectrum and calculating area fractions for the reference spectra and for the unknown spectrum;

(c) establishing a set of equations for each reference sample where the number of equations in each set is equal to the number of said preselected substances and each equation describes the concentration of one preselected substance in terms of its contributions to region areas, each equation having the form

X=C.sub.1x A.sub.1 +C.sub.2x A.sub.2 + . . . +C.sub.nx A.sub.n,

where

X represents the concentration fraction of the preselected substance,

A.sub.1, A.sub.2, . . . A.sub.n are area fractions of the selected regions of the spectrum of the reference sample where n equals the number of regions, and

C.sub.1x, C.sub.2x, . . . C.sub.nx are coefficients associated with the contributions of the preselected substances to the regions;

(d) solving all of the equations established in step (c) for said coefficients;

(e) establishing one set of equations for the unknown sample as was done in step (c) for each reference sample; and,

(f) solving said unknown sample equations for the concentrations of the preselected substances, using the coefficients determined in step (d).

8. The method of claim 6 further characterized with respect to steps (f)(iii) and (f)(iv) in that said relationship is established and said concentrations determined by:

(a) expressing said reference sample concentrations in terms of concentration fractions and arranging the concentration fractions in a concentration fraction matrix, according to said reference samples and said preselected substances;

(b) calculating area fractions from said reference spectra region areas and arranging the area fractions into an area fraction matrix, according to said reference samples and the selected regions;

(c) determining a transpose matrix, which is the transpose of the area fraction matrix;

(d) forming a mathematical quantity using said matrices, as follows: ##EQU4## (e) solving said mathematical quantity to yield a matrix, which consists of correlation coefficients, arranged according to the selected regions and said preselected substances;

(f) calculating area fractions from said unknown spectrum region areas and arranging the area fractions in a matrix; and,

(g) multiplying said correlation coefficient matrix by the matrix formed of said unknown spectrum area fractions to obtain a product which is a concentration fraction matrix which expresses the concentrations of the preselected substances in said unknown sample.

9. The method of claim 6 further characterized in that the substances comprising said unknown sample are paraffins, naphthenes, and aromatics.

10. The method of claim 6 further characterized in that said beam of photons is from a laser source.

11. The method of claim 6 further characterized in that the wave lengths of said beam of photons are closely centered about a value of 6328 angstroms.

12. The method of claim 6 further characterized in that photons are removed from said stream of photons before it is resolved to form a spectrum, the removed portion consisting of photons at the same frequency as said beam of photons and at a higher frequency than the frequency of said beam of photons.

13. The method of claim 6 further characterized in that composite reference spectra are used in performing said comparison, a composite reference spectrum being prepared for each reference sample by providing a plurality of spectra of each reference sample to the computer and averaging each of said plurality of reference spectra.

14. The method of claim 6 further characterized in that a portion of said Raman spectrum is removed and not provided to the computer, such portion consisting of Rayleigh scattered light and the anti-Stokes lines.

15. The method of claim 6 further characterized in that said unknown spectrum and said reference spectra are adjusted to substantially remove false information before said comparison is accomplished.

16. The method of claim 15 further characterized in that said false information comprises a background spectrum, which is obtained in the same general manner as a sample spectrum but with said beam of photons interrupted, and said adjustment to remove false information is accomplished by subtracting the background spectrum intensity from the sample spectrum intensity at each frequency.

17. The method of claim 15 further characterized in that said false information comprises sample fluorescence and stray photons and said adjustment to remove false information is accomplished by means of establishing baselines and discarding that portion of the spectrum which is below the baselines.

18. Apparatus for determining the composition of an unknown sample comprising:

(a) means for producing a beam of photons which is substantially monochromatic and impinges on the unknown sample;

(b) means for collecting photons scattered by the unknown sample into a stream of scattered photons;

(c) means for resolving the photon stream into its component frequencies to form a Raman spectrum of the unknown sample;

(d) means for converting said unknown sample Raman spectrum to digital form and transmitting said unknown spectrum to computing means;

(e) said computing means, which contain reference spectra obtained in the same manner as said unknown spectrum, where the reference spectra are of reference samples whose composition is known; and,

(f) means within the computing means for identifying substances present in the unknown sample by comparing said unknown spectrum to the reference spectra, said computing means accomplishing said comparison by performing the following functions:

(i) inspecting the unknown spectrum and selecting a plurality of separate spectral analysis regions;

(ii) determining a size characteristic associated with each spectral analysis region of the unknown spectrum;

(iii) searching the reference spectra and choosing those reference spectra having in the selected spectral analysis regions features corresponding to those of the selected spectral analysis regions of the unknown spectrum and size characteristics substantially identical to those of the selected spectral analysis regions of the unknown spectrum;

(iv) if more than one reference spectrum is chosen, repeating functions (f)(i), (f)(ii), and (f)(iii), selecting additional regions, until only one reference spectrum is chosen, the substances present in said one chosen reference spectrum being present in the unknown sample;

(v) if no reference spectrum is chosen, establishing a number of working hypothesis, each hypothesis being that the unknown sample consists of a different combination of the reference samples which exhibited the spectra chosen to have features corresponding to those of the selected regions of the unknown spectrum;

(vi) testing each working hypothesis by combining the spectra of the hypothesis to produce a single hypothetical spectrum and comparing it to the unknown spectrum;

(vii) discarding each working hypothesis which is not substantially identical to the unknown spectrum; and,

(viii) if more than one working hypothesis has not been discarded, repeating functions (f)(v) through (f)(viii), selecting additional regions, until only one working hypothesis has not been discarded, the unknown sample composition then being that of said one remaining working hypothesis.

19. The apparatus of claim 18 further characterized with respect to function (f)(vi) in that said testing is accomplished by:

(a) for each working hypothesis, establishing a set of equations consisting of one equation associated with each selected region of the unknown spectrum, which equation describes the area of the region in terms of concentrations of the reference samples and areas of reference spectra regions; and

(b) attempting to solve each set of equations and concluding that each working hypothesis whose set of equations cannot be completely solved or yields unreal numbers is not substantially identical to the unknown spectrum.

20. The apparatus of claim 19 further characterized in that said equation associated with each region is of the form

X.sub.1 C.sub.1 +X.sub.2 C.sub.2 + . . . +X.sub.n C.sub.n =A,

Where

C.sub.1, C.sub.2, . . . C.sub.n =the concentration of the substance of reference samples 1, 2, . . . n in the hypothetical combination of reference samples,

X.sub.1, X.sub.2, . . . X.sub.n =the areas of those regions in each of the reference spectra of the hypothetical combination, and

A=the area of the region of the unknown spectrum.

21. Apparatus for performing a quantitative analysis for preselected substances of an unknown sample comprising:

(a) means for producing a beam of photons which is substantially monochromatic and impinges on the unknown sample;

(b) means for collecting photons scattered by the unknown sample into a stream of scattered photons;

(c) means for resolving the photon stream into its component frequencies to form a Raman spectrum of the unknown sample;

(d) means for converting said unknown sample Raman spectrum to digital form and transmitting said unknown spectrum to computing means;

(e) said computer means, which contain reference spectra obtained in the same manner as said unknown spectrum, where the reference spectra are of reference samples whose quantitative composition is known and where each reference sample is comprised of at least one of said preselected substances; and,

(f) means within the computer for identifying substances present in the unknown sample by comparing said unknown spectrum to the reference spectra, said computer accomplishing said comparison by performing the following functions:

(i) inspecting the reference spectra and selecting a plurality of separate spectral analysis regions;

(ii) determining the areas of the selected regions for each reference spectrum and for the unknown spectrum;

(iii) establishing a relationship between said reference spectra region areas and concentrations of said preselected substances in said reference sample; and

(iv) determining the concentrations of said preselected substances in said unknown sample by applying the relationship established in function (f)(iii) to the unknown spectrum region areas.

22. The apparatus of claim 21 further characterized with respect to element (f) in that said relationship is established and said concentrations determined by the following functions:

(a) selecting a number of spectral analysis regions equal to the number of said preselected substances;

(b) determining the areas of the selected regions for each reference spectrum and for the unknown spectrum and calculating area fractions;

(c) establishing a set of equations for each reference sample where the number of equations in each set is equal to the number of said preselected substances and each equation describes the concentration of one preselected substance in terms of its contributions to region areas, each equation having the form

X=C.sub.1x A.sub.1 +C.sub.2x A.sub.2 + . . . +C.sub.nx A.sub.n,

where

X represents the concentration fractions of the preselected substance,

A.sub.1, A.sub.2, . . . A.sub.n are area fractions of the selected regions of the spectrum of the reference sample where n equals the number of regions, and

C.sub.1x, C.sub.2x, . . . C.sub.nx are coefficients associated with the contributions of the preselected substances to the regions;

(d) solving all of the equations established in function (c) for said coefficients:

(e) establishing one set of equations for the unknown sample as was done in function (c) for each reference sample; and,

(f) solving said unknown sample equations for the concentrations of the preselected substances, using the coefficients determined in function (d).

23. The apparatus of claim 21 further characterized with respect to functions (f)(iii) and (f)(iv) in that said relationship is established and said concentrations determined by:

(a) expressing said reference sample concentrations in terms of concentration fractions and arranging the concentration fractions in a concentration fraction matrix, according to said reference samples and said preselected substances;

(b) calculating area fractions from said reference spectra region areas and arranging the area fractions into an area fraction matrix, according to said reference samples and the selected regions;

(c) determining a transpose matrix, which is the transpose of the area fraction matrix;

(d) forming a mathematical relationship using said matrices, as follows: ##EQU5## (e) solving said mathematical quantity to yield a matrix, which consists of correlation coefficients, arranged according to the selected regions and said preselected substances;

(f) calculating area fractions from said unknown spectrum region areas and arranging the area fractions in a matrix; and,

(g) multiplying said correlation coefficients matrix by the matrix formed of said unknown spectrum area fractions to obtain a product which is a concentration fraction matrix which expresses the concentrations of the preselected substances in said unknown sample.

24. The apparatus of claim 21 further characterized in that the substances comprising said unknown sample are paraffins, naphthenes, and aromatics.

25. The apparatus of claim 21 further characterized in that said beam of photons is from a laser source.

26. The apparatus of claim 21 further characterized in that the wave lengths of said beam of photons are closely centered about a value of 6328 angstroms.

27. The apparatus of claim 21 further comprising means for removing photons from said stream of photons before it is resolved to form a spectrum, the removed portion consisting of photons at the same frequency as said beam of photons and at a higher frequency than the frequency of said beam of photons.

28. The apparatus of claim 21 further comprising means for using composite reference spectra in performing said comparison, a composite reference spectrum being prepared for each reference sample by providing a plurality of spectra of each reference sample to said computer means and averaging each of said plurality of reference spectra.

29. The apparatus of claim 21 further comprising means for removing a portion of said Raman spectrum, such portion consisting of Rayleigh scattered light and the anti-Stokes lines.

30. The apparatus of claim 21 further comprising means for adjusting said unknown spectrum and said reference spectra to substantially remove false information before said comparison is accomplished.

31. The apparatus of claim 30 further characterized in that said spectrum adjusting means comprises means for providing to said computer means a background spectrum for use in accomplishing said adjustment to remove false information, said background spectrum being obtained in the same general manner as a sample spectrum but with said beam of photons interrupted, and said adjustment to remove false information being accomplished by subtracting the background spectrum intensity from the sample spectrum intensity at each frequency.

32. The apparatus of claim 30 further comprising means for substantially removing those portions of the spectra which comprise sample fluorescence and stray photons by means of establishing baselines and discarding that portion of the spectrum which is below the baselines.

BACKGROUND OF THE INVENTION

This invention relates to the detection of substances and measurement of the quantities present. More particularly, it relates to qualitative and quantitative analysis utilizing Raman spectra of the substances under analysis.

The Raman effect, or Raman scattering, is well known. Briefly and simply, when a beam of light impinges on substances, light is scattered. This scattering is of several different types, the predominant type being Rayleigh scattering, wherein the wave length of the scattered light is the same as that of the incident light. In the type utilized in the present invention, Raman scattering, the scattered light is of different wave lengths than the incident light; photons are absorbed by the substance and re-emitted at higher and lower wave lengths. A Raman spectrum of a substance is constituted of Raman scattered light and is spread across a wave length band even if the incident light is monochromatic, that is, all of a single wave length. There is a separate Raman spectrum of a particular substance for, or associated with, each incident wave length. In practice, a monochromatic beam of incident light is always used in Raman spectroscopy because of the difficulties in obtaining spectral separation. When Raman and Rayleigh scattered light is resolved into a spectrum by a spectrograph, Raman lines will appear on both sides of the Rayleigh line. The Raman line or lines on the low frequency side (or low wave number side or high wave length side) of the Rayleigh line are more intense than those on the high frequency side and are called the Stokes line or lines; those on the high frequency side are called the anti-Stokes line or lines. Not all substances are Raman active; there must be a change in polarizability during molecular vibration in order that a substance be Raman active. Substances which do exhibit Raman spectra can be characterized by means of their spectra. Qualitative analysis of a substance can be accomplished by comparison of the locations of its Raman lines with those of known standards. Quantitative analysis can be accomplished by comparison of intensities of Raman lines; this is generally a linear relationship. Of course, spectra which are compared must result from exciting radiation of the same wave length. For purposes of this document, a substance is defined as any composition of matter, including a single element and a mixture or solution of chemical compounds.

Raman spectroscopy has numerous applications and is a major research tool. It is now a rapidly developing area, having been neglected for many years in favor of infrared spectroscopy and ultraviolet spectroscopy. Advances in the equipment available for Raman spectroscopy, particularly the development of lasers as a source of monochromatic light, have provided much impetus. A review of the field of Raman spectroscopy, including theory, applications, potential, and citations to additional literature is provided by two recent publications: Raman Spectroscopy, Long, McGraw-Hill, 1977 and Chemical Applications of Raman Spectroscopy, Grasselli et al., Wiley and Sons, 1981.

Though Raman spectroscopy is an important research technique and is used for qualitative and quantitative analysis, there has not been available a Raman analyzer, that is, apparatus which provides, rather than a spectrum, an output comprising indication of substances present in a sample and, in the case of a quantitative analyzer, numbers denoting the amounts present of the constituent substances of a sample. There has not been available a routine method of analysis utilizing the Raman effect which provides qualitative or quantitative results which need no further processing or interpretation. Further lacking has been universal Raman effect apparatus and methods; that is, those that can be used for a wide variety of samples without significant change to the apparatus being required when different substances are to be analyzed. There are significant advantages in effecting qualitative and/or quantitative analysis using Raman spectroscopy in place of or in addition to conventional analysis methods.

INFORMATION DISCLOSURE

U.S. Pat. No. 4,068,953 (Harney et al.) deals with methods and apparatus for measuring isotope ratios and isotopic abundances using the Raman effect. Apparatus for remote sensing of gaseous materials is described in U.S. Pat. Nos. 3,820,897 (Roess) and 3,625,613 (Abell) and an improvement to the latter patent is disclosed in U.S. Pat. No. 3,723,007 (Leonard).

U.S. Pat. Nos. 3,414,354 (Siegler) and 3,556,659 (Hawes) describe laser-excited Raman spectrometers. A Raman microprobe for producing micrographic images of certain species in a sample is disclosed in U.S. Pat. No. 4,195,930 (Delhaye et al.). An earlier patent of the same inventors covering similar subject matter is U.S. Pat. No. 4,030,827.

Work accomplished using the Raman effect to analyze hydrocarbons is disclosed in U.S. Pat. Nos. 2,527,121 (Dudenbostel) and 2,527,122 (Heigl et al.) and in an article by Heigl et al. entitled "Determination of Total Olefins and Total Aromatics" which appeared in both Analytical Chemistry (Vol. 21, p. 554, 1949) and Proceedings of the American Petroleum Institute (Vol. 27-28, p. 90, 1948). Another reference covering the same work is the report of a conference held by The Institute of Petroleum in London in 1954 entitled "Molecular Spectroscopy".

Additional U.S. patents dealing with Raman spectroscopy are 4,127,329 (Chang et al.) and 2,940,355 (Cary). U.S. Pat. No. 4,270,864 (Barrett et al.) is representative of several patents to Barrett dealing with photoacoustical Raman spectroscopy.

U.S. Pat. No. 4,397,556 (Muller) claims method and apparatus for quality control in which the Raman effect is used.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide methods and apparatus for qualitative and quantitative analysis of substances in gaseous, liquid, or solid form. More specifically, it is an object of this invention to provide the capability of performing analyses rapidly and through a relatively unskilled technician or without a technician in attendance at all. Related is the object of providing methods and apparatus wherein no human judgment and no human interpretation is necessary to produce analyses. It is also an object of this invention to provide methods and apparatus for rapidly analyzing substances which have heretofore required complex procedures involving much sample handling and/or pretreatment by various procedures. Also, it is an object of this invention to provide an analysis which requires only a small amount of sample and to provide a non-destructive method of analysis such that the sample is not altered or consumed. This object is related to that of providing apparatus which consumes little electrical power and does not heat the sample. It is a further object of this invention to provide a method and apparatus to analyze a substance in situ, that is, without withdrawing the substance from a containing vessel or pipeline. It is a still further object of this invention to provide an analysis apparatus which is modular in design and construction, to facilitate troubleshooting and repair. Another object of this invention is