- Email: [email protected]
Short-Wave Near-Infrared Spectroscopy
13. SHORT-WAVE NEAR-INFRARED SPECTROSCOPY
INTRODUCTION Discussion occurs from time to time regarding the beginning and ending of the near-infrared (NIR) region as well as the precise band positions for vibrational information within the NIR spectral region. This chapter is an attempt to clarify this issue relative to a proposed starting wavelength for the NIR spectral region. The usual designation for the visible region includes 380-780 nm. The current "official" position defines the NIR region extending from 780 nm (12,800 cm -1) to 2,500 nm (4,000 cm-l), as specified by IUPAC and published in Pure and Applied Chemistry 57:105-120, 1985 [1]. It may be more appropriate to extend the NIR region to near 695 nm (14,388 cm-1), and there is historical and experimental precedent for this claim. To give historical background on the reasoning behind this suggestion, a brief review of pertinent literature is presented here as a preface to the Experimental section. Current instrumentation provides high quality measurements of hydrocarbons with measurement pathlengths of 10 cm or more. What is considered the visible spectral region, often designated as a measurement region for electronic transitions, also contains vibrational information. The vibrational information occurs primarily as the fourth overtone for C-H stretching and is described in this chapter. William Hershel is the recognized discoverer of the infrared region. His famous work, "Experiments on the Refrangibility of the Invisible Rays of the Sun," read April 24, 1800, at the Royal Society (Phil. Transact. Roy. Soc. 90:284-292) is described as the original discovery of infrared radiation. Herschel writes, "In that section of my former paper which treats of radiant heat, it was hinted, though from imperfect experiments, that the range of its refrangibility is probably more extensive than that of the prismatic colours; but, having lately had some favourable sunshine, and obtained a sufficient confirmation of the same, it will be proper to add the following experiments to those which have been given" [2]. Other early work comparing the effects of atomic grouping on infrared absorption of organic molecules was completed in 1882 by Captain Abney and Lt.-Col. Festing of the Royal Engineers. The work compared the "atomic groupings" of alcohols, halogens, aldehydes, ethers, nitrate, oxalate, sulphides [sic], nitric, carboxylic acids (and salts thereof), glycerine, benzines, anilines, turpentine, salicylate, oil, anhydride, inorganic acids, ammonia, and water, using an arbitrary empirical wavelength scale to compare band positions, strengths, and overall shapes [3].
DESCRIPTIVE ARTICLES OF HISTORICAL INTEREST O.H. Wheeler [4] describes the near-infrared region as extending "from about 2/~ into the visible at about 0.7/~" in a general discussion article (with 11 references). He also noted that "the
133
SHORT-WAVE NEAR-INFRARED SPECTROSCOPY
term 'near infrared' formerly was used to denote the infrared spectrum to 23/~, and to distinguish this region from that of the far-infrared above 23/1." Wheeler credits Rawlins and Taylor [5] with this early use of the term near-infrared. Using a variety of instrumentation, the author identifies near-infrared bands (in microns) in tabular form as follows. For C-H stretch, the fundamental occurs at 3.5, the 1st overtone at 1.8, the 2nd overtone at 1.2, the 3rd overtone at 0.85, and the 4th overtone at 0.7/1. The author identifies band positions for both O-H and N-H stretch as occurring near 2.8 for the fundamental, 1.4 for the 1st overtone, 0.95 for the 2nd overtone, and 0.7/1 for the 3rd overtone. R. F. Goddu and D. A. Delker [6] provide two detailed tables showing (1) the spectra-structure correlations and average molar absorptivity for a number of functional groups for the NIR region, which they describe as extending from 1.0 to 3.1 /l, and (2) maximum recommended pathlengths for 12 solvents (useful for NIR spectroscopy) over the wavelength region 1.0-3.1/1. The authors cite two references in this useful article.
EARLY REVIEWS Professor J. W. Ellis [7] has reviewed work below 3 microns for absorption of organic liquids. The review cites 44 separate works related to studies made prior to June, 1929. In 1929 Professor Joseph W. Ellis wrote, "The region of the spectrum below 3/1, although representing a relatively small portion of the total infra-red [sic] spectrum, is nevertheless quite significant in the study and interpretation of the absorption spectra of molecules. In particular, the absorption spectra of organic liquid molecules shows numerous bands in this region." He goes on to cite work by Coblentz, and Raman related to the observations of bands in the infrared region due to infrared absorptions and the Raman effect. Ellis goes on to refer to earlier work by Puccianti in 1900 with respect to the presence of specific near-infrared bands associated with "molecules [having] a hydrogen atom combined with a carbon atom." Ellis reports that bands relating to "the carbon-hydrogen linkage" were observed at 2.3-2.2, 1.7, 1.4, 1.2, 1.0, and 0.9/1. With this work, and the work of other investigators, the modern science of near-infrared spectroscopy was under way. W. Kaye [8] provides a summary review of the work in near-infrared spectroscopy from the late 1920s to April 1954. The author draws information from 106 references for this review. The author refers to the term "hydrogenic" stretching vibrations as CH, NH, and OH. Work involving measurements in the region of 0.7-3.5/1 is reviewed as pertaining to this "hydrogenic" stretch region. The author presents a Colthup-type chart of characteristic NIR bands and the accompanying references. R. F. Goddu [9] provides an extensive review of near-infrared spectrophotometry prior to 1960. The author cites 110 literature sources of information for this review. The information within this review is organized into instrumentation (and methods), qualitative analysis, quantitative analysis (for C-H, N-H, O-H, thiols, P-H, carbonyls, nitriles, and miscellaneous groups). Also included in the work are inorganic applications, applications for solids and liquids, and future trends. The work provides a number of tables and spectra describing nearinfrared absorption data. K. B. Whetsel [10] reviews the significant work in near-infrared spectrophotometry prior to 1968. The review contains 336 references covering aspects of theory, instrumentation, and sampling techniques. Work for both inorganic and organic compounds is reviewed. Available pre1968 instrumentation is described in tabular form, as is a list of NIR solvents and optical transparency from 1.0 to 3.0 microns. Other tables and figures illustrate such information as the effect of slit width on peak height for first- and second-overtone and combination N-H bands of aromatic amines, NIR spectra of organic compounds, NIR spectra of rare earth ions, and various band locations and assignments for a number of organic and inorganic compounds.
134
EXPERIMENTAL
SHORT-WAVE NEAR INFRARED AS A SEPARATETOPIC Schrieve et al. [11] discuss applications for the short-wave near-infrared (SW-NIR) region, referring to synonyms such as "the far-visible," the "near, near-infrared," and the "Herschelinfrared" to describe the range of approximately 700 to 1100 nm of the EMS (electromagnetic spectrum). The authors cite the increased interest of this spectral region to spectroscopists, particularly those involved with implementing process near-infrared measurements. The lowercost components (sources and detectors) and the inherent reliability of these components are cited as advantages for process instrumentation. In the work, the authors state the major SWNIR absorbances (in nm) for a variety of solvents: acetone (894, 908, and 1016); 1,1, 1-trichloroethane (898, 1004, 1045); dichloromethane (884, 1024); dimethylformamide (914, 1014); glycerol (924, 1012); methanol (916, 1024); n-hexane (914, 928, 1020); tetrahydrofuran (906, 938, 1036); toluene (876, 910, 1020); and water (976). Note: This introductory material, in part, is repeated in Chapter 15.
EXPERIMENTAL The aromatic C-H stretch fourth-overtone band from pure toluene occurs at approximately 695-725 nm (first-derivative zero crossover at 714.8 nm; peak rise above zero baseline at 694.5 nm); methyl C-H stretch fourth overtone near 745 nm; and methylene C-H stretch fourth overtone near 760 nm (Table 13.1 and Figure 13.1). These spectra closely resemble the second- and third-overtone spectra, with fewer features, as the harmonic increases, most likely attributable to fewer sum tone bands. The aromatic C-H stretch harmonic would clearly put 695 within the NIR region. This finding can be verified easily using a 10.0 cm pathlength with pure hydrocarbons or hydrocarbon mixtures. The measurements for this note were made at room temperature (25~ using an FT-NIR instrument, 128 coadded scans per spectrum at 4-cm -1 resolution (Perkin-Elmer Model 2000 FT-IR spectrophotometer). The third- and fourth-overtone measurements were taken using a 10.0 cm pathlength cell with the FT-NIR instrument. The first- and second-overtone measurements can be made using 2.0 cm and 1.0 mm-pathlength cells, respectively. The increased use of high-performance spectrophotometers and descriptions of the existence of C-H and O-H overtones near 0.7 microns (by O.W. Wheeler, W. Kaye, and G.D. Schrieve et al., and as clearly shown in this present note) are indications that an amendment moving the lower starting-wavelength region to 695 for NIR is arguably valid.
Table 13.1 Fourth-OvertoneC-H Stretch Peak Positions forThree Hydrocarbons Hydrocarbon
Aromatic C-H
Methyl C-H
Methylene C-H
714.8 n m
744.5 n m
N/A
2, 2,4-Dimethyl pentane
N/A
748.1 n m
766.0 n m
n-Decane
N/A
750.0 n m
761.4 n m
Toluene
Measured Using a 10 cm Pathlength Liquid Cell.
135
SHORT-WAVE NEAR-INFRARED SPECTROSCOPY
.13-
.12-
.11
I
~-
i
i i
.1-
I
T
!
700 720 Absorbance / Nanometers
"
i
740
=
T
i
I
i
760
780
800
820
Fig. 13.1 The Fourth-overtone spectrum of pure toluene showing aromatic C-H stretch at 714.5 nm, methyl C-H stretch near 744.5 nm, and a combination band near 810 nm (10 cm pathlength cell).
ACKNOWLEDGEMENT The author acknowledges [email protected], the near-infrared users group, for their stimulating discussions and for raising this issue.
REFERENCES 1. [email protected]. Communication with Tony Davies (May 25, 1999). 2. William Hershel. Experiments on the refrangibility of the invisible rays of the sun. Phil. Transact. Roy. Soc. 90:284-292, 1800. 3. Capt.W. de W. Abney, Lt.-Col. Festing reported. On the influence of the atomic grouping in the molecules of organic bodies on their absorption in the infra-red region of the spectrum. Phil. Transact. 172:887-918, 1882. 4. O. H. Wheeler. Near infrared spectra. A neglected field of spectral study. J. Chem. Education 37:234-236, 1960. 5. F. I. G. Rawlins, A. M. Taylor. Infrared Analysis and Molecular Structure. Cambridge, U.K.: Cambridge University Press, 1929. 6. R.F. Goddu, D. A. Delker. Spectra-structure correlations for the near-infrared region. Anal. Chem. 32:140-141, 1960. 7. J.W. Ellis. Molecular absorption spectra of liquids below 3 IX. Trans. FaradaySoc. 25:888-898, 1928. 8. W. Kaye. Near-infrared spectroscopy: A review. I. Spectral identification and analytical applications. Spectrochimica Acta 6:257-287, 1954. 9. R.F. Goddu. Near-infrared spectrophotometry. Advan. Anal. Chem. Instr. 1:347-424, 1960. 10. K.B. Whetsel. Near-infrared spectrophotometry. Appl. Spectrosc. Rev. 2(1 ): 1-67, 1968. 11. G.D. Schrieve, G. G. Melish, A. H. UIIman. The Herschel-infrared--A useful part of the spectrum. Appl. Spectrosc. 45:711-714, 1991.
136
INTRODUCTION Discussion occurs from time to time regarding the beginning and ending of the near-infrared (NIR) region as well as the precise band positions for vibrational information within the NIR spectral region. This chapter is an attempt to clarify this issue relative to a proposed starting wavelength for the NIR spectral region. The usual designation for the visible region includes 380-780 nm. The current "official" position defines the NIR region extending from 780 nm (12,800 cm -1) to 2,500 nm (4,000 cm-l), as specified by IUPAC and published in Pure and Applied Chemistry 57:105-120, 1985 [1]. It may be more appropriate to extend the NIR region to near 695 nm (14,388 cm-1), and there is historical and experimental precedent for this claim. To give historical background on the reasoning behind this suggestion, a brief review of pertinent literature is presented here as a preface to the Experimental section. Current instrumentation provides high quality measurements of hydrocarbons with measurement pathlengths of 10 cm or more. What is considered the visible spectral region, often designated as a measurement region for electronic transitions, also contains vibrational information. The vibrational information occurs primarily as the fourth overtone for C-H stretching and is described in this chapter. William Hershel is the recognized discoverer of the infrared region. His famous work, "Experiments on the Refrangibility of the Invisible Rays of the Sun," read April 24, 1800, at the Royal Society (Phil. Transact. Roy. Soc. 90:284-292) is described as the original discovery of infrared radiation. Herschel writes, "In that section of my former paper which treats of radiant heat, it was hinted, though from imperfect experiments, that the range of its refrangibility is probably more extensive than that of the prismatic colours; but, having lately had some favourable sunshine, and obtained a sufficient confirmation of the same, it will be proper to add the following experiments to those which have been given" [2]. Other early work comparing the effects of atomic grouping on infrared absorption of organic molecules was completed in 1882 by Captain Abney and Lt.-Col. Festing of the Royal Engineers. The work compared the "atomic groupings" of alcohols, halogens, aldehydes, ethers, nitrate, oxalate, sulphides [sic], nitric, carboxylic acids (and salts thereof), glycerine, benzines, anilines, turpentine, salicylate, oil, anhydride, inorganic acids, ammonia, and water, using an arbitrary empirical wavelength scale to compare band positions, strengths, and overall shapes [3].
DESCRIPTIVE ARTICLES OF HISTORICAL INTEREST O.H. Wheeler [4] describes the near-infrared region as extending "from about 2/~ into the visible at about 0.7/~" in a general discussion article (with 11 references). He also noted that "the
133
SHORT-WAVE NEAR-INFRARED SPECTROSCOPY
term 'near infrared' formerly was used to denote the infrared spectrum to 23/~, and to distinguish this region from that of the far-infrared above 23/1." Wheeler credits Rawlins and Taylor [5] with this early use of the term near-infrared. Using a variety of instrumentation, the author identifies near-infrared bands (in microns) in tabular form as follows. For C-H stretch, the fundamental occurs at 3.5, the 1st overtone at 1.8, the 2nd overtone at 1.2, the 3rd overtone at 0.85, and the 4th overtone at 0.7/1. The author identifies band positions for both O-H and N-H stretch as occurring near 2.8 for the fundamental, 1.4 for the 1st overtone, 0.95 for the 2nd overtone, and 0.7/1 for the 3rd overtone. R. F. Goddu and D. A. Delker [6] provide two detailed tables showing (1) the spectra-structure correlations and average molar absorptivity for a number of functional groups for the NIR region, which they describe as extending from 1.0 to 3.1 /l, and (2) maximum recommended pathlengths for 12 solvents (useful for NIR spectroscopy) over the wavelength region 1.0-3.1/1. The authors cite two references in this useful article.
EARLY REVIEWS Professor J. W. Ellis [7] has reviewed work below 3 microns for absorption of organic liquids. The review cites 44 separate works related to studies made prior to June, 1929. In 1929 Professor Joseph W. Ellis wrote, "The region of the spectrum below 3/1, although representing a relatively small portion of the total infra-red [sic] spectrum, is nevertheless quite significant in the study and interpretation of the absorption spectra of molecules. In particular, the absorption spectra of organic liquid molecules shows numerous bands in this region." He goes on to cite work by Coblentz, and Raman related to the observations of bands in the infrared region due to infrared absorptions and the Raman effect. Ellis goes on to refer to earlier work by Puccianti in 1900 with respect to the presence of specific near-infrared bands associated with "molecules [having] a hydrogen atom combined with a carbon atom." Ellis reports that bands relating to "the carbon-hydrogen linkage" were observed at 2.3-2.2, 1.7, 1.4, 1.2, 1.0, and 0.9/1. With this work, and the work of other investigators, the modern science of near-infrared spectroscopy was under way. W. Kaye [8] provides a summary review of the work in near-infrared spectroscopy from the late 1920s to April 1954. The author draws information from 106 references for this review. The author refers to the term "hydrogenic" stretching vibrations as CH, NH, and OH. Work involving measurements in the region of 0.7-3.5/1 is reviewed as pertaining to this "hydrogenic" stretch region. The author presents a Colthup-type chart of characteristic NIR bands and the accompanying references. R. F. Goddu [9] provides an extensive review of near-infrared spectrophotometry prior to 1960. The author cites 110 literature sources of information for this review. The information within this review is organized into instrumentation (and methods), qualitative analysis, quantitative analysis (for C-H, N-H, O-H, thiols, P-H, carbonyls, nitriles, and miscellaneous groups). Also included in the work are inorganic applications, applications for solids and liquids, and future trends. The work provides a number of tables and spectra describing nearinfrared absorption data. K. B. Whetsel [10] reviews the significant work in near-infrared spectrophotometry prior to 1968. The review contains 336 references covering aspects of theory, instrumentation, and sampling techniques. Work for both inorganic and organic compounds is reviewed. Available pre1968 instrumentation is described in tabular form, as is a list of NIR solvents and optical transparency from 1.0 to 3.0 microns. Other tables and figures illustrate such information as the effect of slit width on peak height for first- and second-overtone and combination N-H bands of aromatic amines, NIR spectra of organic compounds, NIR spectra of rare earth ions, and various band locations and assignments for a number of organic and inorganic compounds.
134
EXPERIMENTAL
SHORT-WAVE NEAR INFRARED AS A SEPARATETOPIC Schrieve et al. [11] discuss applications for the short-wave near-infrared (SW-NIR) region, referring to synonyms such as "the far-visible," the "near, near-infrared," and the "Herschelinfrared" to describe the range of approximately 700 to 1100 nm of the EMS (electromagnetic spectrum). The authors cite the increased interest of this spectral region to spectroscopists, particularly those involved with implementing process near-infrared measurements. The lowercost components (sources and detectors) and the inherent reliability of these components are cited as advantages for process instrumentation. In the work, the authors state the major SWNIR absorbances (in nm) for a variety of solvents: acetone (894, 908, and 1016); 1,1, 1-trichloroethane (898, 1004, 1045); dichloromethane (884, 1024); dimethylformamide (914, 1014); glycerol (924, 1012); methanol (916, 1024); n-hexane (914, 928, 1020); tetrahydrofuran (906, 938, 1036); toluene (876, 910, 1020); and water (976). Note: This introductory material, in part, is repeated in Chapter 15.
EXPERIMENTAL The aromatic C-H stretch fourth-overtone band from pure toluene occurs at approximately 695-725 nm (first-derivative zero crossover at 714.8 nm; peak rise above zero baseline at 694.5 nm); methyl C-H stretch fourth overtone near 745 nm; and methylene C-H stretch fourth overtone near 760 nm (Table 13.1 and Figure 13.1). These spectra closely resemble the second- and third-overtone spectra, with fewer features, as the harmonic increases, most likely attributable to fewer sum tone bands. The aromatic C-H stretch harmonic would clearly put 695 within the NIR region. This finding can be verified easily using a 10.0 cm pathlength with pure hydrocarbons or hydrocarbon mixtures. The measurements for this note were made at room temperature (25~ using an FT-NIR instrument, 128 coadded scans per spectrum at 4-cm -1 resolution (Perkin-Elmer Model 2000 FT-IR spectrophotometer). The third- and fourth-overtone measurements were taken using a 10.0 cm pathlength cell with the FT-NIR instrument. The first- and second-overtone measurements can be made using 2.0 cm and 1.0 mm-pathlength cells, respectively. The increased use of high-performance spectrophotometers and descriptions of the existence of C-H and O-H overtones near 0.7 microns (by O.W. Wheeler, W. Kaye, and G.D. Schrieve et al., and as clearly shown in this present note) are indications that an amendment moving the lower starting-wavelength region to 695 for NIR is arguably valid.
Table 13.1 Fourth-OvertoneC-H Stretch Peak Positions forThree Hydrocarbons Hydrocarbon
Aromatic C-H
Methyl C-H
Methylene C-H
714.8 n m
744.5 n m
N/A
2, 2,4-Dimethyl pentane
N/A
748.1 n m
766.0 n m
n-Decane
N/A
750.0 n m
761.4 n m
Toluene
Measured Using a 10 cm Pathlength Liquid Cell.
135
SHORT-WAVE NEAR-INFRARED SPECTROSCOPY
.13-
.12-
.11
I
~-
i
i i
.1-
I
T
!
700 720 Absorbance / Nanometers
"
i
740
=
T
i
I
i
760
780
800
820
Fig. 13.1 The Fourth-overtone spectrum of pure toluene showing aromatic C-H stretch at 714.5 nm, methyl C-H stretch near 744.5 nm, and a combination band near 810 nm (10 cm pathlength cell).
ACKNOWLEDGEMENT The author acknowledges [email protected], the near-infrared users group, for their stimulating discussions and for raising this issue.
REFERENCES 1. [email protected]. Communication with Tony Davies (May 25, 1999). 2. William Hershel. Experiments on the refrangibility of the invisible rays of the sun. Phil. Transact. Roy. Soc. 90:284-292, 1800. 3. Capt.W. de W. Abney, Lt.-Col. Festing reported. On the influence of the atomic grouping in the molecules of organic bodies on their absorption in the infra-red region of the spectrum. Phil. Transact. 172:887-918, 1882. 4. O. H. Wheeler. Near infrared spectra. A neglected field of spectral study. J. Chem. Education 37:234-236, 1960. 5. F. I. G. Rawlins, A. M. Taylor. Infrared Analysis and Molecular Structure. Cambridge, U.K.: Cambridge University Press, 1929. 6. R.F. Goddu, D. A. Delker. Spectra-structure correlations for the near-infrared region. Anal. Chem. 32:140-141, 1960. 7. J.W. Ellis. Molecular absorption spectra of liquids below 3 IX. Trans. FaradaySoc. 25:888-898, 1928. 8. W. Kaye. Near-infrared spectroscopy: A review. I. Spectral identification and analytical applications. Spectrochimica Acta 6:257-287, 1954. 9. R.F. Goddu. Near-infrared spectrophotometry. Advan. Anal. Chem. Instr. 1:347-424, 1960. 10. K.B. Whetsel. Near-infrared spectrophotometry. Appl. Spectrosc. Rev. 2(1 ): 1-67, 1968. 11. G.D. Schrieve, G. G. Melish, A. H. UIIman. The Herschel-infrared--A useful part of the spectrum. Appl. Spectrosc. 45:711-714, 1991.
136