- Email: [email protected]
Industrial applications of vibrational spectroscopy
trendsinanalyticalchemistry, vol.9, no. 3,1990
form of quality control. More rigorous quality control can be attained by including a sample of dilute acid in each batch. General Better pH measurements undoubtedly result if there is some understanding of how electrodes work (see Westcott2’ for a simple account and Bates17 for a more comprehensive treatment). Some manufacturers’ guides to pH measurement are excellent and recommended procedures for using individual electrodes should be followed. Electrodes should not be expected to last forever; normally they will give good service for one or two years, depending on use. Those that are used regularly (daily) normally perform and last better than those used intermittently. It is advisable to have two electrodes in continuous use to allow cross checks and as a guard against sudden failure or damage. References 1 2 3 4 5
R. C. Metcalf, Analyst, 112 (1987) 1573. W. Davison and T. R. Harbinson, Analyst, 113 (1988) 709. W. Davison and C. Woof, Anal. Chem., 57 (1985) 2567. D. Midgley, Atmos. Environ., 21(1987) 173. G. Marinenko, R. C. Paule, W. F. Koch and M. Knoerdal, J. Res. Natl. Bur. Stand., 91 (1986) 17. 6 D. P. Brezinski, Analyst, 108 (1983) 425.
83
7 T. R. Harbinson and W. Davison, Anal. Chem., 59 (1987) 2450. 8 A. K. Covington, P. D. Whalley and W. Davison, Anal. Chim. Actu, 169 (1985) 221. 9 W. Davison and T. R. Harbinson, Analyst, 113 (1988) 1537. 10 A. K. Covington, R. G. Bates and R. A. Durst, Pure Applied Chem., 57 (1985) 33. 11 A. K. Covington, P. D. Whalley and W. Davison, Pure Applied Chem., 57 (1985) 877. 12 A. K. Covington, P. D. Whalley, W. Davison and M. Whitfield. In T. S. West and H. W. Nurnberg (Editors), The Determination of Trace Metals in Natural Waters, Blackwell, Oxford, 1988, p. 161. 13 C. Jones, D. W. Williams and F. Marsicano, Sci. Total Environ., 64 (1987) 211. 14 W. F. Koch, G. Marinenko and R. C. Paule, J. Res. Natl. Bur. Stand., 91(1986) 23. 15 W. F. Koch, G. Marinenko and R. C. Paule, J. Res. Natl. Bur. Stand., 91(1986) 33. 16 W. Davison, C. Woof and E. Tipping, Analyst, 114 (1989) 587. 17 R. G. Bates, Determination ofpH, Wiley, New York, 1964. 18 A. K. Covington, P. D. Whalley and W. Davison, Analyst, 108 (1983) 1528. 19 N. R. McQuaker, P. D. Kluckner and D. K. Sandberg, Environ. Sci. Technol., 17 (1983) 431. 20 C. C. Westcott, pH Measurement, Academic Press, New York, 1978. Dr. Davison is at the Institute of Freshwater Ecology, Windermere Laboratory, The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 OLP, U.K.
observer
Industrial applications of vibrational spectroscopy Kathryn S. Kalasinsky Mississippi
State, MS, U.S.A.
Vibrational spectroscopy has played an ever increasing role in the analytical laboratory. As instrumentation and sampling accessories evolve, the applications to industrial an&ses broaden. Reported here is a brief discussion of the field of vibrational spectroscopy and its applications as reported in the literature as well as several specify sample analyses.
Analytical applications of vibrational spectroscopy have made their biggest impression on society through art work validation’ and forensic science2- . Although these represent only a fraction of the applications, they demonstrate the rapidly broadening 0165-9936/90/$03.00.
field of analyses suited for vibrational spectroscopy. Microscopy has played an important role in expanding the field of vibrational techniques into microspectroscop$-8. The ability to analyze a single fiber or minute paint chip, combined with the specificity and non-destructive nature of vibrational techniques, has made applications such as those mentioned above possible. Industrial concerns that need to closely monitor roducts such as plasticizers’ and P have found vibrational specpharmaceuticals lop1 troscopy to provide specific data rapidly. Biomedical research has also found the utility of vibrational spectroscopy12-14; the biggest problems of this field are not experimental design but interpretation of all the available data. @Elsevier
Science Publishers B.V.
trendsin analyticalchemistry,vol. 9, no. 3.1990
84
Analytical services laboratories that employ vibrational spectroscopy not only have the choice of numerous sample handling techniques but also the choice of sample phase: gas, liquid or solid. These decisions are often more difficult and critical than the actual data interpretation. An example of such decision processes is demonstrated in a ‘shoe box’ sample that was submitted to the Industrial and Agricultural Services Division of a state government laboratory. A shoe store had received a shipment of shoes from a public carrier and the shoe boxes apparently got wet during shipment. After stocking the shoes several of the store employees became ill and had to leave the premises. A sample of one of the shoe boxes was placed in an air tight container and sent to the analytical services laboratory for analysis. Upon receipt of the sample, a chemist opened the container and noted a strong, unidentifiable odor. A portion of the sample was placed in a sample bulb, attached to a vacuum line and freeze-thawed to remove the non-condensable gases. The sample was then frozen out in liquid nitrogen and allowed to begin to warm up slowly. The first volatile gases from the sample were then trapped in an infrared gas cell and analyzed. The resultant spectrum is shown in Fig. 1. Obviously there is atmospheric water and carbon dioxide with an identifiable methanol band at about 1036 cm-‘. In the C-H stretch region the methanol bands are identified and there are also some other unidentified C-H stretches. A portion of liquid from the sample was then condensed into a cold finger on the infrared gas cell and an infrared spectrum was obtained of the equilibrium mixture of gases above the liquid. Fig. 2 shows the spectrum with the methanol peak diminished in size and no
VOLFlTILES
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Fig. I. Infrared spectrum of volatile gases from shoe box sample.
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Fig. 2. Infrared spectrum of equilibrium mixture of gases over liquid from shoe box sample.
other identifiable peaks were observed. It was then noticed that a corrosion had formed on the infrared windows of the gas cell. The sample was pumped out and the final conclusive spectrum was obtained as shown in Fig. 3. The atmospheric adsorptions appeared as negative peaks and a formaldehyde peak at about 1642 cm-’ was clearly visible. All the rest of the C-H stretches were accounted for from the formaldehyde frequencies. The formaldehyde and methanol were existing in an equilibrium mixture in the ‘wet’ shoe boxes. Further checking with the shippers finally revealed that biological samples had been contained in the shipment along with the shoes and one container had broken, leaking formaldehyde on the other packages. Vibrational spectroscopy, especially infrared, has been dubbed as a super sleuth at solving mysteries. Numerous articles15- 7 have described how infrared spectroscopy has solved perplexing problems, or been the conclusive piece of evidence to identify an unknown. Infrared spectrometers have evolved as optional detectors to be used in conjunction with other analytical techniques153’8. Environmental studies are particularly suited for these hyphenated infrared methods. Raman spectroscopy has also evolved as a tool for environmental chemistry1g-21, although to a lesser extent than infrared spectroscopy. Raman spectroscopy has been viewed in the past as an exotic technique22, but with the recent development of Fourier transform Raman23-25, it ought to take its place beside infrared as a complementary tool. Analytical chemistry in the laboratory has certainly felt the presence of vibrational spec-
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B
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Fig. 3. Infrared spectrum of residual gases from shoe box sample.
troscopf6. Quality assurance and quality control have been dominated by infrared techniques2’-2g. Method development and validation3’ for process control is unambiguous. Recently, near-infrared spectroscop*i has seen a resurgence in this area. With the development of mini-computers there was a rise in the use of Fourier transform instrumentation in the scientific field. There are several inherent advantages of Fourier transform infrared (FTIR) spectroscop~2~33, and it is due to these advantages that many sampling accessories evolved. Today, the greatest limitations in data acquisition are typically those dictated by the sample, not experimental design 34. One of the most widely used sampling accessories for infrared spectroscopy is internal reflection3’. This is probably not evident from the literature, but most industrial and analytical services samples are easily handled in this manner. An example of a problem that was quickly solved by attenuated total reflectance (ATR) is shown in Fig. 4. A state had purchased its motor carrier decals from a low bidder and after one month of use the decals were failing. The ATR spectrum of the bid product, a polymer known as Fascal900, is shown as the top trace in Fig. 4. The spectrum of the motor carrier decals shown in the middle is obviously a different material, and it is obvious from the bottom trace that the decals were made of a polymer, Fascal 55OS, whose specifications were known to not meet those required for the bid. Internal reflectance cells for liquid flow systems have recently been developed and have become popular for process stream monitorings697. External reflectance accessories have become extremely effective in measuring thin films on smooth metal sur-
faces for quality control in the computer disk industr38. Diffuse reflectance accessories are extremely popular for infrared experiments and their applications are still growir$‘. One of the most important accessories attributed to the growth of FT-IR is the gas chromatography FT-IR (GCYFT-IR). Much development work has gone into these systems40-43 to make this technique competitive with GC-mass spectroscopy. Fiber optic in situ measurement systems are the latest accessory development for FTIR? Infrared spectroscopy has long been noted for its ‘group frequencies’ and ‘fingerprint region’ for identification45. The ‘fingerprint-ability’ of infrared spectra made routine spectral matching via computer analysis an obvious development46 and a powerful tool. This requires the availability of massive reference databases4’, produced with a certain degree of quality48 and standardization4’. Although much work has been done in these areas, it still lags behind the demand and appears to be a ‘never ending’ problem. Work has been done in developing computer analysis programs using group frequencies50p51 which avoid the problem of a lack of databases. Pattern recognition computer routines have been developed for NMR and mass spectroscopy52-54 and will probably be the next area of development for identification in infrared spectroscopy.
Fig. 4. Attenuated total reflectance infrared spectra of the bid product for a motor carrier decal, the actual decal and the identified material of the decal.
trendsin analyticalchemistry,vol. 9, no. 3,199O
surements can be represented lar to Beer’s Law,
by a relationship
simi-
I= J[X]
Fig. 5. Infrared spectra of a contaminated and normal air sample.
Quantitative measurements by infrared spectroscopy are based on the Lambert-Beer law, A = abc where A is the absorbance, a is the absorptivity, b is the pathlength, and c is the concentration. Computerization of infrared spectroscopy saw a boom in the quantitative applications due to the increased accuracy of measurements and computational powers5. Techniques such as spectral stripping, curve-fitting, and factor analysis evolved56. Extensive development on the area of multicomponent analysis using matrix representations and partial least squares solutions57-64 has helped to repopularize quantitative infrared spectroscopy. Quantitative routines have even been developed to handle special samples65 or sampling apparatus66’67. Quantitative Raman mea-
where Z is the Raman intensity, J is the proportionality constant, and [X] is the concentration of compound Xa. Applications have been shown in several areas69,70. An example of the importance of quantitative measurements can be seen in the spectra shown in Figs. 5 and 6. The office personnel of a manufacturing company began blacking-out at their desks. A sample of the air was obtained in a long-path infrared gas cell. The spectrum of the air from the manufacturing company shown in the top portion of Fig. 5 is compared with the spectrum of normal air on the bottom portion. A difference can be seen at about 2145 cm-i, where carbon monoxide absorbs. A blow-up of the CO region with both spectra overlaid is shown in Fig. 6. It can be seen that the sample is about ten times stronger in carbon monoxide concentration than normal air. Further inspection at the manufacturing company revealed a crack in the gas heater for the building housing the employees. Surface analysis in analytical chemistry is typically associated with electron spectroscopy such as Auger, but vibrational spectroscopy has developed as a prime tool for surface and depth profiling71-73. Surface residues can be studied by diffuse reflectance infrared spectroscopy. Coatings and multilayer polymers can be studied by internal reflection spectroscopy. Nanosamplers have been developed for fiber and dust analysis74. Electrode surface reactions have also been studied by infrared spectroscopy75. Of these techniques diffuse reflectance has probably emerged as the most popular, and much attention has been given to it in the literature76-80. Particular attention needs to be paid to sampling and experimental design when uantitation of diffuse reflectance data is the gaolB 1-83.An example of coating analysis by ATR is shown in the spectra of Figs. 7 and 8. A manufacturer of a vinyl material for office furniture had a customer complaint that the vinyl did not hold up to wear and soiled readily. The manufacturer suggested that the cleaning personnel had cleaned the vinyl coated furniture with a product containing isopropanol, which would remove the protective coating. For the analysis, a spectrum of each of the following was obtained: standard lot sample from the manufacturer; standard lot. sample after treatment with isopropanol; portion of customer’s sample from clean area under furniture where cleaning personnel did not have access with cleaning products; and soiled portion of customer’s sample
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Fig. 6. Infrared spectra of carbon monoxide region of contaminated and normal air sample overlaid.
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Fig. 7. Attenuated total reflectance infrared spectra of vinyl uphoktery material withoutprotective coating.
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Fig. 8. Attenuated total reflectance infrared spectra of vinyl upholstery material with protective coating.
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from area of access to cleaning personnel (soiled portion was cleaned with soap and water as recommended by manufacturer before analysis). These spectra, shown in Fig. 7, are all similar with the exception of the standard lot sample, indicating that the protective coating was not placed on the vinyl before upholstering to the customer’s furniture. The same manufacturer had a similar complaint from another customer where the color had faded out of the vinyl upholstered furniture. In this case spectra of the following samples were obtained: standard lot sample from manufacturer; dark portion retaining original color of customer’s sample; and light portion of faded area of customer’s sample. These spectra, shown in Fig. 8, indicated that the coating was originally on the customer’s sample but had been removed by cleaning products in the faded portion. Conclusions The analytical chemical laboratory has certainly found a place for vibrational spectroscopy. Identification, quality control, and quantitative analysis are but a few of the areas where vibrational spectroscopy has excelled as an analytical tool. Data processing capability which has allowed ‘turnkey’ operation, mathematical manipulation of data and greater spectral sensitivity through signal averaging, has probably been instrumental in enhancing the utilization of vibrational spectroscopy in analytical laboratoriess4. Hyphenated techniques are continuing to evolve. More analyses are being interfaced together for quicker, more complete results with less sample preparation. Automatic data interpretation is also evolving with these newer techniques but not as rapidly as the need requires. This is the biggest area of development in the near future for vibrational spectroscopy. Quantitative analyses are becoming more complex as the sampling apparatuses are able to handle a wider variety of samples with various environmental matrix effects. Developments in these areas have also grown out of the data processing capabilities inherent in the instruments themselves. Method development, though enhanced by the state-of-the-art of analytical equipment, has sometimes become more tedious and confusing as more variables need to be defined in each experiment. However, these same variables allow for more precise and accurate measurements to be devised.
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3 K. Kempfert, Appl. Spectrosc., 42 (1988) 845. 4 M. D. Weiss, Znd. Chem., 9 (2) (1988) 28. 5 J. A. Reffner, J. P. Coates and R. D. Messerschmidt, Am. Lab. (Fairfield, Conn.), 19 (4) (1987) 86. 6 M. Abramowitz, Am. Lab. (Fairfield, Conn.), 19 (4) (1987) 113. 7 P. L. Lang, J. E. Katon and A. S. Bonanno, Applied Spectrosc., 42 (1988) 313. 8 F. Adar, M. LeClecq and R. E. Grayzel, Am. Lab. (Fairfield, Conn.), 14 (3) (1982) 59. 9 J. J. Gibbons, Am. Lab. (Fairfield, Conn.), 19 (11) (1987) 78. 10 J. L. Hopkins, K. A. Cohen, F. W. Hatch, T. P. Pitner, J. M. Stevenson and F. K. Hess, Anal. Chem., 59 (1987) 784A. 11 C. A. Chess and D. J. Gerson, Spectroscopy, l(6) (1986) 46.
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S. Winters, R. I. Leininger, D. Fink, C. R. Jakobsen, Appl. Spectrosc., 35 (1981) 353. Trends Anal. Chem., 5 (3) (1986) 68.
H. Clark en R. E. Hester, (Editors), AdVol. 11, Wiley Heyden, New York, 1984, Ch. 1. 15 D. J. Brown, L. F. Schneider and J. A. Howell, Anal. vances in Infrared and Raman Spectroscopy,
Chem., 60 (1988) 1005A.
16 R. A. Gilhooley, C. P. Richards, J. Warren and K. S. Well, Anal. Chem., 55 (1983) 639A.
T. R. Pofahl and W. R. Croasmun, Anal. Chem., 59 (1987) 1109A. 18 C. A. Cody, L. DiCarlo and B. K. Faulseit, Am. Lab. (Fairfield, Conn.), 13 (1) (1981) 93. 19 A. Campion and W. H. Woodruff, Anal. Chem., 59 (1987) 1299A. 20 M. M. Carrabba, R. B. Edmonds and R. D. Rauh, Anal. Chem., 59 (1987) 2559. 21 A. M. Ahern and R. L. Garrell, Anal. Chem., 58 (1987) 2813. 22 C. Zimba, Spectroscopy, 3 (8) (1988) 8. 23 T. Hirschfeld and B . Chase, Appl. Spectrosc., 40 (1986) 133. 24 H. Buijs, Spectroscopy, l(8) (1986) 14. 25 B. Chase, Anal. Chem., 59 (1987) 881A. 26 M. A. Kaiser and A. H. Ulman, Anal. Chem., 60 (1988) 823A. 27 J. P. Coates, J. M. D’Agostino and C. R. Friedman, Am. Lab. (Fairfield, Conn.), 18 (11) (1986) 82. 28 J. P. Coates, J. M. D’Agostino and C. R. Friedman, Am. Lab. (Fairfield, Conn.), l% (12) (1986) 40. 29 H. Kaplan, Photonics, 22 (3) (1988) 76. 30 J. K. Taylor, Anal. Chem., 55 (1983) 600A. 31 B. Buchanan and D. Honings, Trends Anal. Chem., 5 (6) 17 R. J. McGorrin,
(1986) 154. 32 J. C. Case and W. G. Fately, in J. R. Durig (Editor), Analytical Applications of FR-IR to Molecular and Biological Systems, Reidel, Dordrecht, 1980, Ch. 1. 33 T. Hirschfeld, Appl. Spectrosc., 39 (1985) 1086. 34 R. G. Messerschmidt, Spectroscopy, 1 (2) (1986) 16. 35 C. M. Paralusz, ASTM Standardization News, February
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39 D. E. Leyden and R. S. S. Murthy, Trends Anal. Chem., 7 (5) (1988) 164. 40 G. N. Giss and C. L. Wilkins, Appl. Spectrosc., 38 (1984) 17. 41 P. W. J. Yang, E. L. Ethridge, J. L. Lane andP. R. Griffiths,
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Appl. Spectrosc., 38 (1984) 813. 42 P: *W. J. Yang.and‘P. R. Griffiths, Appl. Spectrosc., 38 (1984) 816. 43 D. E. Henry, A. Giorgetti, A. M. Haefner, P. R. Griffiths and D. F. Gurka, Anal. Chem., 59 (1987) 2356. 44 D. C. Compton, S. L. Hill, N. A. Wright, M. A. Drury, J. Piche, W. A. Stevenson and D. W. Vidrine, Appl. Spectrosc., 42 (1988) 972. 45 A. L. Smith, Appl. Spectrosc., 41(1987) 1101. 46 R. E. Fields, III and R. L. White, Anal. Chem., 59 (1987) 2709. 47 C. L. Wilkins and P. R. Griffiths, Appl. Spectrosc., 42 (1988) 537. 48 P. R. Griffiths and C. L. Wilkins, Appl. Spectrosc., 42 (1988) 538. 49 R. S. McDonald and P. A. Wilks, Jr., Appl. Spectrosc., 42 (1988) 151. 50 H. B. Woodruff and G. M. Smith, Anal. Chem., 52 (1980) 2321. 51 L. S. Ying, S. P. Levine, S. A. Tomellini and S. R. Lowry, Anal. Chem., 59 (1987) 2197. 52 G. L. Ritter, S. R. Lowry, C. L. Wilkins and T. L. Isenhour, Anal. Chem., 47 (1975) 1951. 53 C. L. Wilkins and T. L. Isenhour, Anal. Chem., 47 (1975) 1849. 54 T. F. Lam, C. L. Wilins, T. R. Brunner, L. J. Sol&berg and S. L. Kaberline, Anal. Chem., 48 (1976) 1768. 55 J. F. Kacoyannakis, J. K. Allen and M. L. Parsons, Znd. Chem., 4 (2) (1983) 32. 56 J. L. Koenig, Anal. Chem., 59 (1987) 1141A. 57 D. M. Haaland and R. G. Easterling, Appl. Spectrosc., 34 (1980) 539. 58 C. W. Brown, P. F. Lynch, R. J. Obremski and D. S. Lavery, Anal. Chem., 54 (1982) 1472. 59 H. J. Kisner, C. W. Brown and G. J. Kavamos, Anal. Chem., 54 (1982) 1479. 60 I. E. Frank, J. H. Kalivas and B. K. Kowalski, Anal. Chem., 55 (1983) 1800. 61 M. A. Maris, C. W. Brown and D. S. Lavery, Anal. Chem., 55 (1983) 1694. 62 C. W. Brown, Spectroscopy, l(4) (1986) 32.
63 M. P. Fuller, G. L. Ritter and C. S. Draper, Appl. Spectrosc., 42 (1988) 217. 64 M. P. Fuller, G. L. Ritter and C. S. Draper, Appl. Spectrosc., 42 (1988) 228. 65 A. Hanning, Appl. Spectrosc., 42 (1988) 90. 66 K. J. Hartauer, L. E. Matheson and J. K. Guillory, Appl. Spectrosc., 42 (1988) 699. 67 K. C. Cole, A. Pilon, D. Noel, J. J. Hechler, A. Chouliotis and K. C. Overbury, Appl. Spectrosc., 42 (1988) 761. 68 R. S. Tobias, .Z. Chem. Educ., 44 (1967) 70. 69 D. P. Strommen and K. Nakamoto, Am. Lab. (Fairfield, Conn.), 13 (10) (1981) 70. 70 R. C. Lord, Appl. Spectrosc., 31(1977) 187. 71 S. C. Pattacini and T. J. Porro, Am. Lab. (Shelton, Conn.), 20 (8) (1988) 24. 72 F. M. Mirabella, Jr., Spectroscopy, l(0) (1986) 49. 73 P. R. Griffiths, in J. R. Durig (Editor), Chemical, Biological and Industrial Applications of Infrared Spectroscopy, Wiley, Chichester, 1985, p. 253. 74 F. J. DeBlase, Am. Lab. (Shelton, Conn.), 20 (6) (1988) 96. 75 D. S. Corrigan, L. H. Leung and M. J. Weaver, Anal. Chem., 59 (1987) 2252. 76 M. P. Fuller and P. R. Griffiths, Anal. Chem., 50 (1978) 1906. 77 M. P. Fuller and P. R. Giffiths, Appl. Spectrosc., 34 (1980) 533. 78 I. M. Hamadeh, S. A. Yeboah, K. A. Trumbull and P. R. Griffiths, Appl. Spectrosc., 38 (1984) 486. 79 P. J. Brimmer and P. R. Griffiths, Anal. Chem., 58 (1986) 2179. 80 E. H. Korte and A. Otto, Appl. Spectrosc., 42 (1988) 38. 81 P. J. Brimmer, P. R. Griffiths and N. J. Harrick, Appl. Spectrosc., 40 (1986) 258. 82 P. J. Brimmer and P. R. Griffiths, Appl. Spectrosc., 41 (1987) 791. 83 P. J. Brimmer and P. R. Griffiths, Appl. Spectrosc., 42 (1988) 242. 84 J. L. Koenig, Appl. Spectrosc., 29 (1975) 294. Dr. Kalasinsky ts currently Laboratory Director at ChemAlysis, Inc. in Laurel, MD 20853, U.S. A.
Computer Corner Contributions Contributions of between 400 and 900 words are welcome in the following categories: hardware, software, chemical applications, mathematical tools and interfacing. Please send your papers either to: TrAC Computer Comer, D.L. Massart, Vrije Universiteit Brussel, Fakulteit der Geneeskunde Farmacie, Farmaceutische Scheikunde, Laarbeeklaan 103, B-1090 Brussels, Belgium. or TrAC Computer Comer, A.P. Wade, Department Mall, Vancouver, B.C. Canada V6T lY6.
en der
of Chemistry, University of British Columbia, 2036 Main
form of quality control. More rigorous quality control can be attained by including a sample of dilute acid in each batch. General Better pH measurements undoubtedly result if there is some understanding of how electrodes work (see Westcott2’ for a simple account and Bates17 for a more comprehensive treatment). Some manufacturers’ guides to pH measurement are excellent and recommended procedures for using individual electrodes should be followed. Electrodes should not be expected to last forever; normally they will give good service for one or two years, depending on use. Those that are used regularly (daily) normally perform and last better than those used intermittently. It is advisable to have two electrodes in continuous use to allow cross checks and as a guard against sudden failure or damage. References 1 2 3 4 5
R. C. Metcalf, Analyst, 112 (1987) 1573. W. Davison and T. R. Harbinson, Analyst, 113 (1988) 709. W. Davison and C. Woof, Anal. Chem., 57 (1985) 2567. D. Midgley, Atmos. Environ., 21(1987) 173. G. Marinenko, R. C. Paule, W. F. Koch and M. Knoerdal, J. Res. Natl. Bur. Stand., 91 (1986) 17. 6 D. P. Brezinski, Analyst, 108 (1983) 425.
83
7 T. R. Harbinson and W. Davison, Anal. Chem., 59 (1987) 2450. 8 A. K. Covington, P. D. Whalley and W. Davison, Anal. Chim. Actu, 169 (1985) 221. 9 W. Davison and T. R. Harbinson, Analyst, 113 (1988) 1537. 10 A. K. Covington, R. G. Bates and R. A. Durst, Pure Applied Chem., 57 (1985) 33. 11 A. K. Covington, P. D. Whalley and W. Davison, Pure Applied Chem., 57 (1985) 877. 12 A. K. Covington, P. D. Whalley, W. Davison and M. Whitfield. In T. S. West and H. W. Nurnberg (Editors), The Determination of Trace Metals in Natural Waters, Blackwell, Oxford, 1988, p. 161. 13 C. Jones, D. W. Williams and F. Marsicano, Sci. Total Environ., 64 (1987) 211. 14 W. F. Koch, G. Marinenko and R. C. Paule, J. Res. Natl. Bur. Stand., 91(1986) 23. 15 W. F. Koch, G. Marinenko and R. C. Paule, J. Res. Natl. Bur. Stand., 91(1986) 33. 16 W. Davison, C. Woof and E. Tipping, Analyst, 114 (1989) 587. 17 R. G. Bates, Determination ofpH, Wiley, New York, 1964. 18 A. K. Covington, P. D. Whalley and W. Davison, Analyst, 108 (1983) 1528. 19 N. R. McQuaker, P. D. Kluckner and D. K. Sandberg, Environ. Sci. Technol., 17 (1983) 431. 20 C. C. Westcott, pH Measurement, Academic Press, New York, 1978. Dr. Davison is at the Institute of Freshwater Ecology, Windermere Laboratory, The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 OLP, U.K.
observer
Industrial applications of vibrational spectroscopy Kathryn S. Kalasinsky Mississippi
State, MS, U.S.A.
Vibrational spectroscopy has played an ever increasing role in the analytical laboratory. As instrumentation and sampling accessories evolve, the applications to industrial an&ses broaden. Reported here is a brief discussion of the field of vibrational spectroscopy and its applications as reported in the literature as well as several specify sample analyses.
Analytical applications of vibrational spectroscopy have made their biggest impression on society through art work validation’ and forensic science2- . Although these represent only a fraction of the applications, they demonstrate the rapidly broadening 0165-9936/90/$03.00.
field of analyses suited for vibrational spectroscopy. Microscopy has played an important role in expanding the field of vibrational techniques into microspectroscop$-8. The ability to analyze a single fiber or minute paint chip, combined with the specificity and non-destructive nature of vibrational techniques, has made applications such as those mentioned above possible. Industrial concerns that need to closely monitor roducts such as plasticizers’ and P have found vibrational specpharmaceuticals lop1 troscopy to provide specific data rapidly. Biomedical research has also found the utility of vibrational spectroscopy12-14; the biggest problems of this field are not experimental design but interpretation of all the available data. @Elsevier
Science Publishers B.V.
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Analytical services laboratories that employ vibrational spectroscopy not only have the choice of numerous sample handling techniques but also the choice of sample phase: gas, liquid or solid. These decisions are often more difficult and critical than the actual data interpretation. An example of such decision processes is demonstrated in a ‘shoe box’ sample that was submitted to the Industrial and Agricultural Services Division of a state government laboratory. A shoe store had received a shipment of shoes from a public carrier and the shoe boxes apparently got wet during shipment. After stocking the shoes several of the store employees became ill and had to leave the premises. A sample of one of the shoe boxes was placed in an air tight container and sent to the analytical services laboratory for analysis. Upon receipt of the sample, a chemist opened the container and noted a strong, unidentifiable odor. A portion of the sample was placed in a sample bulb, attached to a vacuum line and freeze-thawed to remove the non-condensable gases. The sample was then frozen out in liquid nitrogen and allowed to begin to warm up slowly. The first volatile gases from the sample were then trapped in an infrared gas cell and analyzed. The resultant spectrum is shown in Fig. 1. Obviously there is atmospheric water and carbon dioxide with an identifiable methanol band at about 1036 cm-‘. In the C-H stretch region the methanol bands are identified and there are also some other unidentified C-H stretches. A portion of liquid from the sample was then condensed into a cold finger on the infrared gas cell and an infrared spectrum was obtained of the equilibrium mixture of gases above the liquid. Fig. 2 shows the spectrum with the methanol peak diminished in size and no
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Fig. I. Infrared spectrum of volatile gases from shoe box sample.
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Fig. 2. Infrared spectrum of equilibrium mixture of gases over liquid from shoe box sample.
other identifiable peaks were observed. It was then noticed that a corrosion had formed on the infrared windows of the gas cell. The sample was pumped out and the final conclusive spectrum was obtained as shown in Fig. 3. The atmospheric adsorptions appeared as negative peaks and a formaldehyde peak at about 1642 cm-’ was clearly visible. All the rest of the C-H stretches were accounted for from the formaldehyde frequencies. The formaldehyde and methanol were existing in an equilibrium mixture in the ‘wet’ shoe boxes. Further checking with the shippers finally revealed that biological samples had been contained in the shipment along with the shoes and one container had broken, leaking formaldehyde on the other packages. Vibrational spectroscopy, especially infrared, has been dubbed as a super sleuth at solving mysteries. Numerous articles15- 7 have described how infrared spectroscopy has solved perplexing problems, or been the conclusive piece of evidence to identify an unknown. Infrared spectrometers have evolved as optional detectors to be used in conjunction with other analytical techniques153’8. Environmental studies are particularly suited for these hyphenated infrared methods. Raman spectroscopy has also evolved as a tool for environmental chemistry1g-21, although to a lesser extent than infrared spectroscopy. Raman spectroscopy has been viewed in the past as an exotic technique22, but with the recent development of Fourier transform Raman23-25, it ought to take its place beside infrared as a complementary tool. Analytical chemistry in the laboratory has certainly felt the presence of vibrational spec-
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Fig. 3. Infrared spectrum of residual gases from shoe box sample.
troscopf6. Quality assurance and quality control have been dominated by infrared techniques2’-2g. Method development and validation3’ for process control is unambiguous. Recently, near-infrared spectroscop*i has seen a resurgence in this area. With the development of mini-computers there was a rise in the use of Fourier transform instrumentation in the scientific field. There are several inherent advantages of Fourier transform infrared (FTIR) spectroscop~2~33, and it is due to these advantages that many sampling accessories evolved. Today, the greatest limitations in data acquisition are typically those dictated by the sample, not experimental design 34. One of the most widely used sampling accessories for infrared spectroscopy is internal reflection3’. This is probably not evident from the literature, but most industrial and analytical services samples are easily handled in this manner. An example of a problem that was quickly solved by attenuated total reflectance (ATR) is shown in Fig. 4. A state had purchased its motor carrier decals from a low bidder and after one month of use the decals were failing. The ATR spectrum of the bid product, a polymer known as Fascal900, is shown as the top trace in Fig. 4. The spectrum of the motor carrier decals shown in the middle is obviously a different material, and it is obvious from the bottom trace that the decals were made of a polymer, Fascal 55OS, whose specifications were known to not meet those required for the bid. Internal reflectance cells for liquid flow systems have recently been developed and have become popular for process stream monitorings697. External reflectance accessories have become extremely effective in measuring thin films on smooth metal sur-
faces for quality control in the computer disk industr38. Diffuse reflectance accessories are extremely popular for infrared experiments and their applications are still growir$‘. One of the most important accessories attributed to the growth of FT-IR is the gas chromatography FT-IR (GCYFT-IR). Much development work has gone into these systems40-43 to make this technique competitive with GC-mass spectroscopy. Fiber optic in situ measurement systems are the latest accessory development for FTIR? Infrared spectroscopy has long been noted for its ‘group frequencies’ and ‘fingerprint region’ for identification45. The ‘fingerprint-ability’ of infrared spectra made routine spectral matching via computer analysis an obvious development46 and a powerful tool. This requires the availability of massive reference databases4’, produced with a certain degree of quality48 and standardization4’. Although much work has been done in these areas, it still lags behind the demand and appears to be a ‘never ending’ problem. Work has been done in developing computer analysis programs using group frequencies50p51 which avoid the problem of a lack of databases. Pattern recognition computer routines have been developed for NMR and mass spectroscopy52-54 and will probably be the next area of development for identification in infrared spectroscopy.
Fig. 4. Attenuated total reflectance infrared spectra of the bid product for a motor carrier decal, the actual decal and the identified material of the decal.
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surements can be represented lar to Beer’s Law,
by a relationship
simi-
I= J[X]
Fig. 5. Infrared spectra of a contaminated and normal air sample.
Quantitative measurements by infrared spectroscopy are based on the Lambert-Beer law, A = abc where A is the absorbance, a is the absorptivity, b is the pathlength, and c is the concentration. Computerization of infrared spectroscopy saw a boom in the quantitative applications due to the increased accuracy of measurements and computational powers5. Techniques such as spectral stripping, curve-fitting, and factor analysis evolved56. Extensive development on the area of multicomponent analysis using matrix representations and partial least squares solutions57-64 has helped to repopularize quantitative infrared spectroscopy. Quantitative routines have even been developed to handle special samples65 or sampling apparatus66’67. Quantitative Raman mea-
where Z is the Raman intensity, J is the proportionality constant, and [X] is the concentration of compound Xa. Applications have been shown in several areas69,70. An example of the importance of quantitative measurements can be seen in the spectra shown in Figs. 5 and 6. The office personnel of a manufacturing company began blacking-out at their desks. A sample of the air was obtained in a long-path infrared gas cell. The spectrum of the air from the manufacturing company shown in the top portion of Fig. 5 is compared with the spectrum of normal air on the bottom portion. A difference can be seen at about 2145 cm-i, where carbon monoxide absorbs. A blow-up of the CO region with both spectra overlaid is shown in Fig. 6. It can be seen that the sample is about ten times stronger in carbon monoxide concentration than normal air. Further inspection at the manufacturing company revealed a crack in the gas heater for the building housing the employees. Surface analysis in analytical chemistry is typically associated with electron spectroscopy such as Auger, but vibrational spectroscopy has developed as a prime tool for surface and depth profiling71-73. Surface residues can be studied by diffuse reflectance infrared spectroscopy. Coatings and multilayer polymers can be studied by internal reflection spectroscopy. Nanosamplers have been developed for fiber and dust analysis74. Electrode surface reactions have also been studied by infrared spectroscopy75. Of these techniques diffuse reflectance has probably emerged as the most popular, and much attention has been given to it in the literature76-80. Particular attention needs to be paid to sampling and experimental design when uantitation of diffuse reflectance data is the gaolB 1-83.An example of coating analysis by ATR is shown in the spectra of Figs. 7 and 8. A manufacturer of a vinyl material for office furniture had a customer complaint that the vinyl did not hold up to wear and soiled readily. The manufacturer suggested that the cleaning personnel had cleaned the vinyl coated furniture with a product containing isopropanol, which would remove the protective coating. For the analysis, a spectrum of each of the following was obtained: standard lot sample from the manufacturer; standard lot. sample after treatment with isopropanol; portion of customer’s sample from clean area under furniture where cleaning personnel did not have access with cleaning products; and soiled portion of customer’s sample
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Fig. 6. Infrared spectra of carbon monoxide region of contaminated and normal air sample overlaid.
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Fig. 7. Attenuated total reflectance infrared spectra of vinyl uphoktery material withoutprotective coating.
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Fig. 8. Attenuated total reflectance infrared spectra of vinyl upholstery material with protective coating.
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from area of access to cleaning personnel (soiled portion was cleaned with soap and water as recommended by manufacturer before analysis). These spectra, shown in Fig. 7, are all similar with the exception of the standard lot sample, indicating that the protective coating was not placed on the vinyl before upholstering to the customer’s furniture. The same manufacturer had a similar complaint from another customer where the color had faded out of the vinyl upholstered furniture. In this case spectra of the following samples were obtained: standard lot sample from manufacturer; dark portion retaining original color of customer’s sample; and light portion of faded area of customer’s sample. These spectra, shown in Fig. 8, indicated that the coating was originally on the customer’s sample but had been removed by cleaning products in the faded portion. Conclusions The analytical chemical laboratory has certainly found a place for vibrational spectroscopy. Identification, quality control, and quantitative analysis are but a few of the areas where vibrational spectroscopy has excelled as an analytical tool. Data processing capability which has allowed ‘turnkey’ operation, mathematical manipulation of data and greater spectral sensitivity through signal averaging, has probably been instrumental in enhancing the utilization of vibrational spectroscopy in analytical laboratoriess4. Hyphenated techniques are continuing to evolve. More analyses are being interfaced together for quicker, more complete results with less sample preparation. Automatic data interpretation is also evolving with these newer techniques but not as rapidly as the need requires. This is the biggest area of development in the near future for vibrational spectroscopy. Quantitative analyses are becoming more complex as the sampling apparatuses are able to handle a wider variety of samples with various environmental matrix effects. Developments in these areas have also grown out of the data processing capabilities inherent in the instruments themselves. Method development, though enhanced by the state-of-the-art of analytical equipment, has sometimes become more tedious and confusing as more variables need to be defined in each experiment. However, these same variables allow for more precise and accurate measurements to be devised.
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