Infrared spectrometry - Analytical Chemistry (ACS Publications)

Anal. Chem. 1984, 56, 349 R-372 R (15dd) Stockerner, J. Z . Lebensm.-Unters. Forsch. 1982, 174, 108-13 (Ger.). (16dd) Williams, M. A.; Humphreys, R. C.; Reader, H. P. Am. J . Enol. Vitic. 1983, 34, 57-60.

(17dd) Woodcock, E. A,; Warthesen, J. J.; Labuza, T. P. J . Food Sci. 1982, 4 7 , 545-9, 555. (18dd) Wu, S. S.;Tseng, M. J.; Wang, K. T. J . Chromatogr. 1982, 242, 369-73.

Infrared Spectrometry Robert S. McDonald General Electric Corporate Research and Development Center’, Schenectady, New York 12301

OVERVIEW OF ANALYTICAL INFRARED SPECTROMETRY Scope of Infrared Spectrometry. Recently this reviewer was reminded of a statement by two pioneers in infrared, G. B. B. M. Sutherland and H. W. Thompson2: “The infra-red spectrum of a chemical compound is probably the most characteristic property of that compound”. This was widely quoted nearly forty years ago, in the days before either commercial grating or FTIR spectrometers. Possibly some would dispute this statement today in view of the enormous advances in other techniques for material characterization, but as A. Lee Smith has stated more recently 3: “It is probably fair to say that infrared is the most nearly universally useful of all instrumental techniques“ (italics by the reviewer). Actually, the usefulness of infrared spectrometry has increased precipitously during the last few years since the famous pronouncement by H. A. Laitenen which likened the history of infrared spectrometry to Shakespeare’s seven stages of man4 with emphasis on the declining years. This only goes to show how difficult it is to forsee or even to recognize the breakthroughs which lead to advances in the art and science of analytical chemistry. It seems particularly unwise to write off any analytical technique as dead, or any revolutionary idea in chemical instrumentation as impractical. Repeatedly, we have seen impossible ideas develop into viable and important analytical methods, Le., FTIR, diffuse reflectance IR, GCIR, LCIR, photoacoustic IR, photothermal beam deflection, near IR reflection, and many others, without which this review would hardly have been worth writing. Strengths and Weaknesses. Once more we review the strengths which make infrared spectrometry so widely applicable and outline some of the problem areas, many of which are being overcome by new techniques and instrumentation discussed by papers cited in the bibliography. The major strength of infrared spectrometry is its applicability to both qualitative and quantitative determination of chemical functionality of a wide variety of covalent bonded chemical substances, including gases, liquids, polymers, and solids, both crystalline and solid. Easy to use spectrometers which are readily available to most university and industrial chemists can produce high quality spectra in under five minutes. Their availability and speed are important factors in a chemist’s productivity. The infrared spectrum maps the internal vibrational frequencies of a substance versus the intensity of interaction of infrared radiation with vibrational motions. A great deal can be learned about a substance by off-hand examination of its infrared spectrum. Presence of various atoms, bonds, and chemical functional groups can be inferred by comparison with a large body of empirical correlations which have been built up over many years. Frequencies and intensities are sensitive to local structure, orientation, physical state, conformation, temperature, pressure, and concentration. This, combined with the chemist’s knowledge of the system, is what makes infrared so useful. On one hand, the spectrum can serve as a fingerprint to identify a reactant, product, or intermediate by matching against known reference spectra. On the other hand, the spectral features can be used to characterize the chemical functionality of substances for which no reference spectra are

INTRODUCTION This review covers the literature for the two year period from late 1981to late 1983 on aspects of Infrared Spectrometry which are relevant to chemical analysis. It is intended to review the state of the art in analytical infrared spectrometry as portrayed by the recent literature. The review is again being submitted to Analytical Chemistry in computer readable form on magnetic tape. However, this time the manuscript was prepared on a personal computer (Kaypro 10) rather than on the large mainframe computer used previously. Selection of References. The initial selection of references was based on CA Selects: Infrared SDectrometrv (Phvsicochemical Aspects) and Infiared Spectrometry ‘(Organic Aspects). Generally, the period covered by the review is defined by Chemical Abstracts, Volumes 96-99 (1982-831, through the December 29, 1983 issue (Jan. 9,1984 issue of CA Selects). Books and reviews have been selected to give broad coverage not only of infrared analysis, but also of other topics which provide support for analytical applications and of technology which shows promise for the future. The research papers have been selected with two points in mind: (1) to cover areas in which new developments are underway, and (2) to give broad coverage to applications where the use of infrared methods appears to be expanding rapidly. Since infrared spectrometry is basically an instrumental technique, considerable attention has been paid to instrumental developments. This review has a strong bias toward papers in English. Since Russian, Japanese, and Chinese journals are difficult for the readers of Analytical Chemistry to obtain and to read, a condition for inclusion of citations to papers in these languages is that Chemical Abstracts contain a useful description of the work or that the basic idea can be grasped from the title. Organization of the Bibliography. The bibliography is divided into 9 main sections. The first two sections, Books (Section A ) , and Reviews (Section B ) were selected on the basis of the type of publication. The remainder of the bibliography is divided into seven main sections according to subject matter, and these are divided into subsections. The text of the review is divided into parallel sections and subsections. Each section and subsection of the bibliography has separate numbering distinguished by literal prefixes. Prefixes for main sections are single letters; those for subsections are double letters with the first corresponding to that for the main section to which the subsection belongs. These sections contain only citations of current papers. Any citations of older papers, oral presentations, etc., are in a separate footnote section between the text and the bibliography whose entries are designated by superscripts in the text. Chemical Abstract numbers are included for most citations except for those of books since the latter never contain information on subject matter. Each citation in the bibliography starts with the complete title in English as given by Chemical Abstracts to facilitate scanning the topics covered by the papers. Inclusion of the titles follows a strong personal conviction of this reviewer that the title is an integral part of any bibliographic citation. In this review, the titles should be regarded as part of the text. 0003-2700/84/0356-349R$06.50/0

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available. It is rare to be able to explain every feature of an infrared spectrum or to make a complete structure roof, even of a pure compound, on the basis of its infrared3spectrum alone. Interpretation of spectra is a skilled art which is part of the repertoire of a growing number of chemists. Computer interpretation of spectra is an active area of research but as yet it is no match for the capability of a skilled chemist. Computer matching of unknown spectra to computerized spectral libraries supplements, and in special cases surpasses, the capability of the chemist. Infrared spectrometry is a technique of intermediate sensitivity. Intensities of major absorption bands of most liquids and solids lie in a fairly narrow range, 0.1 to 1.0 A for a sample thickness of 0.01 mm. It is rare to find an infrared absorption band which is 1000 times stronger than its surroundings, as in the visible or ultraviolet, where the absorption bands may be two or three orders of magnitude stronger than IR bands. For most pure substances, half a dozen or more well defined absorption bands can be measured accurately enough to give a definitive identification for a smear or film less than a thousandth of an inch thick, and less than a millimeter in diameter. Generally, the spectrum provides information on components which are present in percentage amounts, and in relatively simple mixtures. Extensive calibration or separation of components is needed for analyzing complex multicomponent mixtures. Special techiques and favorable circumstances are required for sensitivity in the ppm or ppb range and for measuring the spectra of monolayers. Infrared spectrometers measure directly the fraction of radiation transmitted by a sample, not the amount absorbed. Concentration is proportional to the logarithm of the transmission. By virtue of this logarithmic characteristic, a given dynamic range in transmission corresponds to a much smaller dynamic range in concentration. Thus, an instrumental S/N of 1ooO:l in transmission corresponds to a S/N of 1W1at an absorbance of 1, 1 0 1 a t an absorbance of 2, and 1:l a t an absorbance of 3. Increasing the absorbance is clearly in the direction of decreasing returns. It seems more fruitful to expand the low absorbance region where the relationship between absorbance and concentration approaches linearity. Techniques are now coming into use which measure absorption directly, including IR emission, photoacoustic, and photothermal detection, together with the double modulation techniques of circular dichroism and polarization modulated reflectance. Some of these are proving to be much more sensitive than transmission for special purposes. In laboratory chemical analysis, the interplay between infrared, Raman, NMR, and mass spectrometry, as well as ESCA, X-ray diffraction, and other techniques for determining chemical functionality is very important. Of the major instruments for determining chemical functionality, the infrared spectrometer is the least expensive and the easiest to use. Hence, it tends to be the choice of smaller laboratories with limited budgets. However, for larger laboratories which can afford several, or even all of these techniques, there are choices to be made when it comes to performing specific analyses. Where modern, high field NMR capability for 13C is available, infrared may not be the first choice for organic liquids and solutions. NMR gives a single line, or recognizable multiplet, for each set of equivalent nuclei, whereas infrared gives multiple bands dispersed throughout the spectrum for most functional groups. NMR is clearly the method of choice for compounds whose spectra are previously unknown. Where modern, high resolution GC-MS is available, it may be more desirable for its capability of providing formula weight and empirical formula for the molecular ion and its fragments. Infrared, on the other hand, is able to identify functional groups and their interactions in-situ in a nonvolatile material like a polymer. Where a modern Raman microprobe is available, it would be the choice over infrared for determining the spectrum of a single particle whose dimensions are smaller than IR wavelengths. But FTIR spectrometers generally scan the spectrum more rapidly and provide more effective spectral subtraction capability by virtue of their high accuracy in frequency measurement. Comparing now with X-ray methods, there is no way to calculate a structure from an infrared spectrum as there is from an X-ray diffraction pattern. There is not even a way to calculate the infrared spectrum of most compounds from their structures. However, infrared provides functional group information for non-crystalline solids which 350 R

ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

are beyond the scope of X-ray diffraction. Infrared identification of surface atomic species and their valence states is decidedly inferior to that of ESCA, but on the other hand, infrared has greater capability for distinguishing between organic functional groups on surfaces. Of course, the combination of two or more of these methods is exceedingly powerful, but everyday problems often do not warrant the expense of all-out application of a combination of all of these methods. In the real world of the control lab and plant process control, infrared is used far more than any of the other techniques mentioned in the previous paragraph. Infrared analyzers are widely used for on-line analysis of gases and liquids. However, the very low thickness requirement for effective infrared transmission measurements is a difficult one for viscous or solid samples. Most process analyzers are diode laser or filter instruments. The complexity of FTIR spectrometers has slowed their introduction as process analyzers although their potential is very great. Trends in Infrared Spectrometry. In the previous review of this series, it was stated that there had never been a time when so many talented people were working so effectively to expand the capabilities of infrared spectrometry. The tremendous advances in instrumentation and techniques since then are testimony to the effectiveness of their efforts. Near infrared reflectance between 1and 2.5 pm is the basis of a quantitative analytical method which is expanding rapidly in agriculture and food processing. This method is starting to be applied in the field of process analysis of polymers and other industrial chemicals. Infrared is a major analytical method for the control of purity of silicon for semiconductor devices. It is used for determination of dangling bond terminators in amorphous silicon for solar cells, for evaluating passivating coatings on semiconductor devices, and for evaluating ion implantion techniques. There has been a breakthrough in the use of infrared to study the electrode/ electrolyte interphase in electrochemistry. Automated procedures for measuring spectra as a function of time and/or temperature are proving to be effective in studying biological systems which yielded little information from IR spectra by static sampling. These are being used to study the mechanism of clotting of blood, the interaction of blood proteins with polymers intended for replacement of body parts, for the study of the interaction of cholesterol with triglycerides in the formation of plaque in atherosclerosis, and for the study of the visual pigment rhodopsin. Infrared is being used for remote detection of gaseous pollutant emissions, for the determination of the spatial distribution of trace atmospheric gases, and for the analysis of planetary atmospheres and surfaces. In the polymer area, automated IR measurements are being used for time resolved studies of the rheological properties of polymers, for the investigation of functional group interactions between components of polymer blends, for identification of microscopic defects related to fracture, and for study of the mechanism of polymer degradation via the on-line analysis of time and temperature dependent gaseous degradation products (EGA). There have been major advances in the quality of FTIR instrumentation and in the speed of IR data processing. A number of commercial FTIR spectrometers can produce a complete IR spectrum on a video screen at 2-4 cm-' resolution and with a S/N of 500-1OOO:l less than half a minute from the time of introduction of the sample. FTIR seems at last to be providing on a routine basis the performance which was claimed as much as a decade ago. It has taken a while to learn how to exploit the spectacular capabilities of FTIR. Griffiths (B77)has predicted that grating IR instruments will be entirely superceded by FTIR systems. All of the major U.S. manufacturers of dispersive instruments now have an FTIR product line, and one of them has stopped manufacturing dispersive instruments entirely. The quality of dispersive instruments has improved markedly, partly in response to the competition from FTIR systems. The benefits of computer data processing are applicable to both. Production of the lower cost dispersive systems will probably continue. However, it has been very noticeable during the preparation of this review that the state of the art work in infrared spectrometry is dominated by FTIR systems. The latter provide the best combination of speed, resolving power, S/N,

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Rob.rt S. YcDaald is B mnsubnt on Inrmred spectrmm, and rslated computa appIcB11ons. He praduated rmm Um UnkerSny 01 Mahe In 1941 wkh a E.S. W e 3 In eng1mw physb and r&ed me rn.D. Wee In physical chtmlsby hm MIT h 1952. Alter assoclalbns with Me Stanlord ! R ~ S B B TLCa~m a t a b s of A m m n Cyanamid &. (1942-1946) and mS MIT Spectr'oscopy Labmalay (1946-19511. heblned General ElecMc where ha remalwd mtil his retirement In 1983. His research interests have inckded me study Of SYrfacB hncliooal groups and point defects in solids by inhared spctromelry. I n amnkm to inhared specbomeby. his present interests lncllrje computer processing of spectral data and use Of MmputBrS f M Infamation retrieval. For Some Y M r S he has prepared bbnnlel review of "lnhared Spectrometry" fa A n e l y i b l chsmishy. He is a msmber 01 mS ACS. APS. ASTM. SAS. AAAS. and Me Cobbenu soc(ely. He has served ar lk Board of Governas lw Me CoblenU Socier, and on Me Adviswy Board f M A n a W I C h e m i r h y He Is presenlly a member of me Joint Cornmiltee on AIomIC and Molecular Ropenies fJCAMPI.

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abscissa accuracy. and capability for difficult samples of any kind. Yet, the highest S/N in the near infrared, ea 100000:1, is provided by grating or filter spectrometers, and the circular variable filter systems seem to be competing effectively with FTIR for microscopic samples and for process analysis. For a very large proportion of analytical applications, FTIR and dispersive instruments are very nearly equivalent. Sample preparation is still the rate limiting step in most cases. FTIR spectrometers are becoming smaller and simpler, but they still are not competitive in price with dispersive spectrometers which can be operated without data systems. Performance of some of the smaller FTIR instruments is comparable with or better than that of the larger, higher priced instruments for some applications, and they are decidedly less susceptible to interference by atmospheric water vapor. The sensitivity of ETIR and diode laser systems is so great that insensitive sampling techniques (PAS) or those which severly attenuate the available energy can he tolerated. Effective studies of dilute water solutions are now possible. Use of IR transmitting optical fibers, which now have respectable transmission over many meters, appears to be feasihle. A wide variety of detector arrays is becomin available for use in spatially resolved measurements. Com%inationof Hademard spatial fdtering with FTIR for spatially resolved measurements has been described. Infrared reflectance-absorption (RAIR) and polarization modulation are providing IR sensitivity for surface studies which is comparable with that of ESCA and EELS. Polarization modulation is also being applied effectively for IR circular dichroism. Spatial resolution of IR measurements has been carried out by using one laser for Stark modulation and a second IR laser to measure the spectrum of the gas in the volume where the two beams cross. Current GCIR techniques permit use of capillary columns. The trend is toward complete automation of the GCIR experiment, including on-line computer identification of the GC peaks. A combination of automated matrix isolation with GCIR is comparable in sensitivity with state of the art GCMS. Combination of LCIR and DRIFT is permitting IR detection to he used on-line for chromatographic separations which use polar solvents with gradient elution. System software is becoming more user friendly. Fast search techniques using compressed s ral libraries permit on-line identification of GC peaks. Li ranes of' fully digitized spectra are feasible with the large disks which are currently available. In some cases, these provide a search and identification capability matching or surpassing that of a spectroscopist. Factor analysis and multiple regression techniques are being used effectively for determination of the spectra of ure components of multicomponent mixtures which can only partially resolved by physical separation techniques. Online software is being provided for various methods of resolution enhancement via derivative spectra and Fourier deconvolution. Altogether, the advances in IR instrumentation and techniques are so great that it is impossible to do them justice in this review. The outlook for the future is very bright.

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(A) BOOKS Since Chemical Abstracts provides no abstract of the content of books, ahstract numbers are not generally provided. Wherever possible, a citation to a hook review is included. Volume 9 (AI) and Volume 10 (A2) of the series edited by Clark and Hester have appeared, as well as Volume 10 (A%) and Volume 11 (A26) of the series edited by Durig. These books are relatively dilute in information relative to infrared analysis, but each contains at least one useful chapter. However, the prices are so high that most analytical spectroscopists will be satisfied to consult them in a library. Volume 14 (A19) of the series on Spectroscopic Properties of Inorganic and Organometallic Compounds edited by Adams and Ebsworth has also appeared. Volume 111(As) of the series on Fourier Transform Infrared Spectroscopy hy Ferraro contains a number of chapters of interest to analytical spectroscopists and the price is within reach of the individual. Parker (A3) has published a comprehensive volume on applications of infrared and Raman methods in biochemistry which will probably be of interest to specialists and biologists. Painter, Coleman, and Koenig (A.23) have published a comprehensive volume on the theory and applications of vibrational spectroscopy to polymeric materials which will probably be of interest mainly to specialists and polymer chemists and physicists. Also included under books is an audio visual program for training in infrared quantitative analysis (Al3). The trend toward publication of volumes of spectra for use in special fields continues. Sadtler Laboratories has published a handbmk of infrared spectra of priority pollutants and toxic chemicals (All), and also a handbook of infrared spectra of clays and minerals (AIL?), edited h Ferrarro. The Coblentz Society has published a second ediltion of its desk book of infrared spectra (A4). edited by Craver. Sadtler has published new editions of its indices to the Sadtler library of commercial infrared spectra (A5), (A6). ( A n . Three volumes published by S H E - The International Society for Optical Engineering are included. Volume 289 (A24) covers the 1981 International Conference on Fourier Transform Infrared Spectroscopy. This volume gives a good picture of the state of the art in infrared at the time of the meeting and generally for the early part of the period of this review. This reviewer would recommend it highly if it were still in print. It is not readily available even in most university libraries, hut it is available as a reprint from University Microfilms. It contains a numher of full length papers, some of which have not yet appeared in the journals. S H E Volume 308 ( A l 5 ) covers a conference on infrared standards and calibration, and SPIE Volume 366 (A16) covers infrared References to individual papers in several other SPIE pu hcations ' are included in the research paper section of this bibliography. These represent a relatively incomplete coverage of SPIE publications on infrared technology, the remainder of which should not be overlooked by specialists in infrared instrumentation.

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(B) REVIEWS Most of the reviews are cited in the appropriate section of the text, but a few do not fit under any of the topics which are discussed. Chemical Abstract citations are given for reviews. Abstracts are provided for about a third of the cases; however, in the remaining cases the abstract gives only the numher of references cited in the review.

(C) ANALYTICAL APPLICATIONS Expansion of the use of infrared spectrometry for all types of analytical applications continues, including quality control of incoming and outgoing products, examining competitor's products, monitoring laboratory reactions, and general problem solving. These applications are considered routine and rarely appear in the literature any more. The best sources of information on them are the application laboratories of the infrared spectrometer manufacturers. Improvements in the quality of the instrumentation and development of new techniques are continually opening up new areas, presently, for example, in agriculture, food processing, biochemistry and electrochemistry. We note here mainly areas in which there has been an increase in activity ANALYTICAL CHEMISTRY. VOL. 56. NO. 5, APRIL 1984

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due either to new spectroscopic techniques or to the development of new materials or processes which need the special analytical capability of infrared spectrometry. Results comparable to those of mass spectrometry for I3C abundance is claimed ((71)for an infrared method which is capable of detecting the small differences between C3 and C4 plants. IR measurements on polymers formed on oscillating electrical contacts in air-hydrocarbon environments with deuterated and non-deuterated hydrocarbons shows the extensive involvement of oxygen (C2). Infrared spectra provide a qualitative assessment of the types of constituents and functional groups found in the sludges and extracts of sewage (C3). IR frequencies of 66 compounds of interest in cement chemistry have been tabulated (C4). Forensic use of IR spectra of photocopying toners has been evaluated (C5).Algorithms have been developed for distinguishing compounds and isomers, diastereomers and crystal modifications for fully automatic identification of drugs by computer coupled infrared spectrometry (C6). An infrared analytical method for cloroazepate removes all interferences associated with conversion of the di- to the monopotassium salt, and with other substances which interfere with isolation of pure chlorazepate (C7). These analyses are very exacting because they must stand up under adversary cross examination. Use of IR spectrophotometry for emergency toxicolo ical analyses has been described (CB), and a computer datatase of IR spectra of nonvolatile poisons has been developed for rapid identification of unknowns (C9). Use of IR spectrometry for examination of personal air monitoring badges for organic vapors has been described (CIO).Interference of acetone with breath alcohol determination is eliminated by a two component analysis which displays both concentrations (C11). A facility for identifying volatile condensable contaminants by in-situ FTIR has been described (C12).Deposits less than 200A thick can be identified. Small area and thin fiim analysis is important in the identification of contaminants in semiconductor processing (C13), (C14). FTIR analyses assist in the conservation of ancient paintings (C15) by determining the composition of inconspicuous amounts of material. FTIR techniques for microsample polymer contamination usin direct transmission or micro pyrolysis have been describe$ including the technique for depositing the micro condensate on a tiny area on a salt plate (C16) Trichlorosilanol, a very reactive impurity in silicon tetrachloride, cannot be isolated in pure form for preparation of spectroscopic standards. Its determination is important for the reduction of hydroxyl overtone absorption near 1micron in optical silica fibers prepared from SiCl,. An elegant method for preparing standards for determination of trichlorosilanol in the 200-2500 ppm range has been described. It makes use of the quantitative photolysis of trichlorisilane and oxygen in SiC14(C17). A calibration for OH determination in quartz and silicate glasses allows for the frequency dependence of the intensity of the OH peak (C18). The OH distribution profile in vitreous silica has been measured using a tunable IR laser (C19). IR reflectance has been used to estimate the surface composition of planets (CZO), (C21) and the composition of the atmospheres of various astronomical bodies has been determined by satellite spectrometers (C22), (C23). (CA) Near Infrared Reflectance. The expanding use of near infrared reflectance for the analysis of agricultural products is nothing short of spectacular. This region, from 1 to 2.5 micrometers, contains mainly overtones and combinations of hydrogen vibrations of OH, NH, and CH groups which are common to all types of natural products. Highly precise diffuse reflectance values are determined for a number of wavelengths, often directly on granular or powder samples. Calibration is carried out by multiple regression analysis of data using a learning set of a t least 30 and preferably more known (chemically analyzed) samples which bracket the analytical ranges. The absorption bands overlap so badly that the average spectroscopist would hardly believe that the method would work. But it does work. Reproducibility of results is comparable with that of the chemical methods used to establish the standards, and the infrared method is much easier and faster. Typical spectra of wheat and its components, starch, protein and water, are shown by 352R


Hruschka and Norris in Fig. 1of reference (CAI). The details are discussed in recent reviews (B781, (B93),(B97). A variety of techniques is used for the calculations; two of the newer methods have been compared with the older ones (CAI), (CA2). The theory behind the InfraAlyzer 400 has been described by Day and Fearn (CA3). Since the method is based on statistical correlations, it is necessary to carry out the error analysis on a separate test set of analyzed materials. Failure to do this, or to use too few knowns could result in some surprising errors. Each new product requires a new calibration, i.e., the calibration for wheat cannot be expected to be valid for dry milk powder. Even use of the same calibration for wheat grown in different countries is suspect until proved valid. The use of NIR for the determination of protein and moisture of ground grain is well established and the variation of the calibration from year to year, from place to place (USA to Europe) and from hard to soft wheat has been studied extensively. Osborne, et al. have reported on the use of universal constants for both hard and soft varieties and all growing locations in the UK which required no more than an annual bias adjustment from 1975 to 1979, except for 1976, a year when unusual climatic conditions prevailed in the UK (CA4). The variety of materials which can be analyzed by this method is very large. For example, it has been applied to the determination of fat, protein, and carbohydrate content of cocoa (CA5),fat, protein, moisture in nonfatty substance, and total moisture content of cheese (CA6),moisture, fat, lactose, and protein content of nonfat dry milk (CA7). In the latter case, the attempt was made to differentiate between microKjeldahl and dye binding protein with limited success. Methods have been reported for determining protein, fat and water content of meat (CA8) and meat products (CA9).NIR analysis has been applied to the quantitative analysis of baked products as well as to wheat and flour in the baking industry (CAIO) and the assay of Penicillium oxalicum and Aspergilus awamori fermentation of wheat bran (CAII). One would not expect these methods to work for determining components which have no CH, NH, or OH absorption. But, it is common to determine ash and other inorganic components which have no absorption in the near infrared. Presumably, these components modify the spectra of OH, NH, or CH groups in some useful way, i.e., by shifting absorption bands. In addition to analysis of clearly definable components, NIR has been applied to the determination of composite properties which hardly can be determined directly by chemical analysis. Animal feeds have been examined for 19 parameters, including in vitro metabolizable energy and digestibility (CA12).Endosperm texture, resistance to grinding, and flour color have also been determined for monitoring the breeding of wheat (CA13). This reviewer has two concerns about this technique. The first is that samples may be analyzed, either accidentally or intentionally, by procedures which were developed for different materials. Internal consistency checks are needed, such as reconstruction of the whole spectrum from the components as described by Hruschka and Norris (CAI). This reviewer is reminded of an almost forgotten case in which a fellow graduate student sent his carefully purified diborane sample to a major petroleum laboratory for mass spectrometric analysis, and received what obviously was the result of forcing the mass spectrum of diborane through the C4 hydrocarbon matrix calculation! The second concern is the possibility of applying the technique for indiscriminate correlations,whether association with NIR absorption is plausible or not. The possibilities for abuse by the untrained are enormous. Competent workers in this field analyze their data to understand specific spectral properties which are responsible for their results. The possibilities for extending this method to other materials are very great. Already, use of NIR reflectance for monitoring the compounding of polymers is being developed. The possibility of biological analyses is interesting. In the polymer area, preparation of knowns by blending allows considerably more flexibility than is feasible with natural products like grain. However, the variety of different products to be analyzed poses a serious problem if each requires the usual time consuming calibration procedure. For example a


polymer blending operation may produce hundreds of products and it is probably impractical to prepare a huge set of known samples for each. The problem of reducing this development effort has been discussed (CA2).It is tempting to adapt these correlation techniques for use in the mid infrared where fundamental vibration bands are stronger. But special conditions exist in the NIR. The weak absorption bands in this region are in the linear range, and are not complicated by reflection and dispersion which already cause problems with diffuse reflection of undiluted samples in the mid IR. (CB) Surface Applications. The term surface means different things to different chemists. To the physical chemist, it may mean a region of a few monolayers or less in thickness which is important for catalysis, adsorption, or corrosion. To the organic chemist, it may mean a region of a few microns thickness which is important for adhesion or protective coatings, etc. Advances in sensitivity, and new techniques for enhancin intensity, as well as automated processing of data have mate possible studies on well defined surfaces. Generally the surfaces which are studied by IR fall into two classes: high area materials and materials with well defined planar surfaces. Surfaces of high area materials such as silicas and aluminas and metals supported on such materials can be studied by transmission. Application of IR to the study of such surfaces has been reviewed by Chesters and Sheppard (B4), Haller (B6),Angel1 (B26),Goodwin (B33),and Fripiat (B34).Van Woerkom and De Groot have reviewed emission from catalyst surfaces (B41).Well defined planar surfaces are not readily studied in transmission. They usually require special reflection techniques and are best applied to metal surfaces. These applications have been reviewed by Sheppard (B21) and Allara (B38). Darville (B71)has published a literature review on adsorption on well defined surfaces. A relatively unexploited technique involving surface electromagnetic waves (surface polaritons) has been reviewed by Zhizhin (B43), (B70),Alexander (B44),and a book on surface polaritons has been published by Agranovich and Mills (A2I). In the area of liquid-liquid interfaces, IR spectrometry has been applied to the study of micelle formation in aqueous surfactant solutions (CBI),and to reversed micellar solutions (CB2), (CB3). Cameron et al. (CBI)demonstrated excellent S/N in the CH stretching region for surfactant concentrations well below 0.1 molar even after subtraction of the water spectrum. Clearly, the way is open for a wide variety of studies on dilute aqueous solutions. 'Use of IR spectrometry for the study of floatation reagents has been described by Mielczarski et al. (CB4) and by Kuz'kin et al. (CB5).Diffuse reflectance should be widely applicable to floatation reagents. Giesekke (B82) has reviewed spectroscopic techniques applied to the study of the interactions between minerals and reagents in flotation systems. Transients in the exchange of isotopically enriched CO over Ru catalysts has been studied by FTIR (CB6). Hydrogen spillover on silica has been studied by isotopically enriching a small spot in the center of a silica disk and following the diffusion of deuterium as a function of time and distance with an FTIR spectrometer at 3mm spatial resolution (CB7). The following papers have all used reflection at a high angle of incidence, known as reflection-absorption IR spectroscopy (IRAS or RAIR or RAIRS). Bradshaw (CB8) has compared IRAS with electron energy loss spectroscopy (EELS) in the study of adsorbates in sub-monolayer concentrations on single crystal metal surfaces. Present sensitivity of the two is approximately equivalent. EELS has the advantage of a wider spectral range, while IRAS has the advantages of higher resolution, and potential for use at high ambient gas pressure. Nuzzio and Allara (CB9)have studied the orientation of bifunctional organic disulfides on gold surfaces by IRAS and Boerio et al. (CBIO)have studied organofunctional silanes on alloy surfaces by this and other techniques. The following papers have used polarization modulation for surface studies. Wadayama (CBII), Overend (CB12),and Dowrey and Marcott (CB13) have studied adsorption on metals. Scanlon et al. (CB14) and Hatta et al.(CB15) have studied films on silicon surfaces. Lauer (CB16) has used polarization modulated emission microspectroscopy to study thin films on heated metal surfaces immersed in flowing fuel. (CC) Semiconductor Analyses. Infrared spectrometry continues to be an important method for studyin dopin surface treatments, electrically inactive impurities and Of

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and implantation of insulating layers. It is especially important for amorphous semiconductors which are not easily studied by other methods. Pajot and Debarre ( C C I ) reported new calibration factors for the IR determination of boron and phosphorus in silicon. These impurities are detected by electronic absorption, and the sensitivities are extremely high, of the order of loi2atoms per cm3for a 4 cm thick sample. Determination of boron and phosphorus in silicon b FTIR and laser induced luminescence has been compared (JC2). A new calibration coefficient has also been reported for oxygen in silicon (CC3) after an evaluation in which 70 samples were exchanged among 16 laboratories. The calibration factor for oxygen in silicon for ASTM method F11 has also been corrected. This reviewer suspects that the relatively large correction, approximately a factor of two, came from a misunderstanding concerning the logarithm base of the extinction coefficient. Analytical chemists customarily use absorbance (logl ) whereas physicista customarily use extinction coefficient (log In any case it is prudent to use extreme care to verify the fogarithm base when using absorption intensities from the literature. Oxygen and carbon are not electrically active in silicon, but they have important effects on the properties of integrated circuits. The details are complex, and not fully understood. However, circuit fabricators understand how to modify their processes to accommodate differences in 0 and C concentrations to produce the best yields; hence, the emphasis on determination of these impurities. The determination is complicated by the need to measure both 0 and C in thin slices which may be polished on one or both sides (usually on one side). Measurements on double side polished samples is plagued by enormous interference fringes; methods for removal are discussed in the section on Software. For samples which are polished on one side only, correction for multiple reflection is complicated because the roughness of the unpolished side is not well defined. This roblem has been discussed by Baghdadi (CC4), Graff (C85), Stallhofer and Huber (CC6) and Krishnan (CC7). Implantation of high energy oxygen or nitrogen ions is used to produce insulating layers below the surface of silicon. The presence of these layers causes optical interference effects for transmitted and reflected radiation. The interference patterns contain information on the distribution of implanted ions and their reaction products. A non-linear least squares fitting of reflection data has been reported (CC8) which yields structural and electrical information about the implanted region without damage to the sample. IR reflectance and transmission have also been used to determine the profile of inhomogeneously doped samples (CC9), CCIO). IR techniques are also used to study oxide and nitride layers used to passivate the surface of silicon. Transmission spectra of amorphous SiO, films at oblique incidence show addditional absorption due to longitudinal Si0 vibrations which are not observed at normal incidence (CCI1). Quantitative determination of a- and p-Si3N4 in passivating layers has been reported (CC12) and nitridation of thick (-220nm) oxide films has been studied (CCI3). The validity of the Lambert-Bouger law for transmission of SiOa films grown on Si wafers in a mixture of HC1 and O2at 850" has been verified down to a thickness of 28A (CC14). Etching of silicon wafers for microelectronics is carried out by a plasma in a mixture of a fluorinated alkane with oxygen, typically CF4+ OF The reaction products are SiF,, CO, C02, and COF2.The control of plasma or dry etching by IR analysis has been described by Nishizawa and Hayasaka (CC15) and Poll et al. (CCI6). Amorphous silicon for solar cells is amorphous by virtue of an enormous number of defects. Hydrogen, and sometimes fluorine or chlorine, are used as dangling bond terminators. Oxygen and nitrogen can be present in amorphous layers which are often produced by RF sputtering. The resulting structures are extraordinarily complex and IR spectrometry is perhaps the best way of studying their chemical functionality (CCI7), (CCI8), (CC19), CC20), (CC21). SiH surface functional groups on crystalline silicon have been identified (CC22), (CC23),as well as SiH groups produced by implanting H in crystalline silicon (CC24). Polyacetylene is an organic semiconductor which has been studied extensively by IR spectrometry. The infrared spec-

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trum of polyacetylene is extremely interesting. When it is doped by electrically active impurities, such as I , strong absorption bands appear (stronger than any known uigrational bands); their high intensity indicates electronic absorption. However, on deuteration these bands shift as expected for vibrational bands. A new structural defect, the soliton, has been employed to explain this behavior. Zerbi (CC25) has discussed the analysis of the spectrum of doped and undoped trans-polyacetylene and summarized spectroscopic points which are still controversial. IR spectrometry is a major technique for studying the structure and doping of this material (CC26), (CC27), (CC28). (CD) Coal and Carbon. Traditionally, coal and carbon have been very difficult materials for infrared studies. Like other difficult samples, these two are yielding to FTIR. That is not to say that such studies could not be carried out by dispersive spectroscopy, but they are much easier with FTIR, and hence more likely to be done. Application of FTIR to the quantitative determination of functional groups in coal has been reviewed by Painter (B57) and applications of FTIR in fuel science have been reviewed by Solomon (B58). A set of 21 vitrinite concentrates have been characterized by FTIR, including quantitative determination of hydroxyl groups by combination of acetylation with FTIR procedures (CDI).Diffuse reflectance FTIR has been used to determine the rank of individual coals and the composition of coal blends, as well as the degree of oxidation (CD2). In the latter case, second derivative spectra provided quantitative estimates of oxidation without need of spectral subtraction. A procedure has been described for preparing a film of coal thin enough to yield a normal IR transmission spectrum and strong enough to be oxidized and partially carbonized for subsequent spectral measurements (CD3). A procedure for estimating the OH content of coal using the KBr pellet technique has been described together with a method for minimizing the interference of water (CD4) in KBr pellets. Surface and bulk oxidation of coals has been monitored by FTIR using a photoacoustic detector (CD5) and oxidative weathering of freshly mined Illinois No. 6 coal at ambient conditions has been monitored for two months until the slow oxidation reaction seemed to be complete (CD6).The stability of coal-oil mixtures has been determined by IR (CD7). Reductive methylation of coal with a solution of potassium in ethylene glycol methyl ether has been studied by comparing spectra with the results from deuteromethylation of coal and model compounds ( 0 8 ) . The mechanism and kinetic rates for coal pyrolysis have been studied by examination of the pyrolysis gases from a grid reactor at 500-1800° by FTIR in a form of evolved gas analysis (CD9). The surface of soot generated by the combustion of hexane, taken as a model for petroleum, has been accomplished by FTIR combined with thermal desorption measurements (CDIO) and functional groups in carbon black have been determined by FTIR, using low dilutions in KBr ( C D I I ) . Cocarbonization processes of a petroleum asphalt and coalderived liquids have been followed by FTIR (CDI2). (CE) Electrochemical Applications. For a long time, the only in-situ infrared studies of electrochemical systems were for those which employed organic solvents as electrolyte. Recent advances in technique have made it possible to study systems in which the electrolyte is sulfuric, perchloric, or formic acid. These techniques generally make use of reflection-absorption from a polished metallic electrode covered by a very thin layer of electrolyte. In-situ infrared spectroscopy of the electrode/electrolyte solution interphase has been reviewed by Bewick (B90). Kunimatsu (CEI) has reported on a method which is essentially a linear sweep voltammetry a t a fixed wavelength, repeated at as many wavelengths as are necessary to construct the spectrum, and has used this method to study adsorbed CO produced by chemisorption of MeOH on a smooth platinum electrode between 0.1 and 0.7 V. Two other techniques make use of FTIR to obtain the whole spectrum. In electrochemically modulated IR reflectance spectroscopy (EMIRS) using FTIR, the potential is swept slowly through a wide potential range while repeatedly recording the spectrum. Ordinary difference spectra provide information on the species present at different potentials. This provides chemical functionality information which could only 354R


be inferred indirectly by conventional electrochemical methods. High quality spectra of radical ions of benzophenone, anthracene, and TCNE have been recorded in this manner (CE2).Other systems which have been studied by this method are CO on platinum, rhodium, and gold (CE3) (with similarities to the spectra of the corresponding solid-gas interface), CO on platinum, HSO, and acrylonitrile on Au, and water on Ag (CE4), and reduction of C02 at a Pt electrode in Im H2S04(CE5). Polarization-modulated IR reflection absorption has been used to measure directly the IR spectrum of CO adsorbed on the surface of a Pt electrode in the presence of either 1M H2S04 of 1M HC104while the potential was swept from 15650mV with respect to the normal hydrogen electrode (CE6). Polarization-modulation is a double modulation method which is described in the section on Technique. Electromodulation of SiH and SiOH vibrational bands at a H-implanted Si-Si02 interface using multiple internal reflection has been reported (CE7) and the OH stretching vibration of water in the electrical double layer for thin films of Au(ll1) in 0.5M H2S04with small additions of water have also been studied by modulated infrared-attenuated total reflectance (CE8). (CF) On-Line Analysis. On-line analyzers based on the negative filter, the gas microphone, various kinds of solid filters, and circular variable filter spectrometers have proved themselves in the field over a considerable period of years. Tunable laser spectrometers are a more recent development which is finding use in on-line process control. All of these systems have relatively few moving mechanical parts which is desirable for reliability. NIR reflectance analyzers appear to have considerable promise for control of certain polymer processing operations, but as yet, there is little or no information available in the literature. The same can be said for FTIR and dispersive process analyzers. With computer control, these systems should be more flexible than the simpler analyzers, as is the computer controlled circular variable filter spectrometer. However, there is the nagging doubt about reliability in the harsh environment of the production plant where vibration and fumes abound. Actually, FTIR analyzers are used extensively for routine automated analysis of semiconductor wafers, but in a very carefully controlled environment, far different than the average chemical plant. Most of the literature on instrumentation and applications of process analyzers is in patents, which are not covered in this review. One interesting point concerning IR process analyzers is that it is not necessarily desirable for the process analyzer to resolve the band it measures. The dynamic range in concentration is much higher if the analytical band has positive deviations from Beer’s law. Griffith et al. (CFI) have reviewed the effect of varyin carrier gas on nondispersive IR gas analyzers. Maylott (CF27 has discussed improvement in boiler control by determination of CO, C02 and combustibles by IR analysis. Hartig (CF3) has discussed use of IR spectrometry for control of extrusion and coating processes in the manufacture of flexible packaging materials from various polymers. Wilks (CF4) has described a unique cylindrical internal reflection sampling device with very high energy through-put for on-tream monitoring by IR, designed with water solutions in mind. Jones (CF5) has described NIR process analyzers for determining one component in a binary or multicomponent process stream, for example, water in 1,l-dichloroethane from 0-50 ppm. Mitchell (CF6) has described use of an in-stream attenuated total reflectance IR analyzer for measurement of water in petroleum. DeMattia and Cardis (CF8) have described an on-line application of a dual-wavelength, single beam IR analyzer at critical points in the production of alcohols. Breton et al. (CF7) have described the SPECTRAN 677 process spectrophotometer for the IR, NIR, VIS region with examples of ita use. It is a single-beam dual frequency design, with a thermostatted measuring cell to ensure accuracy of measurement and long term stability. Bezukh et al. have described a multicomponent IR gas analyzer for measuring CO, hydrocarbons, NO, NO2,and C02in exhaust gases, and in thermal power plant and industrial plant waste gases. Jones and Wilks (CFIO)have discussed a C02 laser for analytical purposes. The C02laser can lase at approximately 100 highly intense and monochromaticlines in the 9-11 micron


region. If ever a means is found for switching, rapidly and non mechanically, among these lines, an extremely powerful process analyzer could be the result.

(D) BIOCHEMICAL APPLICATIONS Biochemical applicationsof infrared spectrometryhave been discussed in a comprehensive book by Parker (A3)who describes it as an extension of his earlier book. Only a very small fraction of the biochemical applications can be mentioned here. The selection has been made to indicate the types of application which are presently accessible to study by infrared spectrometry. Traditionally, infrared spectrometry has not been able to deal easily with biological materials, which contain much water, or with water solutions, although there is a great deal to be learned from the vibrational spectra of such systems. As a result of progressive improvement in instrumentation and techniques, some very good work is being done on samples which consist predominantly of water and some of these are mentioned below. For many years, infrared spectrometry has been an important analytical tool for the analysis of urinary calculi, kidney stones, etc. It is necessary for the physician to know the composition in order to assess the cause and prevent its occurrence. Quality control surveys of laboratories using chemical, infrared, or x-ray methods by the Deutsche Gesellschaft fuer Kliiische Chemie were most disappointing (01). Labs using IR spectrometry submitted the largest proportion of incorrect analyses for Ca oxalate and Ca phosphate. On the other hand, in another study, Corns ( 0 2 )has reported that IR spectroscopy using the KBr disk technique, was the single most useful method, being fast, simple to learn, using little sample, and in general permitting positive identification of most of the components found in renal calculi. One conclusion of the DGKC study was that the qualification of the operator is an important factor in the reliability of the results. This should surprise no one. Hesse et al. ( 0 3 )have established a computerized automatic evaluation of IR spectra of such samples, using a library of all substances which occur in urinary calculi. Use of infrared spectrometry and Raman microprobe have been compared by Daudon et al. (04), who discuss the advantages of the latter (which was able to identify over 60 mineral and organic components). As an example of the type of IR study which is now feasible, Jacobsen et al. (05),have monitored events occurring during the thermally induced fibrillogenesis (spontaneous self-assembly of a solution of collagen) in aqueous solution in real time by FTIR. Grant et al. ( 0 6 ) have studied the association of water and metal ions with heparin. Manafait and Theophanides (07)used FTIR to study the interaction of the drug adriamycin with cells. In aqueous dispersions of phosphatidylethanolamines the gel-to-liquid crystal and a higher temperature transition due to simultaneous hydration and chain melting has been observed in temperature studies by FTIR by Mantsch et al. (D8). Lipid phase transitions of fatty acid-homogeneous membranes of the organism A. laidlawii B was investigated by Casal et al. (09) by FTIR. The fatty acid of the membranes was enriched in deuterium by a novel and interesting method. The organism was grown in the presence of avidin, an inhibitor of fatty acid synthesis, in a medium containing pentadecanoic acid-d,. Since it was prevented from synthesizing the acid, it used the deuterated material from the surroundings. This allowed study of the temperature dependence of the CD stretching bands which lie in a window of the water solvent. References to the extensive applications of FTIR to biochemical problems by the group at the National Research Council of Canada, to which Mantsch and Casal and their coauthors belong, can be found in the bibliographies of these papers (D8), (09). Transfer of CO between Fe and Cu of the heme pocket of beef heart mitochrondria cytochrome oxidase has been studied at low temperature by FTIR by Alben et al. (010). Potter et al. (011) have reported that measurement of C-0 stretch bands provides a direct method for characterization of binding sites within intact human red cells. Nauman et al. (012)have reported on use of FTIR for the structural and analytical study of bacterial cell walls and on the possibility of differentiation and identification of whole bacteria. Parker et al. (013)have showed that 26-hydroxycholestero1 forms 1:l H-bonded complexes with the triglycerides triacetin and trilaurin and

suggest the H-bonding property of cholesterol as a possible factor in the mechanism of plaque formation in atherosclerosis. Kisner et al. (014) have reported a method for the simultaneous determination of triglycerides, phospholipids, and cholesteryl esters by infrared spectrometry using their ester carbonyl absorbance peaks. Band overlap and the resultant spectral interference were minimized by using analytical wavelengths on the band shoulders. Gendreau et al. (015) have described various spectroscopic techniques to measure adsorbed protein spectra, including transmission and attenuated total reflection with flowing aqueous solutions of single proteins and mixtures. The data were analyzed by spectral subtraction, derivation, and deconvolution. Gendreau et al. (016) have used FTIR to determine blood protein interactions with Ge and Biomer surfaces. Kellner et al. (D17)have used FTIR attenuated total reflectance for quality control of polyurethane-silicone blood-contact materials in the development of new artificial heart models. Desfonds et al. (018) have described a system for measuring CO transfer in lungs, consisting of a pneumotachograph (IR analyzer) with a microcomputer data system for automated correlation of flow and CO pressure and for determination of the average alveolar CO pressure. Gloor (019) has used IR spectrometry for measurement of hydration of the horny layer of human skin. High hydration values are sometimes found in children, the elderly, and patients with atopic dermatitis. Siebert et al. (020)have described kinetic infrared spectroscopy using a flash photolysis apparatus with monitoring IR beam which allows measurements of relative transmission changes of 0.001 in times of a few milliseconds. Results obtained on the photolysis of CO-myoglobin, rhodopsin and bacteriorhodopsin are discussed. The same authors have presented IR evidence for the protonation of two internal carboxylic groups during the photocycle of bacteriorhodopsin. Gendreau (022)has reported on modifications in detector and software of FTIR for the study of adsorption and desorption kinetics of whole blood proteins on a Ge ATR plate with a resolution time of 0.8s. Fritzche et a1 (023),by study of oriented gels of intact bacterial virus fd by IR linear dichroism, have estimated that the a-helical rods of the coat protein are aligned parallel to the long axis of the virion with an inclination of 3 7 O . Nabredryk and Breton (024)have used polarized IR spectroscopy to investigate the orientation of gramicidin A incorporated in dimyristoylphosphatidylcholine vesicles.

(E) ENVIRONMENTAL APPLICATIONS A program for on-site IR monitoring of occupational ex-

posure ethylene oxide sterilizers in hospitals has been described by Varnell (El).Use of long-path FTIR spectroscopy in kinetic studies of reactive molecules of atmospheric interest has been described by Calvert (E2).Advantages of IR spectroscopy over other methods for identification and determination of asbestos in microgram amounts has been discussed by Luoma et al. (E3). Verified diffuse reflectance spectra for 16 pentachlorobiphenyl isomers have been published by Nyquist et al. ( E 4 Use of factor analysis and least squares curve fitting for analyzing chromatographically separated PCBs has been described by Chem and Gardner (E5).Barbour and Jacobsen (E61have discussed use of FTIR for characterization of both organic and inorganic species in an environmental sample. Trividi et al. (E7)have reported that color IR imaging was useful in detecting chemical waste site locations. Digital pattern recognition techniques show unique reflectance characteristics for liquid chemicals, vegetation, and highways, (EA) Contaminants in Water. Goethel (EAI) has reported on the standardization of official IR methods for determining hydrocarbons in refinery wastewaters, with special reference to the French, German, and Swedish standard methods. Rotteri (EA2)reported that a task force of experts from the oil refining industry concluded that IR spectroscopy is the best available analytical technique for determining hydrocarbons in water and made recommendations on procedures to be used. Gucinski and Goupil (EA3)have reported use of total internal reflection IR to distinguish biogenic components such as proteins or insoluble polysaccharidesfrom anthropogenic hydrocarbons such as oil spills in Chesapeake Bay tributaries. ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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(EB) Aerosols. Bogard et al. (EBI) have described quantitative analysis of nitrate ion in ambient aerosols and application to aerosols from the Los Angeles basin. Johnson et al. (EB2) have studied variation of aerosol acidity with altitude using an ATR impactor installed aboard an aircraft. They detected acidic SO?- in samples collected in the Chicago area. Mudd et al. (EB3)have used a tunable COzlaser to study backscatter signatures of atmospheric aerosols as a function of wavelength. This method did not produce a spectrum comparable with transmission spectra of collected samples, but it allowed differentiation between sulfuric acid and ammonium sulfate aerosols. The sulfuric acid showed a well defined spectral feature near 1000 cm-l, but the ammonium sulfate showed only a continuous spectrum. (EC) Atmospheric Monitoring. Garvey et al. (ECI)have described the Exxon global carbon dioxide measurement system. It employs a computer based IR analyzer system aboard an oil tanker whose route traverses several oceans to determine the geographical distribution of atmospheric and ocean surface CO Zachor et al. (EC2) have reported the minimum detectabe quantities of various trace gases in localized clouds around stationary pollutant sources by FTIR emission. They discuss the problem of contrast between the pollutant cloud and the adjacent background. Pokrowski and Herrman (EC3) have reported on the detection of low concentrations of atmospheric pollutants by a dual beam diode laser spectrometer combined with a 64 meter White cell. The spectrometer has been used on board a research vessel in the North Sea for measurement of HC1 in the plume of incineration ships. HCl detection limit is 100 ppb for 1s measurement time. Herget (EC4) has reported on use of a mobile FTIR system for measurement of aseous pollutant emissions. Zachor et al. (EC5) have discussecftechniquesfor suppression of spatially and spectrally structured backgrounds when using FTIR for detecting and characterizing trace gases in a localized cloud. Hanst et al. (EC6) have reported on the rise and fall of pollutant concentrations during a smoggy 2-day period in Los Angeles using FTIR at 0.25 cm-' resolution with a 1260 m optical path folded along a base of 23 m. Georghiou et al. (EC7) have reported monitoring seasonal and daily variations in formaldehydein urea-formaldehyde foam-insulated houses in St. John's, Newfoundland by a real-time infrared spectrophotometric method. (ED) Atmospheric Trace Gases. Oelhaf and Fischer ( E D I )have investigated the feasibility of detecting 15 trace constituents of the atmosphere (03, COz, HzO,CHI, CO, NzO, NO, NOz,"OB, F-11, F-12, NH3, SOz, HF, and HC1) against atmospheric background by IR limb emission spectra between 4 and lOOpm at various tangent heights in the middle atmosphere. smith (ED2) has summarized the data for 52 atmospheric gas concentration profiles between 0 and 50 km altitude. SPECTROGAZ (ED3) is a computer readable file of spectral data and the interpretation of atmospheric IR transmission spectra from the Air Force Geophysics Laboratory. Resources required to measure concentration profiles on site at various altitudes and as a function of time and location are prohibitive. Considerable effort has been expended to recover the profiles by measurements at fixed altitudes (from the earth or from satellites). Current algorithms for inversion of IR measurements are the onion-peel and spectral inversion techniques. The onion-peel algorithm requires data from an instrument pointed at a number of altitudes while the spectral inversion technique is basically a deconvolution of high resolution spectral lines by inversion of the radiative transfer equation (B89). It is based on the dependence of line width and rotational line intensity distribution on pressure and temperature as a function of altitude. Abbas et al. (ED4)have described an approach which incorporatesthe onion-peel data collection scheme into the spectral inversion technique. Sharma and Zachor (ED5) have reported on tests of an inversion algorithm for spectrally resolved limb emission. Menzies et al. (ED6) have described the Balloon-Borne Laser In Situ Sensor (BLISS), a high resolution diode-laser absorption spectrometer designed to measure concentrations of stratospheric species and their diurnal variations. Murcray et al. (ED7)have described balloon borne remote sensing of stratospheric constituents using a 0.01 cm-l FTIR spectrometer for observing absorption against the solar background, and a liquid helium cooled grating spectrometer for observing 356R


IR emission. Flaud et al. (ED8) have measured daytime variation of atmospheric NOz from ground based infrared measurements using the FTIR spectrometer of the McMath solar telescope at Kitt Peak National Observatory. A rapid increase in NOz was observed between sunrise and noon, followed by a slower increase throughout the afternoon. Zander et al. (ED91have reported ground based measurements of FC-11, FC-12, and FC-22 using a FTIR spectrometer with the sun as source through the 9-13~atmospheric window.

(F) POLYMER APPLICATIONS Infrared spectrometry is indispensable for the analysis of polymers: for control of raw materials, for control of polymerization, for control of processing (blending,orientation, etc.), for analysis of failures, and for developing an understanding of polymer physics and chemistry. It is virtually the only method which can readily study the chemical functionality of both the crystalline and the non-crystalline components of polymer systems. For many years, infrared spectrometry has been used by the polymer supplier to control his product and keep track of the competition, and it has been employed by the polymer user to keep the manufacturer honest, for general problem solving, and also to keep track of his competition. Sample preparation is a major problem in polymer analysis by infrared spectrometry. The polymer analyst needs access to all of the major techniques: transmission, attenuated total reflectance, reflectance-absorption, diffuse reflectance, photoacoustic, emission, polarization, evolved gas analysis, sizeexclusion chromatography combined with IR, microspectroscopy, etc. These are discussed in Section H on Sampling Techniques. The spatial resolution of IR techniques is insufficient for observation of the microstructure of polymer blends and filled or partially crystalline polymers, but a sequence of spectra of a single sample vs temperature, time, or stress-strain gives considerable relevant information. This is nothing more than the time tested approach of the physical chemist, using IR as the measuring device. This technique requires excellent computer data processing capabilities: spectral subtraction, multiple regression, factor analysis, etc. These are discussed in Section I on Software and Algorithms. The reader is referred to a book by Haslam and Willis5for a comprehensive discussion, with many examples, of use of infrared for analysis of polymers. The physical chemical aspects of the application of infrared spectrometry to polymers has been covered in a comprehensive and up-to-date review by Koenig (BIOI).Culler, et al. (B88)have reviewed infrared methods for studying polymer interfaces. Coleman et al. (€387) have reviewed application of FTIR to the study of polymer blends. Painter et al. (A23) have published a book on the theory and application of IR to polymers. Koenig and Kormos (FI) have developed methods for determining the spectra of pure conformational components of amorphous polymers based on factor analysis of a range of samples of different conformational composition. Reversible and irreversible spectral changes have been demonstrated in a temperature study of solvent-cast polyethylene terephthalate between 30 and 320' by Lin and Koenig (F2). Stevens (F3) has followed the cure kinetics of a low-epoxide hydroxyl group-ratio bisphenol A epoxy resinanhy ride system, observing consecutive esterification and simultaneous etherification and both epoxide-OH group and carboxylic acid dimer H bondin Hartshorn (3'4) has studied the kinetics of air-drying of soytean-oil alkyd and linseed-oil coatings, following hydroperoxide formation/decomposition and oxidation and crosslinking paths. (FA) Polymer Degradation. Chen et al. (FAI) have studied degradation and interactions of epoxy resins and their copolymers by comparison of the spectra of copolymers with summation spectra of their respective components. Pearce et al. (FA2) have investigated thermal and thermooxidative degradation of poly(ethy1ene terephthalate). Spectra as a function of time and temperature were analyzed in terms of products formed, rate of formation, and relative stability of various functional groups. Thermal degradation was studied by monitoring the IR spectra of evolved gases. Evolved gas analysis (EGA) is a very powerful technique for studying degradation, flammability, etc. of polymers. It has been reviewed by Lephardt (B57). Webb et al. (FA3) have described in-situ study of photo-

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chemical degradation of bisphenol A polycarbonate coatings on Au and A1 substrates as a function of temperature and ambient atmosphere. Debe and Tushaus (FA4) have studied the bulk-to-surface diffused residues formed by long-term exposure of a polyurethane adhesive to heat and humidity. Such long term experiments place a great premium on the stability of the spectrometer. FTIR is outstanding for such applications because of its laser based frequency calibration. Aceves et al. (FA5) have used IR spectrometry to assess the deterioration of electrical properties in polymeric insulating materials on long term outdoor aging. (FB) Microscopic Defects. Fanconi (FBI)has used FTIR to study changes in end group concentrations during cyclic fatigue of ultrahigh molecular weight polyethylene. Ragimov et al. (FB2) have observed growth as a function of time of a C=C band a t 1630 cm-' in polyethylene subjected to a high electric field. Rowe and Tobazeon (FB3) have reported the reversible appearance of a band at 2910 cm-l in the spectrum of high-d polyethylene sheet subjected to a voltage step. Shabadash and Razmakhnina (FB4) have reported a method for determination of porosity of polymers for pores of size from 10 to hundreds of A by desorption under heat and vacuum in a specially designed cell. Subsequently, sorption of CHC13 was followed up to the saturation point by IR spectrometry. Kim et al. (FB5) have simplified the quantitative determination of OH groups in polymers by use of THF as solvent. Under the conditions used, 30 mequiv/L, all OH groups of the polymer are associated with THF and there are no apparent free or self-associated OH groups. (FC) Polymer Blends. Coleman et al. (B87)have assessed application of FTIR to the study of crystalline compatible polymer blends. One interesting aspect of the stu y of polymer blends by IR spectrometry is that a great deal can be learned from deconvolution of bands in relatively narrow regions of the spectrum, i.e., C=O or CH stretch regions. The molecular vibrations associated with these bands are very highly localized and less sensitive to complications than bands at lower frequencies. The success of deconvolution is totally dependent on the precise freauencv measurement caDabilitv of FTIR spectrometers. Koenig and Tovar Rodriquez ( F C I ) have described use of factor analysis of FTIR spectra to determine the number of comDonents in blends of Dolwhenvlene oxide with Dolvstvrene (compatible) and with pbli@-chiorostyrene) (incompa6ble). Varnell et al. (FC2) have used FTIR to elucidate the specific interactions in miscible polyester-poly(viny1 chloride) blends. Garton (FC3) has used FTIR to study competitive equilibria in miscible polymer blends and low molecular weight analogs. (FD) Polymer Interfaces. Niviroj et al. (FDI)have detected Si-0-metal bonds between an aminopropylsilane coupling agent and metal oxide surfaces by diffuse reflectance FTIR and studied the hydrolysis of these bonds in water. The same authors (FD2) have studied the influence of carbon dioxide in air, pH, and drying conditions on the structure of partially cured films of a hydrolyzed aminopropylsilane on AgBr plates and E-glass fibers. Young et al. (FD3)have reported the feasibility of using FTIR in combination with diffuse reflectance to study graphite fiber reinforced epoxy, polysulfone, and polyimide composites exposed to thermal and radiation environments. Xue et al. (FD4)have investigated the mechanism of polyester fiber reinforcement of rubber by FTIR. They studied chemical reactions between an epoxy functional silane and model compounds representing the main chain of the polyester and its terminal hydroxyl and carboxyl groups. (FE) Polymer Orientation. Levy (FEI) has reported difference spectra, for polarization parallel to the stress axis, of 4 stress sensitive bands of Kapton 50H which reveal information on conformational changes induced by stress. Kissin (FE2)has reported two polarized IR methods for measurement of orientation of isotactic polypropylene, uniaxially stretched at various speeds and temperatures. The method for the crystalline phase, determines precisely the orientation in three orthogonal directions using the polarization characteristics of the 841 and 809 cm-l bands. The method for the amorphous phase, provides semiquantitative information on uniaxially oriented film based on the polarization characteristics of the 1155 cm-' band. Hobb et al. (FE3) have described a method for characterizing surface molecular orientation in three dimensions using a special ATR attachment

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which allows a wide range of well defined incidence angles for depth profiling. Tse and Sung (FE4) have described a sample holder, using a double-edged, square-parallelogram, which permits orthogonal rotation of the sample while maintaining identical contact, for determination of surface orientation of polymers. (FF) Time Resolved Polymer Studies. There are two general methods for time resolved FTIR studies. The first is a straightforward repetitive scan procedure which is limited to approximately 0.1 to 1s time resolution and makes use of the procedure and software used for GCIR. The second, is an exotic procedure in which a separate scan is required for each sample point of the interferogram, in a stroboscopic fashion (see p139-40 of reference (BIOI)for more detail). If sufficient points are taken, if the timing is sufficientlyaccurate, and if the sample does not change with time, the points can be assembled into an interferogram and transformed in the usual manner. Koenig (BIOI)has pointed out that polymers can undergo thousands of cycles if the strain level is low, and the latter condition can be easily met in such an experiment. Fateley and Koenig (FFI) have used this technique with polypropylene which was cycled at 10 hz with an elongation of one to five percent. The spectra showed reversible intensity changes but no frequency shifts. It should be recognized that such experiments require very careful attention to detail if serious errors are to be avoided6. Siesler (FF2) has discussed results of rheooptical FTIR studies at ambient and elevated temperatures for a series of model polyester urethanes in terms of phase separation and segmental orientation. Siesler (FF3)has described the method and apparatus for determining time-resolved spectroscopic and mechanical data simultaneously during deformation and relaxation, and its use for urethane rubbers. Burchell and Hsu (FF4)have described a miniature closed-loop servo-controlled hydraulic tester interfaced to a FTIR spectrometer, and its use for determining microstructural changes in ethylenemethacrylic acid copolymer zinc salt with a time resolution of 1500ps. Lasch et al. (FF5) have described use of this apparatus to follow the microstructural changes in polyethylene and polybutylene terephthalate.

(G) INFRARED INSTRUMENTATION Grangaard ( G I ) has described the laboratory calibration facilities and typical measurements at the Aerospace Guidance and Metrological Center at Newark Air Force Station which has the highest echelon of traceable calibration standards for the US. Air Force, including the laser and IR measurement areas. Hass (G2),(B67) has described methods for measuring the reflectance of front-surface mirrors at various wavelengths and angles of incidence, together with techniques for preparing reflecting films with maximum reflectance and durability. Leonard (G3) has reported the results of an investigation by the University of Dayton Research Institute of many different types of IR polarizers to optimize selection for ellipsometric instrumentation constructed for the Air Force High-Energy Laser program. (GA) FTIR Spectrometer Systems. Philbrick (GAI)has described the data system and software for a commercial FTIR spectrometer. Persson et al. (GA2)have described a liquid nitrogen cooled grating spectrometer for use with a piezoelectric scanned Fabry-Perot interferometer which achieves resolving powers between lo2 and lo6 in the 1 to 5 pm region. Huppi et al. (GA3) have reviewed cryogenically cooled FTIR spectrometers. Kendall et al. (GA4) have described a series of interferometric spectrometers with maximum optical path difference of close to 250 cm, offering maximum resolutions of 0.0025 cm-l and resolving powers of >lo6. Buijs et a1 (GA5) have reviewed FTIR hardware developments, including a description of dynamically aligned interferometers and prephase characterization to permit real time phase correction. Hawkins et al. (GA6) have evaluated the calibration of a FTIR spectrometer by measuring a lar e number of standard lines and found an accuracy of 3 x 10-!cm-l over intervals of several thousand wavenumbers. de Haseth (GA7) has described various operational tests for evaluating the stability of rapid scanning interferometers which should be of interest to many users of FTIR systems. These tests should be useful for all FTIR users for monitoring ANALYTICAL CHEMISTRY, VOL. 56,

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the performance of their systems, and for FTIR purchasers for use in acceptance testing. Several papers have discussed phase correction in FTIR (GA8), (GA9), (GAIO), ( G A I I ) . Phase correction is a very important factor in quantitative intensity measurement and everyone who uses FTIR for quantitative analysis should understand the phase correction algorithms and how to test the phase correction in his own system. Griffiths (B77) has reviewed recent commercial instrumental developments in FTIR. This should be useful reading for anyone intending to purchase a new FTIR system because it describes the differences between the interferometer systems of the major FTIR spectrometers. Birch (BI2) has reviewed recent progress in dispersive FTIR; Minami (BI3) has reviewed FTIR in Japan; and de Haseth (BI5) has written a general review on FTIR. Nafie and Vidrine (B27) have reviewed double modulation FTIR. (GB) Tunable Laser Spectrometers. Lindon and Mantz (B60) have reviewed tunable diode lasers and laser systems for the 3 to 30pm region. Nickles et al. (GBI) have described a narrow band tunable laser source, tunable over the wavelength range 2.8-5.6pm. Rohrbeck et al. (GB2) have described a specially designed CO-laser gain tube which allows operation from 1220-2000 cm-'. Kyro et al. (GB3) have described a computer controlled acoustic color-center laser spectrometer capable of scanning in 100 cm-l sections over the wavelength range from 2.2 to 3.3pm with a resolution of 0.01 cm-l. Bachem et al. (GB4) have described a modular Pb chalcogenide diode laser IR spectrometer for single or multicomponent gas analysis. Davies et al. (GB5) have described a high sensitivity single beam diode laser spectrometer for study of transient molecules. (GC) Dispersive and Filter Spectrometers. F e r n (GCI) has described the design and applications of the InfraAlyzer 500. Spragg and Woodhead (GC2) have described the Perkin Elmer Model 983 dispersive IR spectrometer. Truett and Furlong (GC3) have described the MIRAN-980 computer controlled spectrometer for multicomponent analyses. Matsuzaki et al. (GC4) have described a multichannel IR spectrometer using a pyroelectric videcon detector for spacecraft-borne IR spectroscopy. (GD) Detector Arrays. Stefanovich and Sibille have described a 2-dimensional 32 x 32 InSb IR detector array with sensitivity comparable with that of high performance single element InSb detectors. Bailey (GD2) has described a 128element linear array of InSb detectors with a Si FET switched multiplexer and HFET preamp for readout. McCreight and Goebel (GD3) have reported on Si:Bi charge injection device arrays with background sensitivities comparable to that of good discrete detectors. They also report on a monolithic InSb CCD array and improved Ge:Ga detectors for wavelengths longer than 3Opm. It appears that the technology for infrared detector arrays is moving rapidly. There must be analytical infrared applications which can benefit from this technology, i.e., tiny multicomponent variable filter devices, systems for viewing the distribution of functional groups in polymers by their infrared absorption, various infrared microscope applications, etc. (GE) Absorption Cells. Solomon (GEI) has described an experimental kinetic reactor for the purpose of studying entrained-flow coal pyrolysis-gasification at 700-1200°. Coal is injected into a heated gas stream in a hot furnace. In-situ and external-cell species analyses are performed by FTIR. Miura and Gonzolez (GE2) have described a flow system including an IR cell with the capability of operating either as a pulse microreactor or as a single-pass differential flow reactor for in-situ surface characterization studies of working catalysts. Kinney and Staley (GE3) have described a photoacoustic cell for obtaining mid IR spectra of surface species. (GF) Infrared Optical Materials and Fibers. Hemeda and Tilloca (GFI)have described properties of IR transmitting glass ceramics in the arsenic germanium selenium system. Beswick et al. (GF2)have identified alkaline earth rare earth sulfides as an interesting new class of broad band I!R windows with better thermal and mechanical properties than ZnS or ZnSe. Lucas et al. (GF3) have described the potential of heavy metal fluoride glasses for optical windows and waveguides in the mid-IR. 358R


Miyachita and Terunuma (GF4) have determined the optical transmission of arsenic sulfide fiber in the infrared. Measured attenuation was 164 dB/km at 5.3pm (1dB = absorbance of 0.1). Plotnichenko et al. (GF5)have reported on fiber waveguides from cesium iodide. Optical loss in CsI is less than 0.1 dB/km (0.01 absorbance) for COz, CO, DF, and HF laser wavelengths. Hobrock and Sneed (GF6) have reported on recent advances in IR optical fibers. It appears that some of these fibers would be useful for analytical applications of IR. They should permit placement of the IR spectrometer and its sensor at a location remote from the site of the sample to be analyzed, i.e, for IR analyses at inaccessable locations, or to allow multiple analytical locations to be serviced by a single spectrometer. Plotnichenko and Sysoev (GF7) have described an automated setup for measurement of fiber optical waveguides transmittance spectra in the region 0.8 to 25pm. The setup allows determination of the total optical losses in waveguides from 0.3 to 2 mm in dia. within the dynamic range 0.0022 to 30 dB.

(H) SAMPLING TECHNIQUES Numerous reviews have appeared on infrared sampling techniques. Gough and Scoles (B2)have reviewed optothermal (photoacoustic) infrared spectroscopy, as has Vidrine (B28), (B91). Dignam (B8) has reviewed IR ellipsometric spectroscopy. Birch (BI2)has reviewed dispersive FTIR. Guenthard (B20),and Mamantov et al. (B36) have reviewed IR-matrix spectroscopy. Maulhardt and Kunath (B23) have reviewed diffuse reflectance. Nafie and Vidrine (B27) have reviewed double modulation FTIR. Griffiths and Fuller (B56) have written a comprehensive review on mid-infrared spectrometry of powders. Leone (B69) has reviewed infrared fluorescence. Bewick (B90) has reviewed in-situ infrared spectroscopy of the electrode/electrolyte solution interface. Griffiths et al. (BI00) have reviewed capillary GC/FTIR. A number of novel sampling schemes have been described. Kivinen et al. (HI) have described the stretched polymer method in which benzene derivatives were dissolved in a polymer film and their polarized spectra studied after orientation by stretching the film. Kraentz and Kunath (232) have described use of the Hademard transform technique in combination with FTIR to produce spatially-resolved spectra of the sample. Radiation falls on the sample through a 2-dimensional Hademard mask which is moved under computer control by stepper motors. This is an interesting alternative to raster scanning. Honovich and Dunbar (H3) have described a technique in which cations trapped in an ion cyclotron resonance (ICR) mass spectrometer undergo enhanced visible photodissociation in the presence of IR radiation. They state that the IR wavelength dependence near 9.7-10.7pm (COz laser wavelengths) exhibits features related to the IR spectroscopy of the ions. The effect is attributed to changes in visible-absorption cross section for vibrationally excited ions (by the IR laser). Thus, ICR is used as the detector for measuring the IR spectra of ions. This is an interesting technique, but it is not clear at this time what its analytical possibilities may be. Kellar (H4) has described three mechanisms for laser modulated electron capture detection wherein the sensitivity of electron capture is combined with the selectivity of absorption spectrometry. Lambert (H5) has reported a technique in which IR absorption by oriented adsorbed molecules which possess a 1st order Stark effect is modulated by an alternating electric field, after the manner of microwave spectroscopy. The advantages and limitations of this Electroreflectance Vibrational Spectroscopy (EVS) technique are discussed. Farrow (H6) and Knapp and Hanson (H7) have discussed the technique of optical Stark-modulated absorption spectroscopy to provide spatially resolved concentration measurements of CO in flames. A continuous-wave IR probe beam from a diode laser was crossed with a nonresonant high-intensity beam from a Nd:YAG laser to generate a Stark-induced change in absorption in the crossing volume. This technique has been applied to flames. Staat et al. (H8) have analyzed various random and systematic errors in determination of IR extinction coefficients of liquids. Cameron et al. (H9) have given algorithms for


computation of bandwidths, and center of gravity and least-squares frequencies with maximum uncertainties of hundredths or thousandths of a wavenumber. (HA) Reflection-Absorption. A relatively strong infrared band of a monolayer on a well defined surface such as a crystal plane has an absorbance of 0.0002 to 0.0005. This is not readily observable except under very special conditions. Smooth metal surfaces present a special problem, and an opportunity. The problem is that at normal incidence, radiation forms a standing wave with a node at the surface of a conductor. Since absorption intensity depends on the electric field of radiation at the absorbing species, radiation incident at normal incidence does not interact with adsorbed molecules. This selection rule holds only when E is parallel to the surface and not for radiation at oblique incidence if E is polarized in the plane of incidence (perpendicular to the surface) and only for layers thinner than a quarter of a wavelength. The opportunity is that absorption by monolayers on metal surfaces is strongly enhanced at a high angle of incidence for radiation polarized in the plane of incidence. This enhancement is approximately proportional to the secant of the angle of incidence. It amounts to approximately a factor of 20 near 8 5 O incidence. A polarizer is used to cancel out the component which is not absorbed. This technique is known as reflection-absorption infrared (RAIR or IRRAS). This technique was originally used in the infrared by Francis', and later by Greenler who first realized the enormous intensity enhancement. Relatively large samples are required because the length of the beam image is also proportional to the secant of the angle of incidence. For very strong absorption bands and even moderately thick samples, interpretation is complicated by mixing of absorption with reflection, However, this is not a problem for monolayers. If the plane of polarization in RAIR is alternated rapidly, the absorption is modulated, effectively separating absorption by the surface from all other effects. This technique is known as polarization modulation. It is also described as a double modulation technique. For a FTIR spectrometer, the modulation frequency, 50-100kc, is significantly higher than the highest modulation frequency of the interferometer. Lock-in detection is used. The result is a technique which can readily be applied to well defined metallic surfaces. The results are spectacular because only the absorption by the adsorbed species is modulated. Absorption by gas in equilibrium with the surface and other artifacts are virtually eliminated. Boerio and Gosselin ( H A I )have described the principles of RAIR and the design of experiments for obtaining IR spectra of thin films on metal mirrors. Dignam and Baker (HA2) have described what they call a particularly promising approach which employs a polarizing Michelson interferometer (PMI) and a multichannel detection system. They think this should give a sensitivity improvement of at least 10-fold over the best instruments currently available. Ishitani et al. (HA3) have compared ESCA and FTIR/ RAIR to study oxide layers of the order of lOOA on Cu plates. Generally they find ESCA and RAIR similar in sensitivity for oxidation of copper, with the two providing complementary information. For organic layers, RAIR gave more information on functionality than ESCA. However, ESCA provides more definitive identification of atomic species and their valence states. Greenler (HA4) has compared RAIR with electron energy loss spectroscopy (EELS). RAIR has higher resolution and can operate in the presence of ambient gas, whereas EELS has less stringent selection rules, and a wider spectral energy range. Marcotte (HA5) has compared four experimental approaches for polarized RAIR. He reports the best results for the FTIR polarization modulation technique. Golden and Saperstein (HAG) have reported on a reflection-absorption technique incorporating a switching circuit into the data acquisition electronics of an IBM IR 98 FTIR spectrometer to obtain both reference and PO arization modulated spectrum simultaneously. (HB)Surface Enhanced Infrared Absorption. There have been several reports in recent years of enhanced ATR absorption in the presence of very thin conducting films. For example, Hatta et al. ( H B I ) report ten times enhancement of infrared absorption of p-nitrobenzoate formed on an Ag island film -5nm thick. Only vibrational modes involving a

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change in the dipole moment of the benzoate ion perpendicular to the macroscopic Ag surface were enhanced by ppolarized radiation alone. The mechanism of this enhancement is not fully understood, but it appears to depend on excitation of surface electromagnetic waves in the metal. Ueba and Ichimura (HBZ) discuss use of the Kretchmann configuration to excite surface electromagnetic waves (SEW). This produces a large field at the metal-air interface for IR frequencies. In the Kretchmann configuration two right angled prisms in close proximity to the metal surface are used. Radiation incident normal to the hypotenuse of first prism, at 45O to the surface, exits tangential to the metal surface as a SEW, and is picked up at some distance by the second prism. A surface film can interact with the SEW. Since the distance between the prisms is much greater than the thickness of the film, high absorption intensity is possible. SEWS are related to surface polaritons, which have been extensively studied (AZI), (B43), (B44), (B70). This technique promises high sensitivity by virtue of the long paths which are feasible tangential to the surface. It also promises to be difficult to setup for two reasons: (1) the delicate contact of the prisms with the surface and (2) the very narrow acceptance angle for the IR beam, which is also a function of wavelength. Thus, if the parallel beam from a tunable laser with very low beam divergence is used, the angle of incidence may need to be scanned with the wavelength. Zhizhin et al. (HB3) have compared surface electromagnetic wave spectroscopy (SEWS) with IR reflectance-absorption (RAIR) and shown that SEWS is an order of magnitude more sensitive than RAIR for 4-octadecylphenol Langmuir films on Cu. It appears that this is an area which could blossom forth in the near future. Aravind et al. (HB4) have presented calculations which suggest another geometry for surface enhanced IR spectrosCOPY. (HC) GCIR. On-the-fly acquisition of infrared spectra from gas chromatographs has become a routine analytical technique with sensitivity in the low nanogram range (HCI), (HCZ), (HC3), (HC4). A number of the commercial spectrometers are equipped with data processors which can carry out the Fourier transform and Gram-Schmidt reconstruction in real time. At least one commercial system can carry out a simultaneous library search to identify chromatograph peaks. Searching is discussed in Section I on Software and Algorithms. State of the art GCIR appears to be approaching state of the art GCMS in sensitivity and in utility. It is more or less standard to reconstruct the GC trace itself from the interferogram by the Gram-Schmidt algorithmlo.One might ask why not use a standard chromatograph detector for quantitation and to define the position of the GC peaks. Perhaps the best reason is that the Gram-Schmidt peaks are internally synchronized with the infrared spectra, whereas there is always some uncertainty about the relative timing of the IR signals and those of an auxiliary detector. Considerable effort is being expended on improving the interferogram reconstruction algorithms with gratifying results (HC5),(HC6), (HC7), (HC8). So far, we have discussed GCIR methods which measure the gas spectrum. The possibility of using automated matrix isolation FTIR in obtaining the spectra of GC fractions has been reported by Garrison et al. (HC9).More recently, at the 1984 Pittsburgh Conference, Reedy8 et al. reported on subnanogram sensitivity for GC matrix isolation IR. Five percent of the carrier gas is argon. The capillary column exit is very close to the gold plated surface of a helium cooled, slowly rotating copper drum. The helium is pumped off but the argon freezes down on the surface of the drum, trapping the GC peaks. Time resolution is a few seconds and the drum has a capacity of five hours. The spectrum is measured by reflecting a focussed beam from the gold surface of the drum. The spectra are spectacular because of the low temperature. The specificity can be much higher than with gas spectra, but a new library must be built to achieve the full potential of this technique. (HD) LCIR. The use of LCIR has not yet become a routine operation like GCIR. The major reason for this is that no practical chromatographic solvent is really transparent in the infrared. To be sure, Renzepis and Douglas ( H D I ) have reported on the transparency of liquid Xenon in the infrared and spectra of solutions in other rare gases have been reported, ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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but these are not practical LC solvents. In any case, the usual eluents for reversed-phase chromatography, alcohols, water, and acetonitrile, are not compatible with IR detection. Combellas et al. (HD2) has described the characteristics of a liquid chromatograph interfaced to FTIR which permits spectra of solutes to be obtained using chlorinated eluents and a conventional small volume flow cell. Limit of detection is about lpg injected; incompletely resolved chromatogram peaks can be resolved by spectral subtraction. Brown and Taylor (HD3) have compared LCIR with on-line sampling using a low volume transmission flow through cell for 1-mm internal diameter microbore and 4.6-mm internal diameter analytical scale columns. The combination of FTIR detection with gel permeation chromatography (GPC) is a particularly interesting one. FTIR is sensitive to chemical functionality but not to molecular weight, while the reverse is true for GPC. Thus, the two in combination complement each other very well. Simultaneous determination of molecular weight for individual components in polymer mixtures can be accomplished. Foelster and Herres (HD4) have described use of this technique for the separation and identification of nitrocellulose, alkyd resin, urea resin, and plasticizer from a commercial wood lacquer. Direct observation of the effluent solution has the advantage over the solute isolation techniques described below in that the solute is observed in a constant matrix (solution), uncomplicated by crystallization polymorphs, etc. Shafer and Griffiths (HD5) have attacked the LCIR solvent problem by use of super-critical COz as the eluent. They state that the transparency of COz just above the critical point makes it a nearly ideal solvent, allowing a 1-cm path length. (Let the reader be reminded of the hazard of working with supercritical gases). A more general technique for avoiding the solvent transparency problem is to evaporate the solvent and measure the spectrum of the isolated component by transmission, diffuse reflectance, etc. This is a slow but very useful procedure which has been used since before the development of HPLC. Attempts to automate this procedure have met with some success using a technique developed by Kargers for combining mass spectrometry with HPLC. Duff et al. (HD6) have described a device for on-line removal of aqueous and organic solvents from HPLC effluents by continuous extraction of solutes from the effluent into CHZClz,followed by separation of the aqueous phase and concentration of the organic phase by about a factor of ten. The solute is evaporated to dryness on KC1 powder and its spectrum is measured by diffuse reflectance. Jinno et al. (HD7), (HD8) have described a related technique in which the solute is deposited directly onto a moving KBr plate which is fed through the beam of an IR spectrometer. They used a fixed wavelength, but clearly the technique can be used with FTIR to determine complete spectra of each fraction. They use the term buffer memory for the KBr plate because it retains the sample for further diagnostic treatment, such as chemical derivatization, x-ray fluorescence, etc. (HE) Time Resolved Studies. There are a number of other situations in which the automated scanning capability of computer controlled spectrometers, especially FTIR spectrometers, can be used to great advantage for elucidating the mechanism of chemical reactions and other useful information. Automation of the data collection encourages a dense population of sampling points which facilitates measurement of slopes, detection of changes of slope, detection of induction periods, and differentiation between overlapping reactions. Many of the problems of chemical kinetics can benefit by the multicomponent functional group information provided by such IR techniques. One of the really exciting techniques is Evolved Gas Analysis (EGA). Lephardt (HEI),(B51) has described EGA in some detail, with examples from pyrolysis and combustion of tobacco and pyrolysis of polyvinyl chloride. Basically, the effluent from a thermogravimetric analyzer (TGA) is passed through a heated IR cell in a FTIR spectrometer while the temperature is ramped up continuously. Lephardt used a standard FTIR GC gas cell and software to measure suectra repetitively during tge operation. Dollimore and Hoath xHE2) have reported on an EGA study of the thermal degradation of cellulose. 360 R


The advantages of continuous observation of weight loss or gain by TGA over batchwise weight measurements are well known. EGA provides the same type of advantage over batch type IR measurements. Gas analysis has the advantage over in-situ observation of changes in the solid that the identification of the evolved gas species is often unequivocal, whereas the interpretation of the IR spectrum of the residue is often open to question. Actually, the two techniques complement each other. Rousch, et al. (HE3) have described an automated thermogravimetric analyzer/FTIR interface. Gas solid reactions can be studied as well as decomposition of solids simply by using a reactive flushing gas. The holdup volume of TGA apparatus is rather large, but the heater for pyrolysis GC can be used with a GCIR cell. A word of caution, however, Lephardt (B51)has pointed out that precautions must be taken to prevent carry over of liquid droplets, etc. which can coat the reflecting surface and windows of the GC gas cell. This reviewer’s enthusiasm for EGA comes from unpublished preliminary experiments on low temperature decomposition of solids in a quartz tube inserted in place of the GC column in a GCIR oven, and observation of the transient species in the activation of a catalyst in a flowing activating gas. Use of the EGA technique can be expected to expand rapidly in the study of flammability and combustion, catalysis, and especially in engineering studies at the pre-pilot plant level. Another application of time-resolved IR is the use of heated ATR to monitor polymer transitions by Rousch (HE4) who interfaced a microprocessor temperature programmer to FTIR. The lass transition of epoxy polymers, coated on fiberglass, was letermined by heating while spectra were being obtained. The glass transition was observed as a sharp increase in intensity of all absorption bands, due to flow of the softened epoxy into better contact with the ATR plate. In another application, time-resolved FTIR spectra of glucose anomers in aqueous solution provided new information on vibrational bands of a and /3 anomers. Koenig ( F C I ) has reported temperature dependent studies of polymer spectra combined with factor analysis to determine the spectra of pure conformations which could not be isolated in the pure state. Cameron, Mantsch et al. (D8),(D9) have used this technique effectively in biological studies. (HF)Diffuse Reflectance. Diffuse reflectance of powders is covered extensively by Griffiths and Fuller (B56).Griffiths claimed a t one time that diffuse reflectance FTIR (DRIFT) is the most sensitive IR sampling technique. While there are now other techniques which rival or exceed DRIFT in sensitivity, there is no question that it is the most sensitive technique for routine operation. It is being widely used. Simple, relatively inexpensive, diffuse reflection attachments are commercially available. It requires minimal sample preparation and is one of the first techniques to try on a variety of odd shaped samples which need not be powders. It was once thought that a FTIR spectrometer was required for diffuse reflectance,but Hannah and Anacreon (HFI)have shown that the diffuse reflectance attachments work with a dispersive spectrometer for a great many applications. Serna et al. (HF2) have shown that a certain amount of caution is necessary in interpreting or comparing the spectra of certain crystalline powders which have very strong infrared bands. Surface mode absorptions of such materials are dependent on shape and size of the powder particles. Lack of consistency among reported IR spectra of powdered corundum type oxides is due in part to the different particle shape of the samples used. Van Every et al. (HF3) have described a cell for spectroscopic study of adsorbed molecules on catalyst surfaces. It allows gases to be drawn through a powdered adsorbent at temperatures up to 400° and a catalyst can be activated insitu. Smyrl et al. (HF4) monitored the heterogeneous reaction of lithium hydride and lithium hydroxide with water and carbon dioxide by diffuse reflectance FTIR. They describe a bakeable high vacuum cell, based on a modification of a commercial diffuse reflectance IR cell, together with means for controlling the gaseous and thermal environments. Richter (HF5) has described spectral measurements of directional incidence and hemispherical reflectance with a normal multipurpose FTIR spectrometer in conjunction with


an integrating sphere with a commercial rough texture, gold coated inner surface. (HG)IR Emission. The high sensitivity of FTIR spectrometers makes feasible the measurement of IR emission spectra. While IR emission is relatively simple to measure, there are a number of traps for the uninitiated. The major advantages are lack of dependence on sample shape and the possibility of measuring vibrational bands against a null background instead of as the difference between sample and reference beam. Actually, sample shape is of some importance in the sense that the sample should be optically thin; the spectrum of an optically thick sample is useless because it approaches that of a black body. Hvistendahl et al. ( H G I ) have discussed this point with examples. The null background concept is somewhat of a fiction since even for an optically thin sample, the surroundings contribute to the background unless they are at the temperature of the detector. The spectrometer itself also contributes to the background unless it is at the same temperature as the detector. Thus, the best emission measurements are made with either heated sample and spectrometer and detector at ambient temperature, or with sample at room temperature and spectrometer and detector at reduced temperature. Beware of using a cold detector with a spectrometer at room temperature! Unless the sample is quite hot, emission spectra suffer from low sensitivity in the high frequency region of the spectrum (where the functional group assignments of absorption bands are on the firmest ground). Van Woerkom and De Groot (B41) have described infrared emission spectra from a heterogeneous catalyst system in reaction conditions, and Van Woerkom (HG2)has described insitu study of curing of organic coatings. Chiang et al. (HG3) have studied carbon monoxide on nickel using a liquid helium cooled grating spectrometer to measure emission from a room temperature sample mounted in an ultrahigh vacuum system. (HH) Photoacoustic Spectroscopy. At one time it was thought that photoacoustic spectroscopy in the mid infrared could only be accomplished by FTIR, but Low and Parodi ( H H I ) have demonstrated that useful photoacoustic measurements can be carried out with a single beam dispersive spectrometer constructed from a Perkin Elmer Model 421. However, such measurements are much easier with the normal FTIR spectrometer. Mehicic et al. (HH2)have described analytical applications of photoacoustic spectrometry using FTIR. There are two major advantages of photoacoustic measurements. The first is the same as for diffuse reflectance: almost no sample preparation is required. The second is that sensitive samples can easily be transferred and sealed into the photoacoustic cell under an inert atmosphere. Krishnan et al. ("3) have applied polarized FTIR photoacoustic spectroscopy to determine dichroism of the surface of drawn polyethylene terephthalate film and compared the results with those measured by ATR. The depth of penetration was greater for PAS than for ATR, but the PAS measurements could be performed without touching the surface of the sample with an ATR plate. Griffiths et al. ( H H 4 ) have compared photoacoustic and diffuse reflectance FTIR and concluded that each has its strengths and weaknesses and the strengths of one tend to be the weaknesses of the other. Hence, IR labs which are required to characterize a large variety of samples should have sampling accessories for both. Actually, the disparity between the speed of the two is decreasing as design of photoacoustic cells is improved. Photoacoustic measurements which once took a major fraction of an hour are now accomplished in a minute or two. Photoacoustic spectroscopy has two major disadvantages. The most serious of these is that the background for complete absorption is the energy output function of the spectrometer. This is usually determined by measuring the spectrum of a supposedly totally opaque sample like carbon black. The span between total and zero absorbance must be determined from this background spectrum. As an absorption band becomes stronger, its photoacoustic intensity approaches the background intensity. It is the difference between these two intensities which must be evaluated to determine the absorbance of the sample. This can never be as satisfactory as the case for transmission or diffuse reflectance in which the transmittance of a perfectly absorbing band is zero and the span from zero to 100% transmission can be determined from

internal measurements on the spectrum itself (baseline density method). The baseline density method cannot be used with either emission or photoacoustic spectra. A second problem of photoacoustic spectra (but not emission spectra) is that any infrared absorbing gas, i.e., water or C02,couples much more strongly to the photoacoustic detector than to the surface of a solid or liquid sample. This interferes with use of photoacoustic techniques for studying adsorption of surfaces in equilibrium with IR absorbing gas. This reviewer is familiar with a case in which it was noticed that C02was being evolved unexpectedly from the photoacoustic sample in its sealed chamber. This would never have been detected in a diffuse reflectance or transmission measurement. Teng and Royce (HH5)have discussed the procedure for obtaining quantitative FTIR photoacoustic spectra of solids and liquids. A normalization routine is discussed but its use tends .. to nullify the speed advantage of photoacoustic sampling. (HI) Photothermal Beam Deflection. Photothermal beam deflection spectroscopy is a type of photoacoustic spectroscopy which does not require the sample to be enclosed in a tiny cell. The sample is placed at the focus of the modulated beam from the spectrometer. Heating of the atmosphere in contact with the surface is detected by a mirage detector. This consists of a laser beam passing in very close proximity, parallel to the surface. Deflection of the laser beam passing through the temperature gradient caused by absorption of the infrared beam is detected photoelectrically. This technique is due to Boccarra who first used the technique in the visible. In a more recent paper Fournier, Boccarra, and Badoz ( H I I ) claim that this technique is three orders of magnitude more sensitive than conventional photoacoustic spectroscopy. Low and Lacroix (H12) describe a FTIR spectrometer designed to accommodate a photothermal beam deflection system. Low et al. have also described applications of this technique to measurements on the surface of large objects which could not be studied by conventional PAS W 3 ) , measurements on adsorbent surfaces in opaque regions usually inaccessable to transmission methods (H14), measurements on carbon routinely under rigorously-controlled conditions required for surface studies (HI@,and for characterization of catalyst surfaces (H16).

(I) SOFTWARE AND ALGORITHMS The distinction between the functions performed by the spectrometer hardware and the computer software is less distinct for FTIR systems than for dispersive systems, or, more generally, for time scanning than for frequency scanning systems. In this section, the line will be drawn to include those functions whose algorithms are of operational interest to the spectroscopist. Thus, the software which controls the frequency or modulator drive mechanism, the slits, the mirrors, and the filters and the FFT itself is clearly outside of our present scope. Software for phase correction and for checking noise level, tracking accuracy and photometric accuracy, although falling within the scope of this Section, is discussed in Section G on Instrumentation under FTIR. Generally speaking, the spectroscopist who is staying abreast of the state of the art is interested in understanding the algorithms for every function which affects his work. G. H. Morrison, in an editorial in Analytical Chemistry", has commented on the reluctance of some instrument vendors to share information on software with users. Generally, this reviewer has found the IR vendors willing and eager to discuss their software and algorithms with users. In his specific case, source listings were provided in computer readable form without a non-disclosure agreement. To be sure, the listings were in the form of sparsely commented assembly code, but the authors were willing to discuss the details over the telephone. Availability of these listings provided a ready solution to a number of problems which would have been very difficult to solve without them. Documentation of software remains a major problem. This is not because the software vendors have not tried. The average spectrometer data system has so many functions which can be combined in so many different ways, that it is very difficult to write manuals which are intelligible to the novice while at the same time providing the technical detail needed by the experienced user. ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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There is still another aspect of documentation which seems largely overlooked. This is the function which allows the user to document his sample, its origin, sample handling methods, cell parameters, and the data enhancement procedures used on a spectrum. The instrument manufacturers have done an excellent job of including the instrumental parameters, the date and time, etc., which can be stored in a fixed header on each spectrum. The positions of these parameters in the header is generally compiled into the system, and for compatibility of current data with archived data, this form is for most practical purposes unalterable. Griffiths et al. (11) have published specifications for Infrared Reference Spectra of Molecules in the Vapor Phase as defined by the Vapor Phase Subcommittee of the Coblentz Society Spectral Evaluation Committee. Grasselli, Griffiths, and Hannah (12) have defined criteria for the presentation of spectra from computerized IR instruments. Each includes a long list of information which should be provided with spectra for publication. And, this list is by no means complete because as new sampling and data processing techniques are invented, new types of information will be needed. It is the very strong opinion of this reviewer that all of this information should be entered into the spectral data files at the time the experiment is performed, and not held in someone's head or on a scrap of paper, etc. Usually the file header contains some space for user documentation, but this varies all the way from enough bytes to accomodate the sample number to quite a sizable block. Most systems allow one or two lines. This is far from adequate. Modern data storage techniques are fully capable of handling variable length data fields and addition of new fields as the need arises. To this reviewer's knowledge, only one IR vendor has implemented such a system. Until recently, every IR vendor used a different computer, and much of the software was machine dependent. The trend seems to be toward a use of the same processor by a large number of the vendors. What is more important is that there is a trend toward a standard operating system, Unix or Unix-like, which will facilitate the exchange of software. This is all to the good for the user; the effect on the vendor is less certain. It takes little imagination to forsee the formation of IR software vendors like in the area of sampling accessories, but with a major difference. A hardware accessory can be loaned for trial, or sold on consignment, or rented without too much risk of being copied because the cost of copying is usually greater than the cost of the mass produced hardware item. However, with software, the fist step after purchase is to make a back-up copy which costs virtually nothing. There is also the problem of standardization of the digital representation of the spectral data itself. The frequency accuracy claimed for current FTIR spectrometers is such that spectra should be interchangeable between machines. If not already possible, this should be the common goal. The Joint Committee on Atomic and Molecular Properties is embarking on a program in cooperation with the instrument manufacturers to standardize the form of digital spectra for such exchange purposes. Once the capability of transmission of digital spectra to remote locations via public communication lines becomes simple, a variety of new activities become possible: submission of spectra to specialists for interpretion, better communications between experimentalists and theorists, establishment of archives for reference s ectra, rapid exchange of results between members of a roun -robin study group for analytical methods, settlement of disputes between vendor and customer in product acceptance analyses, etc. The public spectral archive problem also is an important one, Thousands of spectra are stored in the archives of individual companies. In many cases the demand for these spectra does not warrant their inclusion in a commercially available data file. Yet, when the effort has been applied to prepare, purify, and characterize a chemical compound, and carefully measure ita infrared spectrum, the spectrum should be made available to the scientific community at large. In most cases, the owner of such spectra would be willing to share the data if the mechanism existed to do so. (IA) Spectral Libraries and Searching. IR search falls into two broad categories: search of libraries containing high quality spectra and search of libraries containing highly compressed spectra. The latter type of search is used for automated identification of GC fractions where speed and compactness are of the essence.

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Schaarschmidt (B62)has described the state of the art and development trends of computer aided comparison of infrared spectra. The availability of fully digitized high resolution spectra makes possible the matching of reference spectra and unknown spectra with accuracy approaching that of a human. In the case of weak and noisy spectra, computer comparison is superior, especially in the case where a human can distinguish no bands at all. Olsen (IAI),for example, has reported on the the performance of full spectral search to identify spectra lost amidst noise. Koehler et a1 (IA2) have described a system for computer-aided storage, retrieval and analysis of fully digitized IR spectra with examples of applications. Lowry and Huppler (IA3) have described an interactive retrieval system which allows the spectroscopist to retrieve spectra from a library based on the peaks that he feels are most important, with emphasis on the ability to obtain easily the full spectrum of selected reference compounds. Sprouse and Boruta (IA4) have described an automated spectral searching system with automated microfiche display of photographs of full spectra from a library of 100,000compounds. Delaney et al. (IA5) have described a procedure for quantitative evaluation of a library search system which can be used to compare any form of spectral representation or any spectral comparison metric with the results of searching a library of full-intensity, full resolution spectra by a least squares metric. Kwiatowski and Riepe (IA6) have discussed the general procedure of library matching. They point out that for practical use of library search methods in interpreting the spectrum of an unknown, it is essential to distinguish between the search for the spectra of the identical compound, the search for partial structure, and the identification of mixtures. (IB) Compressed Libraries. Present computer data systems cannot quickly search through the largest commercial library of fully digitized spectra which contained 24,000 spectra at the time Sprouse wrote the paper cited above (IA4). For automatic on-line identification of GC peaks, some sort of speedup is necessary. This is accomplished by encoding the data by various techniques or by structuring the data base to search only pertinent regions. The latter is somewhat dangerous since an error which chooses the wrong branch of a structured data base normally results in failure. (The error could be due to an impurity in a reference spectrum, or in the unknown itself.) The largest available library of infrared spectra is the ASTM IR data file which contains nearly 140,000 very highly abbreviated spectra. These data were prepared by encoding absorption peak position by visual examination of spectra. Each spectrum is represented by between 100 and 250 useful bits of spectral information, and about 350 bits of chemical structural information. Many years ago, Erly showed that effective searches could be performed using a subset of this data consisting of 96 bits of IR data (wavelength without intensity) and 32 bits of chemical information. There are many errors in the data, but the library is effectively used in public search systems. Tanabe et al. ( I B I ) have described a new search program for this library which makes use of subdivision into 15 subfiles to inprove the running time. Wood (IB2)has described the SPIR (Search Program of IR Spectra) for the ASTM IR data file operated online by the Canada Institute for Scientific and Technical Informaton (CISTI) of the National Research Council of Canada. Various ways of compressing IR spectra which do not depend on encoding of the data by humans are being developed. Actually, the spectral representation is highly redundant, and various mathematical methods can be used to remove the redundancy. These methods can be divided into two classes, those which operate in the spectral domain, and those which operate in the interferogram domain. Azarraga et al. (IB3) have described an interferogram based search which is claimed to be more tolerant of noise than a spectral domain search. Actually, a relatively small portion of the interferogram contains most of the information needed for an effective search. Novic and Zupan (IB4) have also reported on use of libraries of truncated time domain (interferogram)spectra for accelerating computer searching. They report use of such data for qualitative analysis of mixtures. Hangac et al. (IB5) have reported on use of factor analysis in a Karhunen-Loeve


transformation to achieve a &fold reduction in storage requirements for data in the spectral domain. Basically, this procedure involves use of factor analysis to reduce the redundancy of the spectral data. More recently, Owens and Isenhour (1B6) have reported effective searches using a library prepared by compression of time domain representations by a factor of 100 through single bit quantization. This was done by representing the time domain points, with 1 for positive values, and 0 for negative. In a sense, this is the time domain analogue of the ASTM coding system. Warren and Delany (1B7) described what they call width-enhanced binary representation in the spectral domain. The library contains frequency and width information but not intensity information. They claim adequate library searching is achieved after 25-fold reduction in the number of channels needed to describe vapor phase infrared spectrum. This reviewer tends to view these fast search techniques as useful for screening, but not for absolute identifications. No matter what the quality of the hit index, the user should always verify identifications by comparison between full resolution unknown and known spectra, with a little human judgment thrown in for good measure. This is the only way to prevent an unacceptably high percentage of ridiculous results, unless the range of unknowns submitted to the search procedure is strictly limited to compounds which are in the library. (IC) Computer Assisted Interpretation. Woodruff and Smith ( I C I ) have presented a detailed description of the process by which rules are generated for PAIRS, a computerized interpreter of IR spectra which attempts to parallel the reasoning of spectroscopist.Trulson and Munk (1C2)have described an IR interpreter program based on a table driven procedure. The interpreter reads tables which contain information on peak position, intensity, and shape for the regions which are considered diagnostic for each class functional group. Bink and Van’t Klooster (1C3) described a structure correlation procedure for classification of organic compounds by their IR spectra using pattern recognition and information theory. This procedure was used to separate a class of 40 compounds containing a tertiary butyl group from a data set of 549 IR spectra. Zupan (IC4) has described the generation of a hierarchical tree of 500 IR spectra, using the recently proposed 3-distances-clustering (fractal) method. (ID) Quantitative Multicomponent Analysis. Maris et al. (101) have reported on evaluation of matrix calculations for least squares regression analyses on multicomponent spectrophotometric calibration data using absorbance as the independent variable. Overdetermination by using additional standard mixtures for calibration enhanced greatly the accuracy of most analyses. Bartick et al. (102)described use of a commercially available software package (QUANT) for analysis of copolymer films. Mann et al. (103)examined use of cross-correlation in quantitative analysis for systems with overlapping peaks. Haaland and Easterling (104)discussed improvements in previous least squares regression analysis of IR spectra for quantitative determination of multicomponent mixtures. Spectral baselines and overlapping spectral features are accounted for by the fitting procedure. (IE) Resolution Enhancement. Resolution enhancement and noise reduction (smoothing) are linked to each other in the sense that resolution enhancement increases the effective noise level, and smoothing decreases the resolution. Very low noise is required for effective deconvolution because the noise level increases by about a factor of 10 for a factor of two improvement in resolution. Jones and Shimokoshi ( I E I ) have discussed various aspects of resolution enhancement of spectral data by the method of self-deconvolution. The terms self-deconuolution and Fourier self-deconuolution refer to the same process. The precise connection of the prefii self is not clear to this reviewer. These terms normally refer to deconvolutions which are carried out in the Fourier domain but they can be used to generate convoluting functions which can be applied in the spectral domain. Kauppinen et al. have described smoothing of spectral data in the Fourier domain in one paper (IE2) and deconvolution in another (IE3).Actually, smoothing and deconvolution are essentially apodizations in the Fourier domain with different types of functions. For smoothing, the function decreases with increasing retardation, and for deconvolution

it increases; the precise shape depends on the degree and type of smoothing or deconvolution which is desired. Maddams and Southon (ZE4)have described use of 2nd and 4th derivatives for resolving peaks in overlapping pairs of bands. Yang and Griffiths (1E5) have discussed the relative merits of second derivative and self-deconvolution methods. Sasaki et al. (IE6) have described a method based on principle component analysis and constrained nonlinear optimization for estimating the spectra of pure components from spectra of mixtures. See also (FI). Gillette and Koenig (1E7) have described the application of factor analysis to reduce random noise in FTIR spectra. (IF) Interference Fringe Removal. Interference fringes are an artifact which an infrared spectroscopist has to contend with whenever the sample has two smooth parallel surfaces. One of the computationaltechniques for removing fringes from the spectra of semiconductor wafers is to excise the secondary and tertiary interferograms (signatures) before carrying out the Fourier transform. This never seems to work quite as well as expected, and Baghdadi (IFI) has pointed out the reason why. Removal of the signatures is basically an apodization. Clearly the same procedure must be applied to the background interferogram as well. For very thin samples, such as polymer films, it is often very hard to locate the fringe signatures because they lie in a region of the interferogram which has relatively high amplitude. Moffatt and Cameron (IF2) have suggested a simple means for locating the signatures in such cases.

ACKNOWLEDGMENTS This reviewer wishes to acknowledge the patience and assistance of his wife during the preparation of this review. He is grateful to the editors of Analytical Chemistry for encouragement relative to submission of the manuscript in computer readable form, and to Marianne Brogan (ACS) and Rod Temo

Infrared spectrometry - Analytical Chemistry (ACS Publications)

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