conditions if such a situation should occur, there are many alternative positions that the misplaced chemist may elect. These combined economic forces are more powerful than any force with n-hich they may be opposed. Another point raised by Kolthoff concerns a tendency of industrial laboratories to ask for specialists. This occurrence must be exceptional, for the importance of the greatest breadth to training has long and often been stressed (1, 5 ) . However, such a proposal may sometimes reflect a desire by the employer to permit a graduate student, interested in continuing some particular phase of work, to bridge the transition from school to industry by working in a field related to his university experience. One student who came to discuss a position wanted to work exclusively with polarography. While i t happened that the particular spot in which the man would have started would have been in polarographic analysis, he was discouraged from coming to work, as it was the company’s wish to give men the broadest experience their capabilities permitted, and not to have them continue indefinitely with a single technique, The practice of analytical chemistry is too broad to be attractive to the true specialist. CONCLUSIOYS
The symposia of the analytical division are important in educating analytical chemists in subjects which they never encounter until they begin the practice of analytical chemistry. The summer symposium of a year ago was concerned with standard substances and with some of the problems involved in writing specifications. How many of our newer chemists realize the intimate relationship between the requirements of a specification and the fabrication of an analytical method? To what better text, than the record of that symposium, can one refer a chemist for information of that kind? Ideally the chemist wishes to look a t his measurements in absolute terms, and to believe that a value
is independent of the method or means of the measurement. It is a process of advanced education, and not of uneducation, to teach him that this is seldom true. Whether it be the density of benzene or the results of the Michaelson-Morley experiment, the result cannot be separated from the means by which it is obtained. By attempting to understand the objectives or goals of our educational efforts, we may not clarify the vork of the teacher, but we may help the chemist himself better to qualify for the career he has selected. We may better evaluate the results of the efforts of the practicing chemist by realizing that they are not to be judged by the standards we use for fundamental scientific contributions, but by their own criteria-not necessarily comparable. Clear-sightedness will prevent us from condemning scientific contributions for lacking in economic importance, and practical contributions for lacking in conceptual importance. The two are complementary, mutually indispensable, and supplement each other. LITERATURE CITED
(1) Ashley, S. E. Q., J . Chem. Education, 19, 589-96 (1942). (2) Chirnside, R. C., Analyst, 70,110-18 (1945). (3) Chirnside, R. C., Cooper, B. S., and Rooksby, H. P., G.E.C. Journal, 17,207-16 (October 1950). (4) Clarke, B., Ind. Eng. Chem., 23,13014 (1931). (5) Clarke, B., J. Chem. Education, 14, 561-3 (1937). ENG.CHEM.,ANAL.ED., (6) Clarke, B., and Hermance, H. W., IND. 7,218 (1935). (7) Kolthoff, I. M., Chem. Eng. News, 28,2882-937 (1950). (8) Lundel1,G.E. F.,IND.ENG.CHEX, ANAL.ED.,5,212-15 (1933). (9) Mees, C. E. K., Chem. World, 1, 15 (1912). (10) Rosenbaum, C., Chem. Eng. News,28,3578-81 (1950). (11) Stillman, J. W., J . Chem. Education, 27, 147-50 (1950). (12) Van Brunt, C., Gen. Elec. Rev., 39, 88 (1936). (13) van Nieuwenburg, C. J., Anal. Chim. Acta, 2, 419-24 (1948). RECEIVED for review September 15, 1952.
.4ooepted October 2, 1952.
5th Annual Summer Symposinm-Ingredients of Unknown Constitution
Infrared, Ultraviolet, and Raman Spectroscopy Application to Analysis of Co.mp1ex Materials 0. D. SHREVE, Marshall Laboratory, E . I . du Pont de Nemours & Co., Inc., Philadelphia, Pa.
AXY problems arising in research, development, and manu-
facturing activities require the analytical examination of complex materials, many of which are of unknown or illdefined chemical composition and structure. Such materials include petroleum and coal tar fractions, animal fats, vegetable oils, essential oils and other plant extracts, and natural and synthetic resins. This paper discusses and illustrates with selected examples the unique advantages as well as some of the limitations of infrared, ultraviolet, and Raman spectroscopy as applied to the analysis of such materials. INFRARED AND RAMAN SPECTROSCOPY
Both infrared and Raman spectra have their origin in the characteristic vibration and rotation frequencies of molecules. I n general, each vibration frequency which involves a change in dipole moment will give rise to an infrared absorption band. Each frequency which involves a change in molecular polarizability will give rise to a Raman scattering line. Because of these requirements, an infrared band may be prohibited and a Raman line allowed for a given vibration in a given molecule or vice
versa. Thus, one type of spectrum will sometimes furnish information not obtainable by the other, or a supplementary use of both may be desirable. From the point of view of most general analytical applications, however, the area of overlapping in type of information furnished by Raman and infrared spectra is relatively minor. This fact immediately leads to a consideration of the factors governing a choice between the two methods for general analytical purposes. Until recently the measurement of Raman spectra required long-time photographic exposure followed by microphotometer scanning of the photographed Raman lines. The recent advent of photoelectric techniques in Raman nork, however, has placed Raman spectroscopy on a par xith infrared with respect to time requirements for obtaining a spectrum. Where sampling considerations permit the obtaining of a good spectrum, the Raman technique has some distinct advantages over infrared. Because an infrared spectrum exhibits a multiplicity of absorption bands arising from overtone and combination frequencies, as well as fundamental frequencies, the spectra of most compounds are very complex, and serious band overlapping is usually encoun-
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2
Infrared, ultraviolet, and Raman spectroscopy are first evaluated in a broad general way in terms of the type of fundamental information obtainable by each, the comparative scope and limitations of each, and other considerations governing the utility of these techniques from the practical analytical point of view-. Follow-ing this general evaluation, attention is focused on selected examples involving practical applications of these techniques in the analytical examination of complex materials of ill-defined composition and structure. In this connection emphasis is placed on limitations as well as advantages and the necessity for judicious combination and integration of spectroscopic methods with other methods of analysis in attacking difficult problems.
tered in the spectra of mixtures. For this reason, quantitative infrared analysis of multicomponent miytures can seldom be performed without setting up and solving a series of simultaneous equations based on the absorption laws. h Raman spectrum, on the other hand, usually consists of a relatively small number of sharp, well defined lines arising from fundamental vibrations only, thus minimizing difficulties due t o band overlap. This fact plus the linearity of the intensity scale of a modern photoelectric Raman spectrophotometer represents a distinct advantage in quantitative analytical work. A h o t h e r point in favor of the Raman technique is the fact that the entire vibrational spectrum is available for analytical use, while in the case of infrared the usable range is limited by the absorption of the priam employed. Raman spectroscopy, unlike infrared, is not subject to the disadvantages arising from the susceptibility of rock salt optics and cell materials to water in the atmosphere and in the sample. Despite these inherent advantage5 of the Raman technique, however, infrared methods afford, a t the present time, a more flexible and practically useful approach in general analytical work. Solids, liquids, and gases can all be handled readily by infrared methods, while Raman methods are for practical purposed largely limited to examination of materials in the liquid state. I n general, much smaller samples can be successfully studied by the infrared technique and the presence of color and/or fluorescent properties in the sample presents no difficulty. The tendency of many materials to fluoresce under the intense exciting radiation, and/or the presence of color in the sample, often precludes the possibility of a successful Raman analysis. Because of the greater flexibility and broader scope of infrared methods in diversified analytical applications and other circumstances such as the earlier availability of commercial infrared
spectrometers in this country, infrared methods are today much more widely accepted and extensively employed than Raman methods in analytical work. I n fact, the phenomenal groivth and development and almovt exponentially increasing use of infrared spectroscopy in a diversity of applications during the past decade have brought this technique to a point where analytically useful information can now be rapidly obtained on almost any type of sample even in extremely small amounts and in any state of aggregation. Many of the disadvantages of infrared methods have been overcome or circumvented by the ingenuity and intensive research engendered by the widespread interest and activity in this field. ULTRAVIOLET SPECTROSCOPY
Both infrared and Raman spectra arise from the excitation of vibrational energy levels associated rvith the complex and unique set of interatomic vibration frequencies characteristic of a specific molecule. Selectiveultraviolet absorption, on the other hand, arise,. from exitation of electronic energy levels associated with certain specific resonating groups called chromophores within molecules. The more important practical analytical consequences of thia fundamental difference may be summed up as follows: The scope of ultraviolet spectroscopy is limited by the fact that numerous compounds do not exhibit selective absorption in the ultraviolet region of the spectrum. By contrast, all organic and many inorganic materials will exhibit analytically useful infrared or Rainan spectra, if sampling considerations permit the obtaining of such spectra. (Hon-ever, the limited scope of ultraviolet methods represents an advantage rather than a disadvantage when an ultraviolet-absorbing component is to be determined in the presence of nonultraviolet-absorbing materials.)
k I cn z a Ik2
Infrared Spectrum of Typical Alkyd Resin
T h i n film cast from solution on rock salt plate
.\I1 ultraviolrt spectrum, while to some extent characteristic of the individual molecule, serves primarily to establish compound class and is rather severely limited in it,s ability to distinguish a specific caompound under study from :ill others. An infrared or llanian spectrum, on the other hand, uniquely characterizes :in individual compound and serves also for the detection of many functional groups and atomic configurations not detectable by the ultraviolet method. Bwause water is transparent i i i the ultraviolet, this technique is well adapted to the study of materials in aqueous solution and is relat,ively free from many of the difficulties cbncountered in t.he application of the Raman technique to aqueous solutions. While useful infrared spectra have been ohtained directly in aqueous solution ( 7 ) , i t is usually desirable tp resort to one of the inaiig ingenious alternative sampling techniques now available for solid samples for which suitable infrared transparent solvents are not available. Because of t,heir greater iiiherent sensitivity, ultraviolet methods, where applicable, are usually superior to infrared or Raman methods for the deterniination of very low concentrations of compounds in niistures.
III the. analysis of complez, I mtterials both infrared (or Haiiian) and ultraviolet spectra WAVE LENGTH-MICRONS Mill often contribute t o the Figure 2. InCrared Spectra of Common Oils Used as Modifiers in Alkyd Resin ~olutionof the problem, both Manufacture together lidding more information than can be obtained by either alone. I n the autlior'c n ork, n hic*li involves The three strong baiids in tlic 8- to 10-micron region are characthe analytical examination of an iilnioqt .ibtronomical v x i c t y of teristic of phthalate cFters n i t h some contributions from the complex materials usrd in the surface roatings indudry, many ester linkages of thp oil modifier. The two bands a t 13.5 and problems ai e solved by infrared and/or ultraviolet tcrliriiqurs 14.2 microns arisc from vibration of the +disubstituted aromatic donc. Mole often, hon ever, a judivious combiliation of \prrtroring in the phthalic structure and the doublet near 6.2 niicrons rcopic niethotls supplemented by chemical arid other rnct hods probably arises from overtones of these vibrations. The band of analpis is requircd foi a complete solution. a t 2.0 microns is due to unestcrificd glycerolic hydroxyl groups Oil-Modified Alkyd Resin. The first example has been and that a t 5.8 rnicroiis to carboiiyl groups in both the phthalate chosen to illustrate the iriterdepcndencc of infrared and ultraand oil inodificr structurez. These assignments pluc: the bands violet methods, as ne11 as the dependence of both on chcinical attributable to various carbon hydrogen vibrations a t 3.3, 6.9 methods in analytical w ~ i kthus , pointing up some of the limitaarid 7.3 miwons acrount for all but a few very weak bands of tions as well as the unique advantages of the spectroscopic uncertain origin. Thus, wliile such a spectrum will distinguish methods. The exaniplc refers to the problem of complete anaa phthalate-type alkyd from those based on other dibasic acids lytical charactcrixatioii of an oil-modified alkyd resin, a cornplcx and establish the presence or absence of other modifying resins t y p e ot resin cxtensively used in paints arid varnishes. (such as nitrogen resins, phciiolics etc.), i t yields no information .4n alkyd resin is a polyester type of resin, the esterification about the specific pol) hydric alcohols or oil modifiers present. product of one or Inore polyhydric alcohols with one or more h i ultraviolet spectrum run on the original resin yields little dibasic acids and vegetable oil fatty acids. The most common or no information of analytical value. However, by a proper type of alkyd comprises glycerol as a polyhydric alcohol, phthalic combination of infrared and ultraviolet techniques with chenlical anhydride as the dibasic acidic component, and one or more procedures such a resin can be rather completely characterized nondrying, semidrying, or drying oils as the oil modifier. I n in terms of its various components. recent years, alcohols other than glycerol and dibasic acid? other I n order to accoinpli~hthis, such a resin is resolved into a than phthalic are coming into iricreasing use in such rec;ins. dibasic acid fraction, an oil modifier fraction, and a polyhydric Figure 1 shows the infrared spectrum of an alkyd of the oilalcohol fraction by saponification m-ith absolute alcoholic potasinodified glyceryl phthalate type. sium hydroxide using the conventional Kappelmeicr procedure
V O L U M E 2 4 , NO. 1 1 , N O V E M B E R 1 9 5 2 ( 1 1 ) . This treatniciit prcc.il~it,atcsphthalic acid quantitatively thc potarsiuni salt and thc drying oil acids aiid polyhydric :ilrohol fractions can be isolated from tho filtrate by convcntional c~Iirmic.alprocedures. If a straight alkyd is being analyzed arid ~ilithalic.is the only dibasic acid present, the phthalic content of the original resin is estimated from thc weight of the dried potashiuni pht,halatc precipitate. However, if othcr diba5ic acid? :ire prrscnt or if thc xaniplc contains uiisaponifiable resin, noncmtrifugablc piginents, or other inaterial insolublc in t h e sal)onifiration nicdium, thcse n-ill be ivcighcd 1yit.hthc pot :it r, t,lius rcndc~ingthci graviinctric Kappelxncicr proocdurc for I)hthalic inapplicable. 111 order t o c-ircumvcnt this difficulty :in ultraviolet spcctrophotoiiictric~ method has bccn develope 1 for phthalic: acid in tlie prtwiicc of such interfercwces (83). In this proccdurc thc usual liappelmeicr prcripit,atc is cxtractcd \\-it,ti ivwtcr and, afttr at*itlific.ation, appropriate dilution, arid t ~ d i b r a t i o nthe ~ phthalic acid content of this extract is calcuIatrd from a siiiglc alisorhanicc mcasurement a t 276 niw n-hcw phthxlic: acaiti exhitJits $1 strong alxorption maximuni. This nitthoci, n-hirh is a p l ) l i c d h in thc prcsencc of saturatrd aiid uio~iounsaturatcddiha&ic acids as \\--ell as many other materials whirh defcat, the co~~vrntionitl Kappelnieier mcthod, is being Coinniittec D-1. cvsluated by Snl)-Group S I of .i.S.T.lI. of the niixturc of drying oil fatty a d s isolated from thc filtrate in thtr :tl)ovc. 1)rocdurcusually requires conibined :IS
Figure 3. 1. 2. 3. 6..
Soybean oil Alkali-isomerized soybean oil Alkali-isomerized linoleic acid Alkali-isomerized linolenic acid
1695 chemical and 9pectroscopic technic1uc:s. I n general, infrared and ultraviolet mcthods. supplement each other here, but ultraviolet is the more powerful tool. The limitations of infrared alone in drying oil analysis arc illustrated by the spectra of Figure 2. (These spectrit wcre run on the original oils as glycerides rather than on fatt,y acids from saponification, but the comparative statemcnts made belon. hold equally well for the drying oil acids.) Curve 1 of Figurp 2 is labeled linseed oil (L.O.), cottonsred oil (Cott.O.), and soybean oil (S.B.O.) because the infrared spc:c.tra of these three oils are, for practical purposes, indistinguiehahle. Coconut oil (Cn.0.) can usually be distinguished by tliffercnces in the 8- to 9-micron region as compared with other common oils. Tung oil (Chinawood oil, C.K.0.) andsdehydrated cast’or oil (D.C.O.) always exhibit doublet absorption near 10 microns due to conjugated double bonds not present in ot,her oils. This distinction is not too reliable in practice, however, as oils originally unconjugated may acquire conjugation as a result of past history. Castor oil (not shown) canbedistinguished bystrong hydroxyl absorption and some other differences, but hydroxyl may arisc in other oils from bodying, blowing, or other oridnlive effects, thus rendering this distinction more difficult. Conipletc quaiitit,ative component analysis of an oil in terms of fatty acid composition is impossible by infrared. Ultraviolet spectrophotometry is now widely used in oil arialy& as the result, of work by Mitchell and Kraybill ( I T ) , Brice ( 2 , S ) , and others. The methods used are based on the fact that ( a ) the doubly and triply conjugated polyunsaturated acids occurring in certain drying oils absorb strongly a t 234 and 268 mp, respcct,ively, and ( b ) the nonconjugated nonabsorbing polyunsaturated acids can be converted t o a reproducible extent t o the corresponding conjugated absorbing forms by a so-called alkali-isomerization technique ( 1 7 ) . Figure 3 shows the ultraviolet spect,ra of a sample of soybean oil before (curve 1) and after (curve 2 ) such an alkali isomerization treatment. Bccauer thc original oil (curve 1) contains little or no conjugation, the ahsorption is low. On alkali isomerization, however, t\yo strong maxima develop a t 234 and 268 mp as a result of isomcrizat,ioii of nonabsorbing nonconjugated double bonds and triple bonds, respectively, to the conjugat,ed absorbing forms. Curves 3 and 4 show spectra (after isomerization) of pure 9,12linoleic and 9,12,15-linolenic acids which together comprise the polyunsaturated portion of soybean oil acids. Using the known absorptivity values a t 234 and 268 mp for alkali-isomerized acids, the complete acid composition of such an oil in terms of linoleic, linolenic, oleic, and saturated acids can readily be calculated from the observed absorptivities of the alkali-isomerized sample a t t,hc two analytical wave lengths, together with the iodine value of the original oil. If an oil contains conjugated component’s before isomerization, these can also be determined from the two nieasurcd absorptivities of the original oil when suitable calibration data on pure conjugated acids are available. I n many practical problems, however, complete acid component analysis is unnecessary. If previously determined average absorptivity values at’ 234 and 268 m l on known oils are available, unknowns can be directly identified by comparison of their observed absorptivities a t thcse wave lengths with those of the known samples. While the infrared method is not too useful for the direct idcntification of oils, this technique is a very valuable supplementary tool for revealing t.he presence of styrene, rosin, phenolic: resins, and other materials often blended m-ith or reacted with drying oils in finishes technology. When such materials are’ thus shown to be present, the ultraviolet oil analysis and related’ procedures must be appropriately modified. As an example,, Figure 4 shows comparative infrared spectra of dehydrated castor. oil and a “etyrenated” dehydrated castor oil. The presence of st’yrene is revealed by strong aromatic bands a t 13.2 and 14.3 microns and by other spectral differences. The polyhydric alcohol fraction of an alkyd resin is now handled by chemical methods, although spectroscopic methods may be
1696 potentially useful with suitable preliminary processing of this fraction. I n addition to the chemical separation procedures for complete characterization of an alkyd, other supplementary and/or confirmatory chemical tests are usually necessary, particularly when the sample represents something less straightforward than a glyceryl phthalate-type resin modified with a single drying oil. Compound Type or “Group” Analysis in Examination of Complex Materials. Component analysis for individual compounds presupposes knowledge of the exact constitution of the compounds to be determined. I n the case of complex, ill-defined materials, such information is often unavailable. Information sufficing for the practical purpose a t hand will often result, however, if the ’types and relative amounts of various functional groups present can be established. Many functional groups within molecules give rise to infrared absorption bands whose frequencies remain nearly constant with changes in the molecular weight and remaining structure of the moleculesin whichsuchgroupsreside. Thus an absorption bandobRerved in the spectrum of a complex material canoften be attributed t o a functional group common to a number of molecular species, each of which contributes to the absorption but none of which is unequivocally identifiable. However, the appearance or disappearance of such a band or changes in its intensity with external treatment of the material often afford valuable information in the solution of practical problems. For example, in Figure 2, the infrared spectrum of atmospheric bodied linseed oil has been included for comparison with that of the original oil. A4s a result of oxidative attack, a hydroxyl absorption band has appeared a t 3 microns. More interesting is the fact that a new band has appeared a t 10.36 microns. Infrared spectral studies carried out in the author’s laboratory in collaboration xith workers a t the Eastern Regional Laboratories of the U. s. Department of Agriculture (25) together with the work of others (19,20)have shown that this band in fatty materials arises from unsaturated components having the trans configuration a t the double bond. Thus it can be concluded that atmospheric bodying has caused geometric isomerization of some of the naturally occurring cis unsaturated fatty acids in this oil to the trans form. I n connection with this collaborative study a rapid quantitative method has been developed, based on absorption at 10.36 microns for the determination of trans components in the presence of cis and saturated components in fats and related materials ($4). While this method involves calibration with pure fatty compounds, the trans band can be used to follow the buildup of trans isomers during the autoxidation of unsaturated compounds, such as methyl oleate, even though little information may be available regarding composition of autoxidation mixture or exact nature of various trans compounds produced ( I S ) . Another interesting application of this 10.36-micron trans band in the examination of complex materials is its use in distinguishing Pennsylvania lubricating oils from oils originating in other parts of the country (6). This differentiation is based on the fact that the Pennsylvania oils apparently contain small amounts of trans unsaturated components while the other oils lack this characteristic. Such applications are possible, of course, because of the approximate constancy of the frequency a t which the trans absorption band occurs in different compounds. In general, the approximate constancy of absorption frequency for many functional groups or atomic configurations, regardless of variations in structure of the various species contributing to the absorption, holds also in the ultraviolet and analogous considerations hold for Raman scattering lines. This general principle has afforded a basis for a diversity of applications of infrared and ultraviolet, and to some extent of Raman in the examination of complex materials. While this general principle is well knou-n and has been vr-idely exploited, it is perhaps somewhat less widely appreciated that not only the frequency but
ANALYTICAL CHEMISTRY also the intensity of an absorption band due to a given functional group tends toremain approximately constant in homologous series of compounds, provided the absorption intensity is properly expressed as absorptivity per functional group. This fact opens up many attractive possibilities in the analysis of complex materials, since spectroscopic analyses can be expressed in terms not only of the kind, but also the amounts of compound types present, even though a variety of individual molecular species may contribute to the absorption observed for each type. The concept of functional group absorptivity and approximate constancy thereof stems from the work of Rose (21) and the original infrared frequency assignments made by Fox and Martin for CH, CHI, and CH3 groups in hydrocarbon molecules in the carbonhydrogen fundamental stretching region (5). Resolution of various CH fundamental bands requires the use of a spectrometer equipped with a calcium fluoride or lithium fluoride prism. Rose, however, worked with the overtone bands of these fundamentals in the l-micron region, and was therefore able to use a glass prism instrument. He showed that absorption intensities due to CH, CH2, and CHI remained approximately constant on going from one member of a homologous hydrocarbon series to another. Subsequently, the use of infrared for CH, CHi, and CHI distinction and estimation, using either fundamental or overtone bands, has been the subject of several publications (4, 8, 10, 2 2 ) which have described applications of this type of analysis to the characterization of complex materials. In the petroleum industry, for example, such determinations can provide information of great value relative to the degree of chain branching, amount of ring substitution, etc., which not only throws additional light on the nature of such materials but provides data, which when correlated with practical properties such as octane number, lubricating properties, etc., can be used in the solution of practical problems in this field. Hastings and coworkers (8) have described the application of this type of analysis to the characterization of complex paraffinnaphthene mixtures. Because of the complexity of such mixtures and the unavailability of a sufficient number of suitable calibration compounds, individual component analysis is not feasible. However, with the aid of “average” infrared absorptivity values, obtained on appropriately selected available pure compounds, it is possible to estimate the amounts of the various types of carbon hydrogen linkages present in such a mixture. Using a calcium fluoride prism to obtain the necessary resolution, Hastings et al. have determined CHI, paraffinic CHI, five-membered ring CH2, and six-membered ring CHI in complex systems by appropriate combination of infrared measurements in the 3.5-micron C-H fundamental stretching region, the 7.1- to 7.5micron methyl group absorption region, and the 12.5- to 14.3micron paraffinic CH, absorption region. The potentialities of this kind of analysis are, of course, by no means confined to the estimation of CH, CH2, and CH, in hydrocarbons. Recent work by Pozefsky and Coggeshall (18) indicates that the original Fox and Martin C-H, CHI, and CHI assignments hold reasonably well for compounds containing oxygen or sulfur, thus opening up the possibility of determining such groups in nonhydrocarbon systems. Anderson and Seyfried (1) have also applied the quantitative infrared functional group analysis approach to the determination of oxygenated groups such as carbonyl, hydroxyl, etc. , in complex hydrocarbon synthesis naphthas. I n fact, there has been increasing interest in recent years in the application of spectroscopic methods to the examination of complex products resulting from the oxidative degradation of a variety of materials such as fats and oils, resins, and hydrocarbons. Such applications, however, have been largely confined to the use of long known and more recently published (26) frequency assignments for the qualitative detection of oxygenated groups with lesser emphasis on quantitative estimation. Increasing emphasis on quantitative functional group analysis in connection with such studies can probably be expected.
V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2
WAVELENGTH --MICRONS Figure 4. Infrared Spectra 1.
Dehydrated castor oil.
An interesting example of the application of Raman spectroscopy to group-type analysis is afforded by the work of Heigl and coworkers ( 9 ) , who have developed a rapid Raman spectrophotometric procedure for estimating total aromatics and total olefins in complex hydrocarbon mixtures. I n principle, this method is analogous to that involved in the infrared applications mentioned above. -4fter “average” Raman scattering coefficients have been established from the spectra of a series of appropriate pure aromatic and olefinic compounds, the total aromatic and olefin content is calculated from the integrated scattering intensities of each of two Raman lines arising from vibrations of the aliphatic and aromatic carbon-carbon double bonds, respectively. In the ultraviolet field, also, a number of applications of grouptype quantitative analysis have been published in recent years. Kinder and coworkers ( I I ) ,for example, have developed an ultraviolet spectrophotometric method for the determination of total alkyl benzenes in crude oil fractions. Calibration was carried out in this case by determining absorptivity valups ac 215 mp on several blends representative of those likely to be encountered in practice. The application of quantitative group analysis by spectroscopic methods to a given problem will usually present many difficulties. Because of the assumptions and uncertainties involved, this approach is subject to definite limitations. However, with further extension of background, improvements in technique, etc., this type of analysis seems likely to find increasing use in the examination of complex systems.
Styrenated dehydrated castor oil
While applications of the functional group absorptivity concept to the direct quantitative estimation of functional groups in complex systems is a comparatively recent development, the group-type analysis concept is, of course, involved in all applications where chemical changes are followed by means of bands arising from groups common to several compounds. As in the case of the trans band mentioned previously, such changes can be followed without exact knowledge of the structure of the various chemical species contributing to the absorption. Both infrared and ultraviolet methods are being increasingly used for following changes accompanying polymerization, oxidation, and other chemical reactions. Also, spectroscopic methods have found widespread use in recent years for following the separation of complex mixtures bj- physical techniques such as chromatography, extraction, and distillation, and as criteria for completeness of reaction. An interesting example of the latter type of application has been published by Lipkin and coworkers ( l b ) ,who used ultraviolet absorption as a criterion of completeness of hydrogenation in connection with studies of the catalytic hydrogenation of complex hydrocarbon fractions. Use of Infrared Spectra for Empirical Characterization of Complex Materials. In research, development, and manufacture, i t is always desirable to be able to correlate the behavior, properties, and functions of materials with basic chemical constitution. I n the case of many industrially important raw materials and finished products, however, such correlation is impossible or only partially possible because the materials in question are poorly characterized as individuals in terms of
60 k bp
40 20 -
80 60 -
Figure 5. 1. Phenol-formaldehyde resin.
WAVE LENGTH -M I C RO NS
Infrared Spectra of Phenolic Resins
p - tert-Butylphenol-formaldehyderesin.
chemical make-up. Since such materials are nevertheless dealt with in practice as individual entities, it is necessary that they be characterized by means of physical constants and/or gross chemical properties. Physical properties, such as color, index of refraction, solubility, and boiling or melting range, and chemical properties, such as total acidity, total unsaturation, and saponification number, are widely used for the gross “identification” of such materials and are used as criteria for selection and control of their quality. In applied research and manufacture chemists tend t o “identify” a given complex entity, such as an oil, a coal tar fraction, or a natural resin, in terms of these over-all constants
together with known functional properties, rather than in terms of chemical constitution. Thus type names, trade names, arbitrary code number, etc., tend to replace chemical names to a considerable extent in many industrial activities. As a result of this practice, analytical reports expressed in more or less arbitrary terms rather than in terms of chemical composition and structure often suffice for the solution of practical problems. I n principle, an infrared absorption spectrum affords a very useful means for the arbitrary empirical characterization of a complex material. Such a spectrum should be thought of as, not just one, hut a whole set of unique characterizing constants
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 consisting of the frequencies and relative intensities of the various absorption bands present. In the hands of an expert, such a spectrum will, of course, always reveal some degree of information about chemical constitution. For many practical purposes, however, it can be used with the aid of a file of reference spectra (like the fingerprint of an individual) as a unique characterizing pattern with little or no regard for the compositional and structural features which it reflects. For example, consider the problem of identifying an unknown phenolic resin. Phenolic resins, made by condensing phenols nith aldehydes, are materials of very complex structure. A given resin may be made from any one of a variety of phenols condensed with one of various possible aldehydes. Spectra of three such resins of knon-n compodion are shown in Figure 5 . The spectral pattern observed in the 11- to 15-micron region in curve 1 is qualitatively characteristic of resins based on unsubstituted phenol. The patterns observed in this same region in curves 2 and 3 are qualitatively characteristic of resins based on p-alkyl substituted phenols and p-phenylphenol, respectively. Resins based on other phenols also exhibit unique patterns in this region of the spectrum. Thus the parent phenol used in the manufacture of an unknown resin can often be deduced by comparison of its spectrum with such known reference spectra. Because of the great variety of commercially available phenolic resin.;, hoivever, a match for a given unknown may not be found in a file confined to spectra run on resins of known chemical composition. Nevertheless, if a match can be found among reference spectra run on commercially available resins of known source and behavior properties (but unknown chemical constitution), the “identification” can he reported in terms of supplier’s trade name with resultant solution, in many case$, of the practical problem a t hand. Marron and Chambers ( 1 6 ) have published an interesting paper on the use of infrared spectral patterns in the examination of the complex, ill-defined materials used in the manufacture of printing inks and related products. Infrared spectra are used more or less empirically for monitoring the quality of complex raw materials and finished products in their field of interest. I n the selection and control of raw materials such as waxes, resins, and blended petroleum oils, used in inks, stencils, coated papers for mimeographing, and the like, they maintain reference spectra on materials of standard quality and behavior and compare the spectra of incoming shipments with these stanc!ards. A batch of material showing appreciable spectral departure from thestandard is rejected or diverted to less critical use instead of being tolerated in a substandard product. As llarron and Chambers point out, quality control designed to recognize irregularities in complex finished products before customrr complaints arise is often impossible because it is not feasible t o hold production lots for the extensive time required to verify their composition by conventional methods of analysis. Although it is often impossible to obtain an infrared spectrum directly on a sample of a finished product, such a spectrum can frequently be obtained with a minimum of preliminary treatment of the sample. Such a spectrum, once obtained and compared with a suitable reference standard, will reveal variations in time to allow corrective measures before a large quantity of subatandard product has been made. As an example, while the pigment in an ink prevents direct spectral examination, the spectrum of the supernatant liquid, after one rapid centrifuging, serves for monitoring the composition of the complex organic ingredients used. GENERAL COMMENTS AND CONCLUSIONS
Despite the truly phenomenal progress made in recent years in the use of spectroscopic techniques in the examination of complex materials, “the surface has merely been scratched” in terms of the ultimate potentialities of spectroscopic methods, particularly infrared. Recent advances in sample-handling
techniques, improvement in instruments, and modifications for special types of studies such as microspectroscopy, polarized infrared, and differential spectroscopy have expanded the scope and general utility of the infrared method. Significant improvements in ultraviolet and Raman instrumentation and techniques have also been achieved. Another factor which has added greatly to the effectiveness of these techniques is the recent development of modern punched card and automatic machine sorting methods for indexing and sorting spectra used in qualitative identification Tvork and for correlating spectral character with chemical conatitution ( 1 4 ) . In general, in the author’s analytical work, which deals ektensively with polymeric materials of ill-defined constitution, infrared spectroscopy is by far the most useful single analytical tool. However, this technique affords no universal analytical panacea. Deqpite its great usefulness, problems of any complexity always require companion techniques such as physical and/or chemical separation techniques and various other physical and chemical methods of examination. Perhaps one of the more serious limitations of infrared, in the examination of complex paint systems, is the frequent difficulty or impossibility of directly detecting very small amounts of minor components in the sample. Separation and/or appreciable concentration of such component8 is usually necessary before detection can he achieved. For very small amounts of a given material in such complex systems the greater sensitivity of the ultraviolet method gives this technique the advantage when the situation permits its application. The complexity of paint systems, however, usually precludes the direct appliration of ultraviolet to the original unprocessed sample. LITERATURE CITED
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