ANALYTICAL CHEMISTRY Thompson, H. W., and Harris, G. P., J . Chem. Soc., 1944, 301-3.
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Thompson, H. W., and Sutherland. G. B. B. W.. Trans. Faraday Soc., 41, 197-200 (1945).
Thompson, H. W., and Torkington, P., J . Chem. Soc., 1944, 597-600.
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Thorndike, A.M., J . Chem. Phus., 15,868-74 (1947). Ihid., 16, 211-13 (1948). Thorndike, A. M., Wells, 4.J., and Wilson, E. B., Jr., Ibid., 15, 157-65 (1947). Thornton, Yernon, and Herald, -1.E., ANAL.CHEW,20, 9-10 (1948). Tingley, H. C., and Hanipton, R. R.. J . Optical SOC.Am., 37, 984 (1947). Torkington, P., and Thompson, H W., T r a n s . Faraday Soc., 41, 184-6 (1945). Trans. Faraday Soc., 41, 171-297 (1945). Tuttle, 0. F., and Egli, P. H., J . Chem. Phys., 14, 571 (1946). Vencov. Stefan, Rev. Stiintifica “ V . Adamach,” 28, 248-58 (1942). Vengerov, Mark, Xature, 158, 28-9 (1946). Vergnoux, A. M., Compt. rend., 219, 125-7 (1944). T’oge, H . H.. J . Chem. P h y s . . 16,984-6 (1948).
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Webb, G. M.,and Gallaway. W. S.. J . Optical SOC.Am.. 34, 349-50 (1944).
Whiffen, D. H.. and Thoiiipsori, H. W.,J . C ‘ h m Soc.. 1945, 268-73. I b i d . , 1946, 1006-9.
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RAMAN SPECTRA W. G. BRAUN AND M. R . FENSKE The Pennsylvania State College, State College, P a .
I
S THE: 20 years since its discovery, the Raman effect has provided new approaches for studying molecular structure, the thermodynamic properties of materials, reaction kinetics, and the qualitative and quantitative analysis of natural and synthetic products. This paper is a brief review of the current literature relating to the application of this phenomenon to analytical chemistry. This review covers chiefly the period from 1943 to the present, although some earlier work is included. The theory and application of the Raman spectra have been rather well covered in several monographs on the subject. Particularly good are the books by Herzberg (29), Hibben (30), Kohlrausch (SS), and Sutherland (69). The paper by Glockler ( 2 1 ) brings t h e . work in Hibben’s monograph to July 19-12. Goubeau ( 2 3 )has written a review on the analytical applications of the phenomenon which covers the work t o 1939 very well. T h e paper by Stamm (67) gives a brief but good discussion of the theoi y. APPLICATIOYS
,
The variety of problems to which analytical Raman spectroscopy has been applied demonstrates the versatility and usefulness of the method, and, incidentally, often the ingenuity of the investigators. In cases where the constituents of the material being examined are known qualitatively and where the spectra of the individual compounds present are available for comparison, the analysis is usually straightforward. Hom-ever, when the spectra of the components are not available, or when the materials are of unknown molecular structure, the analysis becomes complicated. This is often the case in the study of natural products, and usually the best that can be done is to compare the spectrum of the unknown with those of structurally similar compounds. Most of the current applications are in the field of organic chemistry, particularly hydrocarbon mixtures. T h e earlier work on the analysis of petroleum products has been outlined by Hibben (SO), Glockler ( d l ) , and Kohlrausch (33), previously cited. Some review papers which have described the field of usefulness of the method in relation to other
physical methods of analysis are by Sielsen (@), Taylor (39), Schlesnian and Hochgesang (69), Adrianov ( I ) , Baroni (b), and Shorygin (6%‘). More specific data on the method as applied to petroleum mixtures are given by Bazhulin ( 3 ) \Tho presented the results of a semiquantitative analysis of a number of petroleum mixtures by the comparison with standard samples, and by Gerding and van der Vet ( 1 9 ) who applied the method t o t h e analysis of binary and ternary mixtures of pentenes. Shorygin (61) studied the composition of light motor fuels containing benzene, toluene, cyclopentane, cyclohexane, cycloheuene, pentenes, and octane isomers. Vol’kenshteln et al. (73, 7 4 ) analyzed Baku, Groany, and Syzran natural gasolines and seve‘ral cracked gasclines with the aid of Raman spectra. 3Iidzushima et al. (37, 38) determined the constituents of several Sanga-Sanga and north Sumatora gasolines. Deln-aulle, FranFois, and Weimann ( 1 2 ) and Okaaaki (45,44)analyzed several fuels prepared by the Fischer method using the Raman technique. Rank, Scott, and Fenske ( 6 3 )outlined a method using a n internal intensity standard and applied this to the analysis of hydrocarbon mixtures. Rosenbaum, Martin, and Lauer (58) report a procedure for analyzing four-component aromatic mixtures using known blends as standards. Fenske et al. (15), using a recording photoelectric spectrograph developed by Rank and Wiegand (54), gave the analyses of several four-, five- and six-component hydrocarbon mixtures and showed how the photoelectric method could be applied to analytical work. Glacet (90) studied the reduction products of crotonaldehyde with magnesium and acetic acid and, by means of Raman spectra, shonwl that the products contained a n ethylenic linkage b u t gave no indication of the presence of a carbonyl group. Pajeau (45) applied the Raman effect to the analysis of a mixture of dibromobutanes in his study of catalytic dehydrogenation in the presence of beryllium sulfate. Chiurdoglu (Y), in eyamining the ring enlargement of cis- and trans-l,2-dialkylcyclopentanes by aluminum chloride, used Raman spectra to analyze the reaction products, and showed that they are transformed into cyclohexanes and paraffins, Ducasse ( l a ) , in his work on the action
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V O L U M E 21, NO. 1, J A N U A R Y 1 9 4 9 of propanol on sodium ethylate, identified the unsaturated compounds and showed that cis-trans-et'hylene structures were present i n the reaction product,. David, Dupont, and Paquot ( 1 0 ) hydrogenated cyclopentadiene and its dimer and identified, using Ita inan spectra, the reaction products as cyclopentene and cyclopeutane. Pajeau ( 4 6 ) used Raman spectra to identify the products obtained in several reactions while attempting to prepare p-tert-butyltoluene and sym-tert-butylxylene using alkylation catalysts. He also (4;)examined the reaction products in the preparation of cyclohesyl-p-chlorobenzene by several niethods to prove they were the same. Gray ( 2 7 ) made a comparative study of the Raman spectra of the different addition compleses of antipyrine and hydroquinone. Kojima (34) used Itaman spectra to follow the copolymerization process between methyl methacrylate and acrylonitrile. Shemyakin (60) outlined the use of Ranian spectra and other optical methods in the analysis and evaluation of dyes and their intermediates. In the field of natural materials RIertens and Hellinckx (36) studied the products of the cleavage reduction of Congo copal and identified many of the hydrocarbons boiling between 30' and 189 C. Pigulevsltil and Ryskal'chuk ( 4 9 ) identified pinene, d-a-carene, and 1- ~3-carenein their investigation of the fractions of Pinzcs sihestris turpentine boiling between 157" and 172" C. Harispe-Grandperrin and Harispe (28) distilled the essential oil from the fion-er heads of I n d a crithrnoides and by means of Raman spectra showed it to contain p-cymene and d-a-phellandrene. Dupont and Yvernaut ( 1 4 ) obtained the Raman spectra of the methyl estem of fractions prepared by alcoholysis of butter, castor oil, and poppy oil. and showed from the position of the C=C line that oleic acid has the cis configuration while elaidic acid has the trans configuration. It \vas found also that the CSand C, fractions From butter gave Raman lines corresponding t o a C-C bond. Tvernault ( 7 6 ) , in his study of the esters of butter fat, found the same acetylenic bonds. Coniparativrly little worlc has been done on the application of b i n a n spectra t o inorganic materials. Summaries of the work are given in the pwvionsly cited monographs and by Simon (63, 64). The theory arid applications of the spectra of crystals are given by Born a n t i 13rntll)urn ( J ) , Cfihannes (51, and Taboury (70). One of the principal difficulties in using the Raman spectra of inorgzrnic compounds is the changcx in the spectra caused by the ionization of the materials in water solution. The extent of this change was used by Itedlich and co-workers ( 5 6 , 5 7 )to determine the degree of ionization of nitric arid perchloric acids. Simon ( 6 3 ) showed the possibility of analyzing O.l'% pyrophosphoric acid in anhydrous ortho acid and Stamm ( 6 7 )showed how the method could be used for the analysis of sodium nitratosodium nitrite solutions in water. Less than 1% of the Ranian work which has appeared in the literature relates to the spectra of materials in the gaseous state and virtually no work has been done using this method for analytical work. The principal difficulty is the long esposure time required to obtain measurable spectra. Sielsen and Ward (42) used a n arrangement whereby vapor-state spectra v-ere obtained in 12 to 3G hours whereas liquid-state spectra were obtained in 5 to 30 minutes. Kirby-Smith and Bonner ( 3 2 ) describe an apparatus for obtaining the spectra in 3 to 6 hours a t 2 t,o 3 atmospheres using a high-intensity light source, a large scattering volume, and a large-aperture spectrographic camera. Because of the long exposures and the insensitivity of the Raman method to the detection and quantitative determination of minor constituents in a mixture of gases, this procedure has found little favor among analysts, and the infrared absorption techniques are used almost exclusively for this. O
APPARATUS
I n the period shortly before 1939 several European optical manufacturers produced a few Raman spectrographs which are
described briefly by Kohlrausch ( 3 3 ) . The war interrupted the development and production of these units and until very recently Raman equipment has been off the market. Many investigators have assembled their own apparatus and there are a considerable number described in the literature. Most of these are described also in the previously cited monographs. Industry has been hesitant in applying the method because of the lack of commercial equipment, and the time necessary to make analyses. Industrial analysts have resorted to the infrared and ultraviolet absorption techniques because good commercial equipment, designed for rapid analyses, is available. Recently one manufacturer introduced a complete line of instrumentation for recording Raman spectra photographically, and this should make the use of the method somewhat more attractive. Several instrument manufacturers are developing photoelectric recording spectrographs which should place this method on a n equal basis with the absorption methods with regard to rapidity of making analyses. Spectrograph. The choice of spectrographic equipment for Raman work is usually a compromise between one t h a t allows large dispersion and good resolution and one that takes a relatively short time for recording the spectra. Because of the small amount of light available by Raman scattering, the effective aperture should not be greater than f10 and should approach f3 to fG, if possible. Dispersion should be better than 35 A. per mm. in the spectral region to be used. The length of time for recording spectra is limited by convenience, temperature changes in the optical system, vibration of the spectrograph, and stray light. In all but two of the instances given below, the spectra were recorded photographically. Glass prism instruments are used almost to the exclusion of grating types because of the higher light intensity available. While some Raman spectra have been recorded in the ultraviolet region which requires the use of quartz optics, this region is not used in analytical work because most of the molecules are either not transparent here or are not stable. Liquid p r i s m are not recommended because of the large temperature coefficient of refractive index. Rank, Scott, and Fenske (53) used a 3-prism instrument with an f4.5 objective yhich gave a dispersion of approximately 32 A. per mm. a t 4500 8. Rosenbaum, Martin, and Lauer (582 used a three-prismo instrument whose dispersion was about 6 A. per per mm. a t 5000 mm. a t 4000 A. and 20 Stamm ( 6 7 ) describes the construction of an instrument employing an echelette-tvpe grating and an objective of f3.6. The dispersion is 36.97 A. per nim. Photographic methods of recording spectra are time-consuming and are always attended by the inherent difficulties of develop ment and photometry. Rank and Wiegand (54) developed a direct-recording, semiautomatic, photoelectric, grating spectrograph which v,as used by Fenske et al. (15) and which gave reproducible spectrograms in ahout 30 minutes. A nine-stage photomultiplier and direct current amplifier system were used. Chien and Bender (6) used a similar arrangement xvith a prism instrument. Light Sources. The intensity of scattered light is proportional to the fourth power of the frequency of the exciting line and hence it is advantageous to use as high a frequency line as possible. Practically, however, the analyst is limited in the frequency because of the necessity of using quartz equipment and t o the absorption, fluorescence, or photo-decomposition of the sample. The most common source of light for analytical work is the mercury aTc which provides, among others, the 5461, 4358, 4047, and 2537 A. lines. The use of the 4358 A. line is general, but is dependent upon the elimination of the 1047 A. line a s a s i m u t taneou: source of energy. It has been suggested ( 6 7 ) that the 5461 .A. line be used for colored substances that absorb the 4358 A. line but do not absorb in the 5461 A. region. The mercury lamps should be of a reasonably high intensity
.
a.
14
and yet operate a t a suf€iciently low pressure to produce a narrow exciting line. In general, there is a broad continuum in m e r c y y arcs caused by the mercury fluorescence excited by the 2537 A. line, which is strong in the region occupied by the Raman spectrum excited by the 4358 A. line, and the intensity of this continuum increases with pressure. Commercial double-jacketed mercury arcs operate a t a constant intensity but the pressure will be considerably higher than that of a single-jacketed arc. Thus, the single-jacketed arcs will produce sharper lines and a less intense continuum. However, with single-jacketed arcs the intensities are much more susceptible to temperature changes. Rosenbaum et al. (58)use a type H-11 lamp in which the mercury content is reduced to such a n extent that the arcs are operated with the mercury completely vaporized. Consequently, they are not temperature sensitive and can be operated cooler than the commercially available arcs with a consequent reduction of the heavy continuum Rank and PllcCartney (52) have measured the intensity of the background a t 4420 A, for several lamps. Although these lamps were operated a t various currents, the data show that the background intensity is proportiona: to the mercury pressure and also that the intensity of the 4358 A. line is dependent on the temperature and pressure conditions at both electrodes. Filters. I n order to isolate a single monochromatic exciting line and remove the continuum in the region of the Raman spectrum, filters usually are placed between the light source and the sample cell. Most commonly they are liquid solutions and although they are quite generally used, comparatively little work has been done to find the best materials and the optimum concentrations. Kohlrausch (33)and Goubeau ( d 3 ) list a number of ' the solutions which may be used and Stamm (67) gives extensivr transmission data for solutions of sodium nitrite, praseodymium chloride, potassium ferricyanide, cupric nitrate, neodymium chloride, and Rhodamine 5GDN Extra ofrom which appropriatr filters for isolating the 4358 and 5461 A. mercury lines can be elected. Some work has been done to substitute more stable solid filters for the solutions which are generally used. Crawford and Horwitz (8)used a Wratten 2-A gelatin filter wrapped arouud the Raman tube. Glockler and Haskin (22) studied filters by centrifugally casting dyed plastic dopes on the inside of glass or methacrylate tubing of slightly larger diameter than the Raman tube. The authors claim that these filters are more convenient when applied directly to the Raman tube since the number of reflecting surfaces is reduced. Glass filters have been used but the results are not so satisfactory as liquid filters. Interference filters produced by metal evaporation should be satisfactory but no work on these has appeared in tfie literature. Sample Cells. Most investigators have used a modification of the sample tube originally designed by Wood with volumes ranging from 1 t o 50 ml. I n some cases micro equipment using glass capillaries has been necessary ( 2 3 ) so that volumes of 0.02 to 0.05 ml. could be examined but alignment of the tube with respect to the spectrograph becomes critical. Recently Nielsen (40) developed the theory of condensing lenses for Raman tubes of small volume in order that the maximum amount of light may enter the spectrograph. This should help in the design of equipment so that small samples can be used. Modifications in the tube design are principally in the jackets for temperature control and filter solutions and in the windows Some of these are described by Goubeau (25) and Kohlrausch (33). Fenske et al. ( 1 5 ) describe a sample tube with two concentric jackets, one in which a praseodymium filter solution is placed and another through which cooling water can be circulated. Nielsen and Ward (4.9) describe a tube in which liquid and gaseous samples can be heated up t o 300" C. and Vodar and co-workers ( 7 2 ) describe a low temperature arrangement in which a sample is maintained a t - 150" C. for 200 hours with a
ANALYTICAL CHEMISTRY variation of +0.1" during the recording of the spectra. Woodward and Tyrrell (75) have devised a cell in which solutions containing hydrogen fluoride can be examined using fluorite as a window. Freed and co-workers ( 1 7 ) recommend windows of synthetic saphire (aluminum oxide) for use with hydrogen fluoride SAMPLE PREPARATION
Contributing to the background scattering, which is an annoying and often limiting factor in the recording and photometry of Raman spectra, is the fluorescence due to the molecular structure of the sample itself or due to dust, oxidation products, or contamination caused by stopcock lubricants, rubber, material extracted from corks and plastic bottle caps, etc. Treatment of the sample by some chemical or physical method prior to the examination of the spectrum is almost always necessary. Among the chemical methods, Kohlrausch (55)lists shaking of organic sulfur compounds with mercury to remove colored impurities and the treatment of iodides with mercury or freshly reduced copper to remove iodine. Goubeau ( 2 3 ) recommends the use of ferrous sulfate to remove peroxides from mixtures of unsaturated hydrocarbons while Gerding and van der Vet ( 1 9 ) adsorbed the peroxides on charcoal. The physical methods of sample treatment are distillation, recrystallization, adsorption, filtration, and sublimation. Where the samples are sufficiently volatile the fluorescent impurities are most easily removed by a simple distillation carried to dryness t o avoid any fractionation and collected directly in the Raman tube ( 1 5 ) . Kohlrausch (33) describes several procedures for making the distillations. Stamm ( 6 7 ) describes a simple vacuum distillation apparatus. Gerding and Nijveld (18) used a distillation flask attached directly to the Raman tube from which they distilled liquid sulfur dioxide. Vacuum distillations and even steam distillations may be necessary where the materials are relatively high boiling or are temperature sensitive. More stubborn cases of fluorescence may be removed by a treatment before distillation with activated charcoal (24), activated alumina (53),by distillation over clean metallic sodium (58), or by passing the vapor through hot activated silica gel before condensation (15). Kahovec and Wagner ( 3 1 ) describe an apparatus for the continuous purification of alkylhalides by distillation and passage over copper while the spectra are being examined. I n the examination of solutions, particularly of inorganic salts, filtrations have been used. Kohlrausch (33) describes a procedure wherein a solution is shaken with an adsorbent to decolorize it and then filtered through either paper or sintered glass. In the use of paper, extreme care must be observed to remove all paper particles from the filtrate. Simon and Feh6r (65) used this procedure in their evamination of the salts of the phosphorus acids. Simon and Schulze (66) used a glass chamber containing a sintered glass disk attached directly to the Raman tube wherein they twice recrystallized orthophosphoric acid to remove the pyroacid. After the recrystallization the acid was decanted into the sample tube. Small amounts of materials such as nitrobenzene, hydroquinone, resorcinol, or pyrocatechol (223) may be added to quench fluorescence in certain cases. POLARIZATION MEASUREMENTS
1Irasurements of depolarization factors have been given in almost all work involving the Raman spectra of pure compounds. The actual determination of these values is tedious and rarely sufficiently accutate for analytical work. Recent improvements in apparatus and technique may make it possible t o use these data for more reliably determining materials qualitatively and quantitatively. Crawford and Horwitz (8) give an excellent review of the various methods used for determining these data photographically and describe an apparatus with which they can
V O L U M E 2 1 , N O . 1, J A N U A R Y 1 9 4 9 be obtained. Douglas and Rank (18) have studied the errors arising in the measurement of the depolarization factor of highly depolarized lines and measured some values on a photoelectric spectrograph. With these improvements better values may be obtained. QUANTITATIVE ANALYSIS
The intensity of a Raman line attributable to a substance in a mixture is a function of the concentration of t h a t substance and quantitative analyses are possible when the intensity-concentration relationships are known. For mixtures of structurally similar, nonpolar compounds i t has been found t h a t the intensityconcentration relationship is almost linear (15, 26, 53). For mixtures such as acetic or chloroacetic acids and water (55, 71), benzene and dioxane (68), and electrolytes in water (35) the relationship is not a linear one and the position and width as well as the intensity of the line may vary with the composition. I n these cases analyses are considerably more difficult. T h e determination of intensities has, for the most part, been by some photographic photometry method and the techniques adopted are similar to those used in emission spectroscopy (16, 48). The procedures are usually time-consuming and are always attended by the inaccuracies inherent in the photographic technique. Within the past 3 years the trend has shifted to the use of photoelectric Raman spectrographs, with which the intensities may be recorded directly, since this equipment is capable of greater speed and photometric accuracy. Stamm (67) and Goubeau (33) have reviewed critically the various schemes of analysis once the intensities of the lines and the intensity-concentration relationships are known. The method which an analyst must choose depends on the type of samples, the number of components, the equipment at hand, the time available, and the accuracy desired. Grosse, Rosenbaum, and Jacobson (86) used perhaps the simplest method in which the spectra of the unknown and a series of knowxi mixtures having the same components were photographed under similar conditions. T h e blackness of the lines were compared visually to give the composition of the unknown. Crigler (9) compared microphotometer deflections for the spectra of a series of known mixtures with the spectrum of a n unknown material. These are both only semiquantitative schemes and the results depend t o a large extent on the experimental conditions, length of exposure, steadiness of light source, and uniformity of plate development. A somewhat more refined method for binary systems used by Stamm ( 6 7 ) and Gerding and van der Vet (19) uses the ratio of the intensities of two suitable lines, one for each compound in the mixture. These ratios, plotted graphically against concentration for a series of known blends, give a calibration curve from which analyses may be made. Ternary mixtures may be analyzed by additional calibrations (18,25). Rosenbaum et a l . (58) developed a method whereby a fourcomponent mixture could be analyzed. Rank, Scott, and Fenske (53) attempted to decrease experimental inaccuracies by the use of a n internal standard (carbon tetrachloride) in their work on hydrocarbon analyses. They defined a scattering coefficient as the ratio of the intensity of the hydrocarbon line in question to the intensity of a particular line of this internal standard. I n this method all intensities can be referred to the same intensity basis. Objections are: (1) the contamination of the sample and pure materials by the standard, and (2) the possible overlapping of the standard lines with those of the sample. Fenske et al. (15) recording photoelectrically, referred the intensities of the lines of all pure materials and unknown samples to t h a t of the 459 em.-’ line of carbon tetrachloride. For those cases where the intensity-concentration relationship was linear the analysis was made by a simple proportion between intensities. As many as nine components have been determined in a mixture using their method.
15
An unfortunate deficiency in Raman analytical work, as in all other empirical methods of analysis, is t h a t intensity measurements depend on certain instrument constants and, accordingly, each spectrograph must be calibrated with a complete set of the known pure materials which will be found in the samples to be analyzed. Rank (50, 51) has discussed the chief sources of error in intensity measurements and developed equations t o enable polarization corrections to be applied to measurements taken on one instrument so that they can be used on another. Chien and Bender ( 6 ) suggested intensity corrections for the spectral response of the photocathode when recording spectra photoelectrically. Limitations. Qualitatively Raman spectra have been used t o detect constituents in concentrations as low as 0.1% in exceptionally favorable cases. Generally the minimum detectable concentration is about 1%. Quantitatively, these spectra should not be used for concentrations below about 5% when the material scatters light strongly; when the scattering is weak, the minimum roncentration is about 10%. LITERATURE CITED
Adrianov, A. F., Priroda, No. 9, 45 (1940). Baroni, E., Wien. Chem.-Ztg., 46, 156 (1943). Baahulin, P. A,, T r u d y Vsesoyuz. Konferents. A n a l . K h i m . , 3, 105 (1944). Born, M.,and Bradburn, M., Proc. Roy. Soc. ( L o n d o n ) , A 188, 161 (1947). Cabannes, J., Rev.sci., 80, 60 (1942). Chien, J., and Bender, P., J . Chem. P h y s . , 15,376 (1947). Chiurdoglu, G., Bull. soc. chim. Belges, 53, 55 (1914). Crawford, B. L., and Horwits, W., J . Chem. P h y s . , 15, 268 (1947’1. Crigler, E. A., J . Am. Chem. Soc., 54,4207 (1932). David, S., Dupont, G., and Paquot, C., B u l l . soc. chim., 11, 561 (1944). Delwaulle, M. L., Francois F., and Weimann, J., Chimie & industrie, 56, 292 (1946). Douglas, A. E., and Rank, D. H., J . Optical S O C .Am., 38, 281 (1948). Ducasse, J., Bull. soc. c h i m . , 11, 333 (1944). Dupont, G., and Yvernaut, F., Ibid.,12,84 (1945). Fenske, M. R., Braun, W. G., Wiegand, R. V., Quiggle, D., McCormick, R. H., and Rank, D. H., ASAL. CHEM.,19, 700 (1947). Forsythe, W. E., “Measurement of Radiant Energy,” chapt. VIII, New York, McGraw-Hill Book Co., 1937. Freed, S., McMurry, H. L., and Rosenbaum, E. J., J . C h e m . Phys., 7, 853 (1939). Gerding, H., and Nijveld, W. J., Rec. trau. chim., 56,968 (1937). Gerding, H., and van der Vet, A. P., Ihid., 64, 257 (1946). Glacet, C., Compt. rend., 222,501 (1946). Glockler, G., Reus. Modern Phys., 15, 111 (1943). Glockler, G., and Haskin, J. F., J . Chem. P h y s . , 15, 759 (1947). Goubeau, J., in “Physikalische Methoden der analytischen Chemie,” by W. Bottger, Leipaig, Akademische Verlagsgesellschaft, 1939. Goubeau, J., 2 . a n a l . Chem., 105, 161 (1936). Goubeau, J., and Thaler, L., A n g e w . Chem., 54, 26 (1941). Grosse, A. V., Rosenbaum, E. J., Jacobson, H. F., IND.ENG. CHEM..ANAL.ED., 12,191 (1940). Gray, M . E., B u l l . soc. chim., 12,607 (1945). Harispe-Grandperrin, M., and Harispe, J. V., Bull. soc. chim. biol., 26, 192 (1944). Heraberg, G., “Infrared and Raman Spectra of Polyatornic Molecules,” New York, D. Van Nostrand Co., 1945. Hibben, J . H., “Raman Effect and Its Chemical Applications.” New York, Reinhold Publishing Corp., 1939. Kahovec, L., and Wagner, J., 2. p h y s i k . Chem., B42,123 (1939). Kirby-Smith, J. S., and Bonner, L. G., J . Chem. P h y s . , 7, 880 (1939). Kohlrausch, K. W. F., “Ramanspektren,” T’ol. 9 of “Hand und Jahrbuch der Chemischen Physik,” ed. by A. Euken and K. L. Wolf, Leipsig, Xkademische Verlagsgesellschaft Becker & Erler. 1943; J. W. Edwards, Ann Arbor, Mich., 1945. Kojirna, K., J . Chem. Soe. J a p a n , 64, 496 (1943). Kujumselis, Th. G., 2 . P h y s i k , 110,742 (1938). Mertens, E., and Hellinckx, L., B u l l . soc. chim. Belges, 50, 99 (1941). Midsushima, S., and Tobiyama, T., J . Chem. soc. Japan, 65, 374 (1044). Midzushima, S., Tobiyama, T., and Shirakawa, H., I b i d . , p. 549.
ANALYTICAL CHEMISTRY
16 Naylor, W. H., J . Inst. Petroleum, 30, 256 (1944). Nielsen, J. R., J . OpticaZSoe. Am., 37, 494 (1947). Nielsen, J. R., Oil Gas J., 40, No. 37, 34 (1942). Nielsen, J. R., and Ward, h’.E., J . Chem. Phys., 10, 81 (1942). Okazaki, H., J . Chem. Soc. J a p a n , 60, 559 (1939). Ibid., p. 1269. Pajeau, R., Bull. soc. c h i m . , 9,741 (1942).
(54) (55) (56) (57) (58) (59)
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Ultraviolet Absorption Spectrophotometry E. J. ROSENBAUM, Sun Oil Co., Norwood, Pa.
r
133 advent of the Beckmaii quartz spectrophotometer (IO) just before the recent war marked a turning point in the analytical applications of ultraviolet absorption spectrophotometry, particularly in this country. Before that time a limited amount of work in this field had been carried out, usually on problems for which a n alternative solution was impractical or nonexistent. The conventional technique involved photography of spectra, and a n arc between metal electrodes was the most frequently used radiation source. I n the late thirties the use of photoelectric radiation detectors and hydrogen arc sources was developed. One of the best researches of that time was the work of Ilogness, Zscheile, and Sidwell (21), who presented a treatment of the fundamentals of absorption spectIophotometry which is still useful today. The commercial availability of a oompact, convenient, and relatively inevpensive spectrophotometer of adequate resolution and stability has rapidly led to a great increase in the application of ultraviolet absorption spectra to chemical analysis, and this increase is partially reflected in the growing literature on this subject. Few instruments have as dominating a position in their field as does the Beckman qnartz spectrophotometer at the present time. INSTRUMENTAL DEVELOPMENT
There has bean a considerable amount of interest in employing the Beckman spectrophotometer in ways for which i t presumably \$as not intended. For example, one development ( I S ) led to a iecording spectrophotometer with a n electron multiplier phototube for scanning the whole spectral range from 200 to 400 mp in terms of per cent transmittance. This was made possible by automatic change of slit width and correction for change of sensitivity of the phototube with wave length. Another type of change (27) resulted in a modified amplifier containing a high impedance direct current to alternating current converter whose stability made the instrument useful for the analysis of a flowing sample at fixed wave length. Several adaptations of the basic instrument t o analysis by means of fluorescence have been described (9, 16). One investigator (68) has reported the nse of a mercury arc line source instead of the more usual hydrogen arcs
which emit a continuum. This is possib!e, of course, only when the mercury lines happen to lie within the absorption bands of the substance under investigation. Although the Beckman spectrophotometer is widely used, some workers use other types of spectral apparatus. ‘Applications to analysis have recently been reported for a grating spectrograph (26), a Bausch & Lomb quartz spectrograph (a), and Hilger quartz spectrographs (18, 25). One investigation (11) \t‘as directed a t the stabilization of the electronic circuits used in conjunction rvith a Coleman spectrophotometer in order to detect and measure small changes in the absorption of a sample. The widespread application of spectrophotometric methods requires a good source of solvent (often a saturated hydrocarbon) without appreciable absorption in the spectral range of interest. A practical investigation (19) points out the usefulness of silica gel in removing the last traces of contaminants in otherwise satisfactory solvents. Two papers (17, 29) are concerned with the absolute calibration of Beckman spectrophotometers and the comparison of data obtained from these instruments with those obtained from other spectrophotometers. They both indicate t h a t the Beckman spectrophotometer showed up favorably in the comparison. One paper (15) deals with the intercomparison of a number of Beckman spectrophotometers and leads t o the reasonable conclusion that for most accurate results in quantitative analysis each instrument should be calibrated individually, although fair results can be obtained by using the calibration data obtained from one instrument to carry out analyses with another. APPLICATIONS
Ultraviolet spectrophotometric methods have proved to be important for biochemists and analytical chemists dealing with animal and vegetable fats and oils, and these methods continue to be widely used. h number of investigators have reported analytical studies on fats, fatty acids, and oils (4, 6 , 7 , 8, 24) and on vitamin A (20, 8 3 , . Another broad field of application of ultraviolet spectroscopy is the analysis of hydrocarbons containing aromatic rings or conjugated double bonds in the presence of saturated compounds or
