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)
I b i d . , 12, 637 (1945). Pajeau, R., Compt. read., 215, 578 (1942). Pierce, W. C., and Nachtrieb, N. H., I N D .E X . CHEM.,ANAL. E D . ,13, 774 (1941). Pigulevskii, G. V., and Ryskal’chuk, A . T., Compt. rend. mad. 8Ci. U.R.S.S., 44, 372 (1944). Rank, D. H., ASLL.CHEW,19, 766 (1947). Rank, D. H., J . Optical SOC.Am., 37, 798 (1947). Rank, D. H., and McCartney, J. S., I b i d . , 38, 279 (1948). Rank, D. H., Scott, R. W., and Fenske, M .R., IND.ENG.CHEW, ASAL. ED.. 14. 816 119421. Rank, D. H.; and Wiegand, R. V., J . Optical SOC.Am., 36, 325 (1946). Rao, N. R., I n d i a n J . Phys., 17, 326 (1943). Redlich, O., and Bigeleisen, J., J . Am. Chem. Soc., 65, 1883 ( 1943). Redlich, O., Holt, E. K., and Bigeleisen, J., I b i d . , 66, 13 (1944). Rosenbaum, E. J., Martin, C. C., and Lauer, J. L., IND.EX. CHEY.,ABAL.E D . ,18, 731 (1946). Schlesman, C. H., and Hochgesang, F . P., Oil Gas J . , 42, No. 36, 41 (1944).
(60) Shemyakin, F. M.,T r u d y Vsesoyuz. Konferents. A n a l . K h i m . , 3, 192 (1944). (61) Shorygin, P. P., J . P h y s . Chem. U.S.S.R., 15, 1072 (1941). (62) Shorygin, P. P., Uspekhi K h i m . , 13, 90 (1944). (63) Simon, A, Angew. Chem., 51, 783 (1938). (64) Simon, A., Z . Elektrochem., 49, 413 (1943). (65) Simon, A . , and FehBr, F., Z . anorg. u . allgem. Chem., 230, 289 (1937). (66) Simon, A . , and Schulze, G., Ibid., 242, 313 (1939). (67) Stamm, R. F., ISD.E N G CHEW, . ANAL.E D . ,17, 318 (1945). (68) Sushchinskii, M . &I.,Compt. rend. acad. sci. U.R.S.S., 33, 18 (1941). (69) Sutherland, G. B. B. hl., “Infrared and Raman Spectra,” London, Methuen and Co., 1935. (70) Taboury, F. J., Bull. soe. c h i n . , 10, 205 (1943). (71) Traynard, Ph., Ibid., 12, 981 (1945). (72) Vodar, B., Jardillier, Y., and Mayence, J., Compt. rend., 222, 1493 (1946). (73) Vol’kenshtein, M. V., and Shorygin, P. P., T r u d y Vsesoyuz. Konferents. Anal. K h i m . , 3, 90 (1944). (74) Vol’kenshtein, M . V., Shorygin, P. P., and Shomova, N. N . Zavodskaya Lab., 9, 860 (1940). (75) Woodward, L. A , , and Tyrrell, H. J. V., Trans. Faraday SOC., 38, 513 (1942). (76) Yvernault, Th., OZBagineim, 1, 189 (1946). RECEIVED Sorember 8. 1948.
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
17
V O L U M E 2 1 , N O . 1, J A N U A R Y 1 9 4 9 mono-olefins. For esample, this technique i.5 suitable for the dctermination of butadiene in mixtures of Ca hydrocarbons, particularly at low concrntrations ( 2 7 ) . Similarly, it has been used, for the determination of cyclopentadiene and methylcyclopentadiene (26) and for the analysis of CS aromatic mixture ( 1 8 ) . A quantitative analyak of ternary mistures of naphthalene, 1inethyhaphthalrne, and 2-methylnaphthalene (12 ) has been reported. An ultraviolet method for the determination of benzene and tolurnr in gasoline ( 1 , 2 ) has been written by an A.S.T.M. group (Committee D-2, Subcommittee XXP, Section F). This does not begin to esliaust the possibilities of ultraviolet spectrophotometric methods of analysis. Acetone at low concentrations ha.: b w n accurately determined ( 5 ) . The analysis of mixtures of phcnol anti the isomeric cresols has been described ( 2 6 ) . llisturcs of aniline, Al--niethylaniline,arid S,S-dimethylaniline have hcen successfully analyzod (28). The value of thc ultravio1t.t method in the determination of certain inhibitors ill polymers has heeri demonstrated ( 3 . In order to dccidv whether ultraviolet spectrophotometry would be useful in a particular analytical problem, it is helpful to have at hand the spectra of the compounds of interest. The appearance in the literature of the ultraviolet absorption spectra of many compounds is obviously very valuable. As an example, one recent paper (22) presents the spectra of about a dozen anthracene derivative.. The catalog of ultraviolet spectra issued by .hierican Petroleum Institute Research Project 44 at the Sational Bureau of Standards (14) is particularly useful for hydrocarbon analysis, although the spectra of some nonhydrocarbons are also included. TERMINOLOGY
absorbance to the product of c3ncentiation arid optical path length is called the “absorptivity” (symbd a ) Thia quantity is a specific property of a material or substance and the suffix “-ivity” denotes that fact. With these term3 and their wmbols, Beer’s law can be expressed as follows: A = - logr abc where T is the tiansmittance, h i, tlie optical path length, and c i s tlie concentration. Widespread adoption of these terms would semi to br desira1,lt..
LITER-iTURE CITED Soc. Testing JIat,erials, ”Compilation of d.S.T.N. Standards on Petroleum Products and Lubricants,” p. 690, 1948. Am. Soc. Testing Materials, Proceedings, 1948. Banes, F. IT., and Eby, L. T., ISD.ENG.CHEM.,.IN.~T, ED., 18, 535 (1946). Barnes, R. H., Rusoff, I. I., Miller, E. S.,and Burr, G. O., I b i d . , 16,385 (1944). Barthauer, G. L., Jones, F. \-., and Metler, -1.V., Ibid., 18, 354 (1946). Beadle, B. W., and Kraybill, H. R., 6.Am. Chem. SOC.,66, 1232 (1944). Brice, B. .\., and Swain, M. L., J . Optical S O C .4m., . 35,532(1945). Brode, W.R . , Patterson, J. W., Brown, J. B., and Frankel, J., IND. ENG.CHEX.,AN.AL.ED..16,77 (1944). Burdett, R. A , , and Jones, L. C., Jr., J . Optical S O C .A m . , 37, 554 (1947). Cary, H. H., and Beckman, A . O., Ibid., 31, 682 (1941). Chance, B., Rev. Sci. Instruments, 18, 601 (1947). Cleaves, A. P., Carver, M. S., and Hibbard, 11. K., S a t l . Advisory Com. Aeionautics, R e p t . TN1608 (1947). Coor, T., Jr., and Smith, D. C., Rev. Sci. Instruments, 18, 173 (1947). Demmerle, R. L., Chem. Eng. .Yews, 25, 904 (1947). Ewing, G. W., and Parsons, T., Jr., . ~ N . A L .CHEhf., 20, 423 (1948). Fletcher, M .H., White, C. E., and Sheftel, J,l. S.,ISD. ENQ. CHEM., .\SAL. ED.,18,204 (1946). Gibson, K. S.,and Balcorn, M .M., J . Optical S O C .Am., 37,593 (1947). Gordon, R. R., Powell, H., and Tadayyoii, J., -1.Inst. Petroleum, 33, 103 (1947). Graff, M. M.,O’Connor, It. T., and Skau. E. L.. IND.ENQ. CHEM..AXIL. ED.. 16. 5,56 (1944). Halpern,’G. R., Ibid.; 18; 621 (l946j. Hogness, T. R., Zscheile, F. P.. and Sidmell, A. IZ.,J . Phys. Chem., 41, 379 (1937). Jones, R. N., Chem. R ~ P 41, . , 353 (1947). Xeal, R. H., and Luckinann, F. H., ISD. ENG.CHEM.,hzra~. ED..16, :358 (1944). O’Connor, R. T., Heinzelnian, D. C., Freeman, -1. F.,and Pack, F.C . , Ibid., 17,467 (1945). Powell, J. S., and Edson, K. C., .\x.AL. CHEM., 20, 510 (1948). Robertson, W.W.,Ginshurg, N., and Matsen, F., ISD.Ezra. CHEM.,.~N.AL. ED.,18, 746 (1946). Roienbaurn, E. J., and Stanton, L., rlra~. CHmf., 19,794 (1947). Tunnicliff, D. D., Ibid., 20, 828 (1948). I‘andenbelt, J. >I., Forsyth, J., and Garrett. .1.,IND.ENQ. CHEX.AXAL.ED..17.235 (1945). RECEIVED Kovember 16, 1948. .\in.
~I
For many years it has generally been recognized that the tcrminology of absorption spectrophotometry is in a n unsatisfactorv Ptate. Such poor terms as “optical density” (almost invariably abbreviated t o density) arid “extinction coefficient” (absorption coefficient, absorption index) have become entrenched through use. With a large and increasing number of m-orkers (many of whom are not spectroscopists) using qpectrophotometric niethods for chemical analysis, the need has become more apparent for a set of ternis less Confusing than that employed up to now and more consistent with usage in other branches of physics. Several organizations, including the Xational Bureau of Standards, the American Society for Testing Materials, and the Society for h p plied Spectroscopy, have become aware of the need and are considering suggestions for improving the situation. -4set of terms for the quantities involved in Beer’s law which has much to commend it has been used in a proposed A.S.T.M. test method ( 1 , 2 ) . The word “absorbance” (symbol A ) is used for the logarithm of the reciprocal of the transmittance, which has previously been called the optical density. The ratio of the
X-RAY ABS0 RPTlON H E R M A N A. L I E B H A F S K Y
General Electric Company, Schenectady, ,V. Y
I
S THE past, when the analytical chemist has used x-rays. he has taken advantage primarily ,of their diffraction ( 1 5 ) by crystalline substances, or of the emission of characteristic spectra. Within recent years, hoxTever, it has become possible for the first time t o measure x-ray absorption a t once precisely and conveniently. As a consequence, x-ray absorptiometry (23) will probably become increasingly important as a method of chemical analysis and of chemical control, for the distinctive characteristics of x-rays lead to results unobtainable with the radiant energy now commonly used for these purposes. Meas-
urements of x-ray absorption will not noticeably alter the sample, and a single measurement can often be made in a matter of seconds once the sample is in the beam. FUNDAMENTAL INFORMATION
The advantages and limitations of x-ray absorptiometry are in general deducible from the known characteristics of x-rays, for a thorough discussion of which the reader is referred to several excellent books ( 5 , 7,1.4,67). rln attempt is made here to illustrate these characteristics by comparing the absorption of x-rays by
