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ANALYTICAL CHEMISTRY
(4) Light, A. E., IND.ENG.CHEM., ANAL.ED., 14, 42-3 (1942). (5) Martin, M. E., Kelley, X.L.,and Green, M . W., J . Am. Pharni. ASSOC., 35, 220 (1946). (6) Mitchell, J., Smith, D. M., and Bryant, W. M. D., J. Am. C h e m Soc., 63, 573 (1941).
(7) Niederl, J., and Niederl, V., “Micromethods of Quantitative Organic Analysis,” New York, John Wiley & Sons, 1942. ( 8 ) Oppenauer, R. v., Rec. t m u . chim. PaUS-BaS, 56, 142 (1937). (9) Petit, G., Bull. doc. France, 1948, 141. (10) Sabetay, S.,Bull. 8oc. chim. (51, 5. 1419 (1938). (11) Seaman, W., Woods, J. T., and Massad, E. A., ANAL.CHEW, 19, 250 (1947). (12) Shriner, R. L., and Fuson, R. C., “Systematic Identification of Organic Compounds,” p. 168, New York, John Wiley & Sons, 1948.
(13) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” New York, John Wiley k Sons, 1949. (14) Smith, G. B. L.,and Wheat, T. G., IND. ENO.CHEM.,ANAL.ED., 11,200 (1939). (16) Stillman, R. C., and Reed, R. M., PerjuumeTy Eeeent. Oil Record, 23,228 (1932). (16) Veibel, E., A c h Chem. S c a d . , 1, 54 (1947). (17) Veibel, S.,Blaaberg, A., and Sterns, H. H., Damk Tids.F a n . , 14,184 (1940). RECEIVED November 14, 1949. Presented before the Division of Analytica and Micro Chemistry a t the 116th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. Based upon a portion of a senior research project submitted by Anthony .M. Trociolo in partial fulfillment for the degree of bachelor of science, Department of Chemistry, Illinois Institute of Technology, January 1950.
Double-Beam Infrared Spectrophotometer Chemical and Biochemical Applications JOHN U. WHITE1 The Perkin-Elmer Corporation, Glenbrook, Conn. The value of a versatile double-beam recording infrared spectrophotometer in solving chemical problems is illustrated with several examples. Reproductions of original records show the advantages resulting from high speed, resolution, stability, and flexibility in the measurement of spectra, analysis of samples, and determination of structure. The possibility of compensating for scattering losses in mulls and for solvent absorption in solute study is particularly useful.
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N T H E past the utility of infrared spectroscopy in chemical problems has increased directly with the ease of reducing the observations to useful form. Even though the uniqueness of infrared spectra has been known for many years ( 6 ) ,the necessity for point readings and curve plotting limited its use to university research and more recentb to simple industrial analyses with point reading instruments (6). With the advent of commercial automatic recording spectrometers ( 4 ] .14, 16, IT), infrared spectroscopy became easy and fast enough for general industrial we. The records obtained were useful directly for identification and analysis, but had to be replotted on some absolute scale for easy understandability in conferences and publications. To avoid this labor of replotting, instruments have now been designed to record directly in graphs of uniform wavelength or wave-number scales us. per cent transmittance ( 1 , 8, IS, 18, bf, $8). When these features are combined with the high resolut,ion and versatility of modern instruments (8)it is possible to record directly a number of typa of spectra that would otherwise be difficult and time-consuming to measure. The purpose of this paper is to show how such an instrument may be applied to a group of diversified chemical problems. Many of them have been donebefore onsingle-beaminstruments, but without the ease and directness of the present work. Special attention is paid to the advantages obtainable from the use of the doublebeam principle under a wide variety of operating conditions. I n each case the application is presented rn a brief description with reproductions of instrument recordings of per cent transPreeent address, The W h i k Development Corpomtion, 90 Linooln Ave., Stamford, Conn.
mittance us. wave length. The problems considered and the special features illustrated are: Type
Of
Stiy:2re
Example o-Toluidine
Spectra of isomers
a- and
Product purity
1% ethylbenzene in
Steroid spectrum
pconidendrin
styrene Dehydroisoandrosterone in chloroform
Features Illustrated Speed, noise level, and scattered light Resolution, compensation for scattering Elimination of styrene spectrum by compensation Elimination of solvent spectrum
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Figure 1. Optical Layout and Block Diagram
V O L U M E 22, NO. 6, J U N E 1 9 5 0
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is proportional t o the 8 uare of the slit width for wide slik, but falls off more rapixly as the slits become narrow. To ensure good operation of the balance motor the amplifier gain control must be readjusted whenever the resolution control is changed. The response speed control changes the time for full scale d e flection of the pen between 4.5 and 110 seconds, thus reducing the noise level of the records when accurmv or hizh resolution
The spectra were d l measured on the Perkin-Elmer Model 21 recording infrared 8peotroDhotometer. As B detailet
the absorption Tby solventia variable space liquid cell bas fillea with solvent, placed in the reference beam, and adjusted until absorption bsnds known to belong only t o the solvent were removed as completely as p s i b l e from the spectrum. O-TOLUmME
The spectrum of 0-toluidine was measured as B cspillary layer between two salt plates. The spectrum shown in Figure 3 was made in 14 minutes per run with 0.2 r.m.8. microvolt for full scale deflection. To show the amount of scattered light, the zero line was measured with an opaque shutter from 2 t o 6 microns, a glass one from 6 t o 9.5 microns, and P. lithium fluoride one from 9.5 t o 15 microns. The effectiveness of the grating filter that was used from 7.5 to 15 microns t o reduce the scatter is indicated by the fact that 100% absorption of the 13-micron band coincides with the indicated zero line. This spectrum was measured without B compensating plate in the reference beam t o avoid interference between the spectrum and the IOrecorded with the cell removed. The latter indicates the noise level to be expected in the spectrum, the accuracy with which the two beams are ballsneed over B wide spectral region, and the amount of interference from atmospheric water vapor st this scanning speed without great precautions t o dry the inside of the instrument.
The instrument operates on the principle described by Wright
($2) in which two beams of light are taken s arately from a
~ourceand combined by 8 wetor mirror that Zernately trmsmits one and reflects the other into a single direction. One beam passes through the sample and the other through an adjustable wedge aperture and, if desired, a compensating cell. If the intensities in the two beams are equal their composite formed by the sector mirror has a steady intenkty; if the intensities are not e ual, the composite beam fliokers as the sector mrror moves. %he amount of flicker, indicating the degree of off-balance, is detected by the thermocouple, amplified, and used t o drive the intensity balance motor in the proper direction t o restore balance. The pen records continuously the position of the wedge required for balance. The monoehromtor used to select radiation of different wave lengths from the composite beam is essentially the same as t,he one described by Barnes (4). Its prism is of rook salt, 60 mm. high and 75 mm..on each face. The instrument's operating controls m e &I1 indicated in Figure 1 exoept for the amplifier gain and source intensity adjustments. $he wave-length scanning speed is manually adjustable meohaniodly and electrically and also automatically adjustable through a connection to the amplifier that slows down the scanning speed whenever balance is not maintained between the two . it possible .. :t feat? spectra rapidly i? rFgions beams. . Thi! makes where there 1s no a~sorptionana siowiy wnenever aetsuea spec,are present. The degree of this automatic speed suppression adiustable. The record-
P
AND E-CONIDENDRIN
The alphs and beta forms of eonidendrin are solids with me16 ing points of 255-256" and 210-211' C., reapectively. Both samples were measured in the solid state as mulls in Nujol. I n Figure 4, s ectrum A is the beta isomer BS normdly mess! the scattering losses in the powder. Spectrum wed, including a% C is the same sample approximately compensated for scattering losses by a ground salt plate in the reference beam. The curvature of the spectrum at short wave lengths has been pretty well corrected, although the difficulty of matching the scattering accurately makes it unwise to try to measure exact trmamittmce v ? l y , u n d e r t+se condjtiyp The B s p e c p " is that o;,t$e sipna. Isomer, a s 0 co~recreaIOT somtenng. 1% mows very mue similarity to the other, Although both isomers have been a8-
:ords c w be made on uni'm scales of 1, 2, 5, et.a., : h a per micron. The slit cam continuously just8 the slit widths t o 'e approximstely cann t energy OUtDUt from and 15 microns. By Lnging the resolutionoon81 in the slit control cirt, all slit widt.hs may be lltiplied or divided by xessive factan of thk lare root of 2. The lest slits are eight times !narrowest. The energy
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Figure 3.
Infrared Spectrum of o-Toluidine with Correlations between Absorption Bands and Molecular Groups
ANALYTICAL CHEMISTRY
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per micron. It is especially interesting because it shows additional detail not found in the other record. At the six wave lengths marked on the spectrum, new bands are resolved or clearly indicated. This and the increased separation of the other absorption bands indicate finer structure than is usually observed exce t in the spectra of light gases. Any reduction in the resoging power of the spectrometer would have resulted in the lass of possible information. This oint is emphasized because general statements have sometimes teen made that high resolution is not required for liquid or solid samples.
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ETHYLBENZENE IN STYRENE
A 1% solution of ethylbenzene in styrene is shown in Figure 5 as an application of the doublebeam infrared spectrophotometer to an analytical problem of product purity.
A is a blank run with no absorption cell. B is the spectrum of a capillary layer of ethylbenzene. A and B are both offset compared to the other spectra to make room for C , the differential spectrum recorded when the variable cell containing pure styrene was placed in the referenre beam. Comparison of the two shows that all the major bands outside the shaded area6 were observed in the compensated spectrum. However, when the transmittance in the reference beam fell below IO%, the instrument ceased to operate properly. The bottom air of each spectra are those of the sample, D, and of pure styrene, measured in the normal manner. The sample is in a fixed 0.1mm. cell, the styrene in the variable cell set to 99% of the thickness of the sam le cell. Because of scratches on the windows of the variabg cell, the pure styrene seems to absorb more than the mixed sample everywhere except at 13.4 microns. At this wave length the strong ethylbenzene band reduced the sample's transmittance and caused the two records t o cross. The sensitivity wm 1 microvolt for full scale deflertion, the full scale response time 12.5 seconds, and the scanning time 27 minutes. The filter was used a t wave lengths longer than 7.5 microns, a3 the effects of scattered light increase in proportion to the amount of light absorbed from the reference beam.
E,
DEHYDROISOANDROSTERONE 5
In recent years there has been considerable infrared u ork with steroids (11) and an active increase is expected because of the recent interest in ACTH and cortisone. The solvent most generally used for this work is carbon disulfide because even in 3-mm. thickness it has extensive transmittance windows. However, the more highly oxygenated steroids are not soluble in carbon disulfide, and chloroform is the best substitute. Unfortunately, in thicknesses greater than 0.3 mm. chloroform has
IO
WAVE LENGTH IN MICRONS
Figure 4. Infrared Spectra of Nujol Mulls of a- and 8Conidendrin A . &Conidendrin uncompensated E . a-Conidendrin' compensated for scattering C. 8 - C o n i d e n d r i n ' c o m ~ n s a tfor ~ scattering D . @-Canidendrincempensated for scattering, high resolution
signed the same structural formula (9,10). the differences in their infrared spectra indicate that there must be some marked difference between them. These three records were made in 11 to 12 minutes per run with 0.2 r.m.8. microvolt for full scale deflection. The conditions were identical, except that the source temperature was increased in the compensated runs to help make up for the selective scattering losses at shorter wave lengths, and the gain w m increased slightly to comnsate for general energy rsses. These records have unusually low noise levels and water vapor interference. The zero runs were made with an opaque shutter throughout and without the scattered light filter. Spectrum D is a repeat of C at seven tenths the previous slit width with the slowest response speed and a scanning rate of 1 hour
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Figure 5. A.
N o sample. B .
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Ethylbenzene in Styrene
Ca illary la er of pure ethylbenzene. C. 1 % ethylbenzene in styrene, compensated. D. ethyl&nzeme in styrene, uncompensated. E. Pure styrene Cell thickness 0.1 rum.
f%
V O L U M E 2 2 , N O . 6, J U N E 1 9 5 0
171
This work, however, requires critical examination of the solvent question. This has already been made for single-beam instruments (19) in which any solvent is automatically rejected from general consideration if it has many absorption bands. E\cept in restricted spectral regions they are, in fact, limited to carbon tetrachloride, carbon disulfide, long-chain paraffins, methylcyclohexane, and a few others. The results illustrated above show that a solvent can be used reWAVELEHGTW IN MICRONS gardless of the number of its Figure 6. Infrared Spectrum of 1 to 2% Solution of Dehydroisoandrosterone in absorption bands in any reChloroform gion where its transmittance A . With compensation is greater than 10 to 30%, deB. Without compenaation pending on the resolution and Cell thickness 1 mm. speed of spectral scanning desired. When examined according to this criterion, many common solvents are useful over so many strong absorption bands that it is not practical without most of the 2- to 15p region, and only limited a r e a are uncompensation methods to observe the absorption of a small usable. It would appear that a number of solvent pairs could concentration of solute. T o illustrate these points, dehydrobe established which (1) are good solvents for a given class of isoandrosterone was studied a t a concentration of 1 to 2% in organic compounds, and (2) have complementary transmittance chloroform. windows. These solvent pairs should be established in the literaFigure 6 shows the spectrum of the dissolved sample, A , with ture with the regions of usability and nonusability designated. compensation by an equal amount of solvent in the reference It is desirable that solvent use be standardized, for the specbeam and, B, without compensation. The thickness of solution trum of a solute may vary with the nature of the solvent, and a was 1 mm. The curves were run in 17 minutes each at a senpublished solute spectrum may be valid only for the eolvent sitivity of 0.4 r.m.s. microvolt and a response time of 12.5 used. For example, a series of very nice correlations of absorpseconds for full scale deflection. Higher amplifier gain ww tion frequency us. position of carbonyl substitution on the steroid used for the compensated run than for the others. h the filter nucleus was established (18)using carbon disulfide as a solvent. was not used. the failure of the 13-micron chloroform band to These correlations were observed to shift slightly in carbon reach zero indicates the amount of scattered light in the monotetrachloride, but they were almost entirely destroyed when chromator alone. observed in chloroform ( 7 ) . It seemed that the interaction of The most interesting part of this spectrum lies between 10 the carbonyl group with the polar solvent brought most of the and 12 microns, where the sample has a group of characteristic carbonyl frequencies to the same value. Thus in choosing paired absorption bands. A strong solvent band in this region has been solvents, it might be &ell to select the most nonpolar possible, eliminated in the compensated run, presumably to the extent bearing in mind that double-beam compensation permits solvent indicated by the removal of the 2.5-micron chloroform band, The resulting spectrum is satisfactory except in the shaded areas, thickness much greater than hrts been previously considered acceptable. It is also apparent that solvent compensation where the solvent’s transmittance was less than about 20%. requires the use of an accurate variable-thickness absorption cell. In these regions the instrument was insensitive and drifted slowly up to the right because of a small electrical signal introduced to ACKNOWLEDGMENT make the pen responsive a t zero transmittance. When these spectra were measured, the humidity was high enough to make In conclusion, the author wishes to thank V. Z. Williams of the water bands pronounced in the 6-micron region. the Perkin-Elmer Corporation for help and suggestions in preparing this paper, R. C. Gore of the American Cyanamid ComSOLVENTS pany for samples of o-toluidine and a- and 8-conidendrin, The spectra shown illustrate the advantages to be gained for R. F. Stamm of the American Cyanamid Company for the chemical research and analysis from the use of a wide range of sample of ethylbenzene, Konrad Dobriner of the Memorial instrument variables. The principal one is the possibility of Hospital for the sample of dehydroisoandrosterone, and Norman extending into the infrared the benefits of solvent compensation Wright of the Dow Chemical Company for the sample of styrene. that are now realized in instruments for the visible region. LITERATURE CITED Because of the solvent difficulty in the infrared, the art of mineral oil or perfluoro lubricating oil mulls has been developed for study (1) Baird, W. S.,O’Bryan, H . M., Ogden, G . , and Lee, D., J . Optical of solid samples. This technique is usually satisfactory for qualiSoc. A m . , 37, 754 (1947). (2) Baker, E. B . , and Robb, C. D., Rev. Sci. Instruments. 14, 362 tative work b u t very difficult for accurate quantitative analysis. (1943). The experimental technique is difficult, and changes in the crystal (3) Barnes, R. B., Gore, R. C., Williams, E. F., Linsley. S. G . , and form of a compound often change its infrared spectrum. Only Petersen, E. M., IND. ENG. CHEM.,ANAL. ED., 19, 630 one paper (3) has appeared describing quantitative analysis in (1 947). (4) Barnes, R . B.,McDonald, R . S.. Williams, V. Z., and Kinnaird, the solid state by use of an internal standard, and it is evident R. F., J. Applied Phys., 16, 77 (1945). that the method is tedious and requires a skilled technician. (5) Brattain, R . R . , and Beeck. O., Ibid., 13, 669 (1942). Now the use of solvent compensation will permit much more (6) Coblentr, W. W., Carnegie Inst. Pub., Washington, D. c., quantitative work with solid samples. Bull. 35 (1905).
ANALYTICAL CHEMISTRY
772 (7) Dobriner, K.,and Jonee, R. N., private communications. (8) Gore, R. C., McDonald, R. S., Williams, V. Z., and White, J. U:, J . Optical SOC.A m . . 37,23 (1947). (9) Hayworth, Richardson, and Sheldriok, J. Chem. Soc., 1935, 1576. ( I O ) Hayworth, Sheldrick, and Mavern, Ibid., 1935, 636. (11) Jones, R. N.,and Dobriner, K., Vitamins and Hormonca, 7, 293-363 (1949). (12) Jones, R. N.,Williams, V. Z., Wahlen, hl. J., and Dobriner, K., J . Am. Chem. Soc., 70,2024 (1948). (13) Lehrer, E., 2.tech. Phybik, 23, 169 (1942). (14) Liston, M. D., J . Optical Soc. Am., 37, 515A (1947). (15) Liaton, M. D., and White, J. U., Ihid., 40,36 (1950).
(16) MeAlister, E. D., hiatheson, G.. L.. and Sweeney, W. J., Rea. Sn'. Instrumenta, 12, 314 (1941). (17) National Technical Laboratories, Bull: 153 (May 1946). (18) Sutherland, G.B. B. M., and Thompson, H. W., Trans. Fareday SOC.,41, 171 (1945). (19) Trans. Faraday Soc., 41, 171 (1945). (20) White, J. U.,and Liston, M. D., J . Optical SOC.A m . , 40, 29 (1950). (21)Wild, R. F.,Rev. Sci. Instrumenfa, 18,436 (1947). (22) Wright, N.,and Herscher, L. W., J . Opfical SOC.Am., 37, 211 (1947). RECEIVED December 7, 1949.
BECKMAN FLAME SPECTROPHOTOMETER P. T. GILBERT, JR., R. C. HAWES, AND A. 0. BECKMAN National Technical Laboratories, South Pasadena, Calif. A new flame spectrophotometer, now commercially available, is characterized by a simple detachable atomizer handling ramplea of 1 ml. or less, a heated spray chamber for evaporating the spray, a versatile burner for oxygen-gas or other flames, and construction of the atomizer-burner unit ae an attachment for the Backman quartz spectrophotometer. Individual measurements can be made in very rapid sequence, and the precision of such measurements is a few tenths per cent of full scale. Interference effects and methods o f circumventing them are discussed. Flame spectra and detection limits are given for several dozen elementr, and excitation characteristics and methods of analysis are illustrated for a few cases.
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LAME spectrophotometry, as a quantitative analytical technique, has only recently aroused widespread interest in this country. Although fhne spectra have been used nearly 100 years for the qualitative identification of elements, the literature discloses little work on the use of flame spectra for quantitative determinations prior to 1929,when Lundeglrdh (7)published his first treatise on the method. A number of European papers on flame spectrophotometry have appeared since that time, but in this country it was not until 1939 (6) that any work was published, and only within the past 3 years has any apparatus been described which differs significantly from that of Lundeglrdh. Lundeglrdh and most of his followers employed a spectrographic technique. The solution to be analyzed waa sprayed into a flame placed in front of a spectrograph. After exposure, the photographic plates were developed and the optical densities of various spectral lines recorded thereon were measured. With the aid of suitable calibration data the optical densities could be correlated with chemical concentrations with relative accuracies of a few per cent. The photographic step introduced delay and inconvenience and also limited the accuracy of the method to the reproducibility of photographic emulsions. To overcome the disadvantages of the spectrographic method, direct-reading flame photometers were described as ea'rly as 1935. Instruments of this type were made by a t least three manufacturers in Germany prior to the war. In 1945 the first direct-reading flame photometer was described in this country ( 1 ) . Since that time many papers have appeared, describing applications and apparatus. Judging from the recent literature, it appears that the apparatus heretofore commercially available leaves much to be desired with respect to convenience and speed of operation, accuracy of measurement, and freedom from spectral interference in multicomponent solutions. To overcome these shortcomings the instrument described in this paper was designed. It has high optical resolving power with attendant freedom from the interference resulting from unresolved overlapping spectral bands. It provides high photometric accuracy. It is simple and fast in operation. Determinations
can be made in a few minutes, and no cleaning is required between samples. A single drop of sample is consumed in making a reading, and detectable concentrations may be as low as a few parts per 108 for the alkali metals and somewhat higher for other elements. The instrument has already been used for the determination of twenty elements and undoubtedly can be used for many more. Briefly, the features which distinguish the present instrument, described in greater detail below, from other direct-reading flame photometers, include : ( 1) a one-piece, high-suction, concentric atomizer requiring no rinsing and providing exceptionally constant and low rate of consumption of sample; (2) a heated spray chamber, which completely evaporates the spray, enhances luminous intensity, and improves stability of performance; (3) use of an oxygen-natural gas flame; and (4) construction of the atomizer-burner unit as a separate attachment to be used in conjunction with the Beckman quartz spectrophotometer whose utility for other purposes is not interfered with by its use as a flame sDectrouhotometer. DESCRIWION OF BECKMAN FLAME SPECTROPHOTOMETER
A satisfactory flame spectrophotometer must have adequate resolving power to differentiate between the spectral emission of the element being determined and the emissions of any interfering substances that may be present. For general utility and precise measurements an essential part of a flame spectrophotometer is necessarily a monochromator which provides a continuous selection of wave lengths with resolving power sufficient to separate completely most of the easily excited emission lines, and freedom from scattered radiation sufficient to minimize interferences. These requirements are provided by the monochromator of the Beckman Model DU spectrophotometer (4). As there are many hundreds of these spectrophotometers already in use throughout the world, it was decided to design a flame spectrophotometer which could take advantage of this monochromator and the aensitive and accurate photometric equipment of these instruments. In a paper presented in November 1947 before the Soil Science
