infrared Spectrometric Quantitative Analysis of Multicomponent liquid Hydrocarbon Mixtures J. W. KENT AND J. Y. BEACH Richmond Laboratories, Calijornia Research Corporation, Richmond, Calif. A method is described for making rapid accurate analyses of liquid hydrocarbon mixtures for all components. The method is a combination of distillation, infrared spectrometric, and calculation procedures. It has been applied to a wide variety of mixtures containing paraffin and isoparaffin hydrocarbons boiling between 28" and 124' C. Tests of Beer's law and optical density additivity are presented for several synthetic mixtures. The accuracy of the method is shown to be approximately 1% for each component.
I
S STUDIES of the isomerization, alkylation, or polymeriza-
tion of hydrocarbons, a major deterrent to a clear under,>tandingof the reaction mechanisms and the equilibria involved has been the lack of a suitable method for analyzing quantitatively the composition of complex liquid hydrocarbon mixtures. The principal reason for failure to develop a satisfactory method of analysis based on the measurement of the usual physical prc~pertiesis that many hydrocarbons have approximately the iame physical constants. Accordingly, different,iations are difficult to make. I n spite of this, Rossini et al. (4, 6, 11) have analyzed certain hydrocarbon mixtures, utilizing precise measureruents of the temperature, the density, and the refractive index of narrow-boiling-range distillation cuts obtained from a very efficient fractionation of the sample. From these data they have 1)tbt.n able to determine t,he major constituents of several alkyl;ttc> and hydrocodimer samples. The time, the cost, and the newssarv equipment, however, prevent such a procedure from lwinp universally acceptable. The present, paper reports a set of FTorkable procedures for the quant,itat,ive analysis of multicomponent liquid hydrocarbon niistures, based on infrared spectrophotometry. The method i h R combination of distillation, infrared, and calculation procrdure. The infrared step is a modificat,ion and extension to liquids of the methods developed by Brattain, Rasmussen, and C'ravath (1). Applications have been made to paraffinic and isoparaffinic liquid mixtures for all constituents boiling between 28" C. (2-met,hylbutane) and 124' C. (2,2,5-trimethylhexane), escept, 2,2,3,3-tetramethylbutane. These examples illustrate procedures which can be applied to mixtures containing naphthenes, olefins, and aromatics in addition t,o paraffins. The possibility of analyzing liquid mixtures of isomeric hydrocarbons by infrared methods was first shown by Oetjen (8). This kind of analysis has also been reported by Thompson, Sutherland et nl. (IS'), by White and associates ( l e ) ,by Sweeney (12), and by Fry et al. (5). Nielsen and Smith ( 7 ) have analyzed liquid nitroparaffin mixtures. While there are many points of similarity twtween their methods and the authors', there are also important differences. I n particular, the method of calculation, the complexity of the mixtures which can be analyzed, and the presentation of the instrument data will be of interest to workers in t'he field of hydrocarbon research.
and which is applicable regardless of the concentrations of the isomers. The distillation procedure finally selected will necessarily be a compromise among distillation efficiency, distillation time, and the isomclric separat,ionswhich must be made by the fractionation. The distillation column used for this work is a glass tube 20 mm. in diameter and 1 meter long, packed wit,h single-turn a / 3 2 inch glass helices. The column is lagged with a layer of magnesia 5 em. (2 inches) thick. The column head is of the total reflux-intermit'tent take-off type usually operated a t a reflux ratio of 20 to 1 with a take-off rate of 1 cc. per minute. The charge to the still pot is usually 300 cc. of a sample together with 50 cc. of a paraffinic chaser stock which has an initial boiling point of 150" C. Under these conditions, a sample can be completely fractionated into the required cuts in 8 hours.
In the early developmental stages of the procedure, samples of the same size were distilled uader conditions of higher reflux ratios and slower t,ake-off rates. Analysis, by the infrared procedure described, of the cuts from tivo aliquot parts of a given material distilled under these rapid conditions of higher distillation efficiency showed that the over-all results duplicated well within experimental error, and it was concluded that litt'le if anything was t,o be gained by carrying out the longer distillation. This conclusion is not, surprising, in view of the fact that analysis by infrared met,hods of cuts from one of Rossini's distillat'ions (15) shows that even with their highly efficient fractionation procedure, narrow cuts may contain as many as five components, while Whit,e (14) has shown that distillation of liquid mixtures in columns of about 75 plates may yield cut's containing appreciable concentrations of components whose boiling points are
Table I. Cut
Temperature So. Range
Remarks
c.
28- 72
72- 85 85- 98 98- 9 9 . 5 99.5-1 11
EXPERIMENTAL 111-114
Distillation Procedure. The distillation step is for the purpose I J ~simplifying the mixture, so that the spectrometric step will nut be too difficult. One of the primary requirements of this :cnalytical method is the establishment of a fractionation protedure which is rapid, yet efficient enough to make sufficient separation of isomers for obtaining best spectrometric accuracy,
Distillation Cuts
S t a r t collecting cut a s soon a s boiling point rises from isopentane plateau If mixture contains much 2,2,4-TMPa, collect abput 20% of this hydrocarbon in this cut If mixture contains a large amount of 2,2,4-ThIP this fraction will be pure 2,2,4-ThfP. I t should consist of, about 69% of 2,2,4-TMP present This cut will contam remaining 20% of 2.2,4T N P Dresent. This fraction should be cut sharpl$,at 111' C. This fraction should be cut sharply a t 114" C
114-116 __ ._. .
8 9
0
b
290
116-120 120-124
This cut can be analyzed successfully ~prily i f sufficient 2,2,5-ThIHexb IS present to blork" higher boiling hydrocarbons T M P trimethylpentane. T>IH'eeu, trimethylhexane.
V O L U M E 19, NO. 5, M A Y 1 9 4 7 as much as 10' ( ' . outside the boiling range of the cut. In general. therefore, it is usually impossible to know a priori just how niany and which of the possible components in a mixture are prestmt in a given distillation cut,. This greatly affects the number of rompounds for n-hich analyses must t x made, and the c.ut,s must kw analyzed for ten, eltw~ii, or twelve cwmponentx. If 300 cc. of sample are not available, the distillation is made with a Podbielniak Super-Cal column IIodel S o . SC 706-D. This column is 13 mm. in diameter and 90 cm. (36 inches) long, equipped with Heligrid packing. The distillation head is of the total reflux-intermittent x- a p o r phase takeoff t y p .
291
Table 11. Components and CharacteristicWave Lengths in RIicroiis for Nine Distillation Cuts of Paraffin Hydrocarbons Boiling Range,
2-Methylbutane n-Pentane 2,2-Dimethylbutane 2,3-Dimethylbutane 2-hIethylpentane 3-.Methylpentane n-Hexane 2,2-Dimethylpentane "4-Dimethylpentane 2,2.3-Trimet hylbutane 3,3-Dimethylpent ane 2,3-Dimethylpentane %Met hylhexane S-Methylhexane (3-Ethylpentane I n-Heptane 2,2,4-Trimethylpentane 2.2-Dimethvlhexane 2.5-Dimethblhexane
28-72 10.26 10.99 12.80 8.92 13.52 10,49 11.27 8.00 12.37 9.05
...
... ... ...
...
... ... . . I
, .
,..
...
... ,..
72-85
85-98
...
...
...
...
...
8.71 10.49 11.27 8.00 12.37 9.05 12.69 8.92 11.02 12.97 , . .
...
... ...
... ... ... ... ...
98 - 9 9 . 5
...
99 5-111
...
...
... ...
...
... ...
, . .
..,
... ... ... .. ... .. ... ...
...
,..
..
...
...
... 10'. i 6 12.3i
9.05 12.69 8.92 13.71 12.95 11.14 13.83 7.78
... ,.. ... ... ... ...
... ...
... ...
...
...
8.92 13.71 12.95 11,14 13.83 7 78 9 05 8.53 8.19 10 04
... ...
...
...
... ... ..
... . . ...
B'i,
13.71 12 9,5
. . ...
1 3 , 8.3 7 78 9.05 13.27 8.19 13.05
...
...
9.62 9 92 10.97 11.14 13.49 10 51 8 53 12 69 11.26 10 35
... ... ...
...
..
, . .
...
For alkylate or hydrocodimer samples distillation cuts are made in the manner indicated in Table 1. For special mixtures which are known to contain a large amount of certain constituents it is possible to alter the distillation procedure to simplify the composition for the spect,rometric analysis. For example, most alkylates cont,ain large quant,ities of 2,2,4-trimethylpentane. By distilling about ZOY0 of the estimated concentration of this octane into t,he 85" to 98" C. c u t , it was found that a "blocking effect" was obtained and no compounds boiling higher than 2,2,4-trimethylpentane were di4lled into this cut. A heart, cut of the iso-octane plateau comprises about 607G of the 2,2,4-trimethylpentane, while the remaining 2OYO is taken into the 99.5' to 111' C. cut n-hich again, because of the blocking effect, contains no compounds boiling at, temperatures below 99" C. This blocking effect occurs for a mixture only when one compound is present in the \vhole sample in a concentrat,ion of 20Y0 or greater. Infrared Technique. The infrared spectrometer used in these .-tudies is basically the I.R.-l routine infrared spedrophotometer manufactured by rational Technical Laboratories. The original instrument has been modified considerably in keeping rrith the general requirements of high accuracy in optical density, D, at characteristic wave lengths and high speed of obtaining vomplete approximate spectra, preliminary to selecting the vharacteristic wave lengths. Clearly, the best accuracy in D is obtained by a null method of measuring thermocouple output. For this purpose the authors used a potentiometer, calibrated in terms of D, in conjunction with a high-sensitivity galvanomtLter. K i t h this simple device, questions of linearity of galvanomt,ter and amplifier are avoided. In contrast to the methods 1rhic.h depend on automatically recording the entire spectrum, this null method allows one to perform and to check the measurerntsiits in a short time. The I and 10measurements are made in quick succession with different settings of the same potentiometer. The measurements of ZOare made with a dummy cell consisting of a single rock salt window equal in thickness to the total thickness of the two sample cell windows. I n this way much of the reflection by the sample cell is compensated for. Inasmuch as both the calibrating liquids and the mixtures t o be analyzed are measured in the same inanncr. the error in ZO introduced by the use of this dummy
...
... ...
... ...
... ..
... , . .
.. ..
, . .
"Methyl, 3-ethylpentane 2,3-Dimethylhexane 4-Methylheptane 3,4-Dimethylhexane 2-hlethvlheDtane
...
116-120, 120-124
...
...
...
111-114 114-116
... .. ...
,..
...
C.
...
...
, .
.. 9162 9.92 10.97 11 14 13 49 10.51 13 70 12 69 11 26 10.35 8 14 8 28
cell should sufficiently cancel u u t 111 the ~tnnlvticaliebults The over-all consistency of the results appeals t o justify this vien. The sample cell replaces the usual gas cell and IS constructed from two circles of rock salt 8.9 cm. (15/16 inches) in diameter and 0.475 cm. ( 3 / 1 6 inch) thick separated by an amalgamated copper spacer cut from 0.075-mm. (0.003-inch) sheet. The circles and spacer are clamped firmly between two metal plates fitted with neoprene, amalgamated lead, or amalgamated copper gaskets. One metal plate and the adjacent rock salt circle are drilled a t top and bottom to permit introduction or withdrawal of the sample from the cell. The usual false energy corrections have been dispensed ithin the authors' measurements. This simplification is accomplished by the use of a glass shutter in place of the lithium fluoride shutter and the magnesium oxide filter. The false energy consists mostly of scattered short wave length ( < 2 ~ ) radiation, which is transmitted by the glass shutter. Thus the scattered radiation reaching the thermocouple for given wave length and slit-width settings is nearly the same before and after the shutter is raised. Only that part of the false energy which fails to get through the shutter can affect the change in thermocouple output, measured by the potentiometer. Reflection of short Q-ave lengths and transmission of long wave lengths are small for glass. I n the authors' instrument, using a cell 0.10 mm. thick filled with carbon tetrachloride as a total absorber and having the spectrometer set a t 13.40y, with spectral slit width of 0 . 1 5 ~no~ deflection (*0.5 mm.) of the galvanometer was detected when the glass shutter was placed in or out of the optical path (IO corresponds to a 100-mm. deflection). Thus the amount of scattered light having a wave length greater than 2q is very small. Even in less favorable cases, the error in I and IC,due t o the use of the glass shutter should largely cancel in the final analytical results. The slit widths were selected to give adequate experimental accuracy a t each wave length. The optimum settings were obtained a t the outset by decreasing the slit width until the minimum accurately measurable galvanometer deflection (100 mm.) was obtained, using the dummy cell. Thus slit widths of 0.065 to 0.44 mm. were used between 8 and 14p, corresponding to spectral slit midths of 0.04 to 0.15,~. I n order to facilitate the selection of characteristic wave lengths, provisions were made for automatically recording the entire spectrum. The thermocouple output was amplified electronically and fed into a continuous recorder. The prism and Littrow mirror arrangement was continuously and automatically rotated a t constant angular velocity during the recording operation The continuous drive could be set manually a t any n-ave length chosen for the quantitative multicomponent analysis; this feature was also useful in accurately exploring narrow spectral regions by the null method
292
ANALYTICAL CHEMISTRY -
Table 111. Tests of Beer’s Law and Additivity Law Wave Lengths ( F ) Tested, between 0 and 100% 8.92, 11.27, 1 3 . 8 7 “
liquids they have not been tested sufficiently to permit any useful generalization.
To justify the above simple calculation procedure, which was finally adopted, the authors examined a variety of binary liquid paraffin mix2,4-Dimethylpentane-n-hep- 12.37, 13.855, 1 3 . 8 7 “ tane tures and found that in general the optical 3-Methylpentane-%methyl1 0 . 4 9 , 13.04, 11.27, 13.5 Z a density is a sufficiently linear function of concenpentane 2,2-Dimethylbutane-2,3-di8 . 4 5 , 12.80, 8 . 9 2 Phillips Petroleum Co. tration, a t least for certain wave lengths. Table methylbutane General Motors Corp. 2,2,3-Trimethylbutane-2,3- 8.92, 9 . 0 3 , 9 . 0 5 , 9 . 2 3 “ General Motors Corp. I11 gives the compositions and wave lengths for dimethylpentane which measurements were made over the entire 2,2,4-Trimethylpentane-n7 . 7 8 , 10.68, 1 3 . 8 7 a Rohm 8: Haas Standard heptane Fuel concentration range. A t the wave lengths marked I$-estvaco Standard Fuel with a superscript deviations became detectable a D u s . C plot deviated from linearity by 1 t o 5 % . around 50 mole 9 0 and reached as high as 5% at 100 mole %. At all other wave lengths the deviations from linearity were less than 1%. In all cases tested, the wave length selected for the analytical CALIBRATIONS AND CALCULATION PROCEDURE method showed satisfactory linearity, as can be seen by comparing Tables I1 and 111; indeed the linearity test should be considered &s Using the automatic recorder, a wave-length calibration was the basis for choosing between two wave lengths which are equally made with ammonia, carbon dioxide, water vapor, 2,2,4-trigood in other respects. Some of the compounds analyzed were not methylpentane, and n-heptane as calibrating substances. The available in sufficient quantity to permit a linearity test, but the wave-length positions of the absorption bands of these comset of binary mixtures in Table I11 should be representative. pounds were obtained from the data of Oetjen and Randall et al. (9, IO). To facilitate the solution of the set of simultaneous equations for each of the nine cuts, an experimental model of the electric From the recorded spectra of all the hydrocarbons which might computer designed and constructed by the Consolidated Engibe present and from more accurate data obtained by the null neering Corp. of Pasadena, Calif., was kindly made available for method in certain useful regions where two or more compounds use in their laboratory. This instrument has since become had interfering absorptions, it was possible to select a wave available commercially. With the aid of this device, the reciprocal matrix for each set was obtained quickly and accurately length which is characteristic of each compound and for which from the calibration coefficients, E$,. These reciprocal matrices all other compounds of the mixture are considerably weaker then have the form: absorbers. I n general, the extinction coefficients of the interXI = cii Di CIZ Dz . . . . . C I D~n fering compounds were kept to less than one half that of the Xz = czi Di ~ 2 D 2z . . . . . C Z Dn ~ principal absorber, although occasionally nearly equal extinction coefficients for two compounds were unavoidable. In these cases, . . i ~ n D2 2 . . . . . ~ n Dnn Xn = ~ n D, the differencesa t other wave lengths were great enough to allow an analysis. In choosing the characteristic wave lengths, the where the (cij)’s are constants which are called “concentration factors”. With this set of equations only a simple sum of positions of the most intense absorptions are not always the best. products need be performed to obtain the values of the conOften, other wave lengths are preferable because the optical centrations, Xi, of the various components in the mixture when density is more closely a linear fuhction of the concentration or the experimentally measured values of optical density Di, are because, for the types of mixtures encountered, D more closely substituted in these equations. The use of the e1ectri)cal computer for obtaining the reciprocal matrix appreciably reduces approaches the optimum values for the best accuracy of measurethe time required, especially when the number of components ment. is eight or more. Another way of making these calculations with In Table I1 are shown the characteristic wave lengths selected the computer is to solve directly each set of n equations for each for each component in each of the nine cuts considered here. analysis, keeping the calibration coefficients E in the machine unless other types of problems intervene. Two sets of wave lengths are given for analyzing cut 4 because if it is known that 3-ethylpentane is absent, determinations with This analytical method of calculation is a safer one than those the alternative set may be made more accurately. In a new methods which rely on rough estimates of the interferences, such and unfamiliar type of mixture, however, analysis of cut 4 as by the extrapolation of the background of a recorded spectrum. should be made for all ten possible components. Indeed, one is forced into as rigorous a treatment of interferences The calibrating compounds were National Bureau of Standards as possible when the number of components in the mixture standard samples produced through the cooperative program of becomes larger than three or four. the ilmerican Petroleum Institute and the National Bureau of Standards (3). RESULTS Components 2,3-Dimethylbutane-n-hexane
Source of Material Phillips Petroleum Co. General Motors Corp. California Research Corp. IVestvaco Standard Fuel Phillips Petroleum Co.
The optical densities of each compound a t all the necessary wave lengths were measured accurately by the null method; for a mixture of n components there are n2 calibration determinations. The resulting values of D for the pure compounds, obtained with a fixed cell thickness, t , were called the “calibration coefficients”, E . Since E = t., where B is the extinction coefficient, we can use these values of E in all subsequent computations of analytical results, provided t has remained constant during the calibrations and during an analysis. The simplest possible procedure for calculating concentrations from the observed optical densities of multicomponent mixtures would be one based on a linear relation between D and C for each component in the presence of the others. This case would involve the straightforward, if laborious, solution of n simultaneous linear equations, preferably by the reciprocal matrix method of Grout ( 2 ) . However, such linear relations imply that Beer’s law is applicable to the mixture and that the optical density is composed additively of the D’s of each component, Such conditions are by no means always met and in the case of
++
++
++
+
+
+
The final and most rigorous test which can be applied t o any analytical method, of course, is the analysis of synthetic mixtures.
Table IV.
Synthetic Mixture Analysis
SynHydrocarbon thetic Analysis dnalysis Analysis Analysis voz. % Vol. % Vol. % Vol. % Vol. % 20.0 19.6 2,2,4-Trimethylpentane 2 0 . 0 19.7 20.1 19.4 19.2 19.5 19.4 2,3-Dimethylpentane 18.8 20.2 20.8 20.4 21.0 2.2.3-Trimethylbutane 2 0 . 2 -0.7 -0.4 -0.2 0.0 2,2-Dimethylpentane 0.0 3-Ethyipentane 1.0 1.0 1.0 1.1 1.0 2,4-Dimethylpentane 20.0 19.6 20.0 19.9 19.7 3,3-Dimethylpentane 0.0 0.4 -0.3 0.1 0.0 0.1 -0.4 2-Methylhexane 0.0 -0.1 -0.3 2-Methylhexane 0 0 0.0 -0.3 0.3 0.0 19.5 20.2 n-Heptane 20.0 20.0 20.1 Mean average error
Average Error
% 0.2 0.6 0.3 0.3 0.0 0.2 0.2 0.2 0.2 0.2 0.26
V O L U M E 19, NO. 5, M A Y 1 9 4 7
.
If accurate analyses of such mixtures can be made, the assumptions which have been made in setting up the procedure are justified. Such a synthetic mixture containing six hydrocarbons has been analyzed, using the appropriate ten-component matrix by four independent sets of measurements with the results shown in Table IV. According to these results, this ten-component analysis can be made with an accuracy of 1 % on each component. In cases where this cut is a small percentage of the total sample, the accuracy on the basis of the original sample is much higher. The small negative answers mean zero and arise because the results are obtained by solving the equations which can give answers on either side of the correct answer (in this case zero). These procedures have been in use in this laboratory for some time for making complete analyses of mixtures of paraffin and isoparaffin hydrocarbons for all components.
293
(2) Crout, P. D., Trans. Am. Inst. EZec. Engrs., 60,1235 (1941). (3) Demmerle, R. L., Chem. Eng. News, 24, 2020 (1946). (4) Foraiati, A. F., Willingham, C. B., Mair, B. J., and Rossini, F. D., J . Research Nutl. Bur. Standards, 32, 11 (1944). (5) Fry, D. L., Nusbaum, R. E., and Randall, H. M., J . Applied Phys., 17, 150 (1946) (6) Glasgow, A. R., Jr., Streiff,A. J., Willingham, C. B., and Rossini, F. D., Petroleum Refiner, 25, 93 (1946). EXG.CHEM.,ANAL.ED., (7) Nielsen, J. R., and Smith, D. C., IND. 15, 609 (1943). (8) Oetjen, R. A., thesis, University of Michigan, 1941. (9) Oetjen. R. A.. Kao. C. L.. and Randall, H. M.. Rev.Sci. Instruments, 13, 515 (1942).
(10) Oetjen, R. A., Randall, H. M., and Anderson, W. E., Rev. M o d . Phys., 16, 260 (1944). (11) Rossini, F. D., Mair, B. J., Foraiati, A. F., Glasgow, A . R., and Willingham, C. B., Oil Gas J.,41, No. 27, 106 (1942). (12) Sweeney, W. J., IND.ENG.CHEM., AXAL.ED.,16, 723 (1944). (13) Thompson, H. R., and Sutherland, G. B. B. M., Trans. Faraday Soc., 41, 197 (1944); and information distributed by British
Central Scientific Office, Washington, D. C.
ACKNOWLEDGMENT
The authors iyish to express their appreciation to Xorman Bauer of this laboratory for valuable assistance during the preparation of this paper.
(14) White, J. U., unpublished communication on work done in
Esso Laboratories, Standard Oil Development Co. (15) Willingham, C. B., and Rossini, F. D., J . Research Natl. Bur. Standards, 37, 26 (1946).
LITERATURE U T E D
(1) Brattain, R. R., Rasmussen, R. S., and Cravath, A. M., Applied Phys., 14, 418 (1943).
J.
PRESENTED a t the October 1945 meeting of t h e California Section, AMERICAN CHEMICAL SOCIETY.
Application of Infrared Spectroscopy to the Analysis of liquid Hydrocarbons J. J. HEIGL, M. F. BELL, AND J. U. WHITE’ Esso Laboratories, Standard Oil Development Go., Elizabeth, N . J . The application of infrared spectroscopy to the analysis of hydrocarbon mixtures is described in detail. Emphasis is placed upon accuracy, rapidity, and simplicity of the “base-line” technique which uses spectra superimposed on the radiant energy background. An analysis of an octane fraction illustrates development of the procedure. The method can be applied to a wide variety of samples. Instrumental requirements for satisfactory accuracy are listed.
S
TANDARDIZED infrared spectrometric analyses which can be used to obtain accurate results rapidly are now invaluable in laboratory and pilot-plant development of methods for the production of high-quality gasoline. These simplified procedures can be generally used in the analysis of hydrocarbon mixtures. T h e present paper describes the technique which has been applied t o samples containing principally paraffinic hydrocarbons with from five to nine carbon atoms. The method described is based on the proportionality between concentration and absorption peak height above a spectrum reference line. These “base-line” optical density measurements (16) are applied in preference to optical density measurements utilizing a reference cell determination, because of advantages in speed of spectrum scanning, calibration, and calculation. The steps employed in developing a procedure include determination of the spectra for all compounds present in the mixture t o be analyzed, selection of absorption peaks characteristic of the individual compounds, measurement of the base-line optical densities a t the wave lengths of the peaks selected, and derivation of the equations to be used in analyses. The analysis of a typical gasoline sample is carried out by fiist separating the sample into fractions containing four to eight COI11ponents, by means of a precision distillation column. Analysis of 1
Present address, Perkin-Elmer Corp., Glenbrook, Conn.
the fractions involves (a) proper blending of cuts of similar composition, ( 6 ) qualitative determination of the compounds present by inspection of the complete spectrum, (c) measurement of the baseline optical densities a t the selected wave-length intervals, (d) substitution of these values in the appropriate equations, and ( e ) solution of the equations and calculation to obtain the final results. A commercial spectrometer equipped with sodium chloride optics is employed to obtain spectra for the 7.5- to 14-micron region. The accuracy of the meth6d established by the analysis of synthetically prepared samples averages 1yo; the time required for an individual infrared determination is one hour, which includes scanning of the spectrum and calculation of the results. The total time requirement for an alkylate sample is approximately 25 manhours for distillation, blending of cuts, and infrared analysis. DEVELOPING T H E ANALYTICAL PROCEDURE
The infrared spectrum, superimposed on the radiant energy background, is used directly in both the determination of calibration data and the subsequent sample analysis. Because of the decrease in radiant energy as scanning toward the longer wave length progresses, the slit widths must be increased at several wave-length positions. Such increases in the slit setting are made when the recorded spectrum falls to 5Oy0 of full chart reading, re-
