Aromatic Molecular Weight Distribution and Total Aromatic Content Determination by Mass Spectroscopy H. E. LUMPKIY AND B. W. THOMAS Humble Oil and Rejining Co., Baytown, Tex. In a refinery producing motor gasoline and aromatic solvents, rapid and accurate determination of total aromatics and aromatic molecular weight groups is frequently necessary for refinery process control and for evaluation of pilot unit products in process development studies. The distillation, silica gel, acid absorption, and specific dispersion methods may become laborious. The mass spectral patterns for the aromatic compounds are sufficiently different from the other compound types in a hydrocarbon mixture to allow determination of aromatics only in a com-
plex mixture. A procedure is presented which has an elapsed time requirement of 45 to 60 minutes and an accuracy of about &lqo for the different aromatic molecular weight groups and for total aromatic content. This procedure may be adopted by any group possessing an analytical mass spectrometer. The molecular weight distribution determination feature, in particular, will effect a considerable saving in time requirements, as a distillation step must be employed with conventional analytical procedures in order to obtain similar information.
0
VER the years many methods have been devised for the
determination of aromatics in wide boiling petroleum fractions. The ASTM procedure (I) employs a mixture of sulfuric acid and phosphorus pentoxide to absorb olefins and aromatics; olefins are calculated from bromine number, and aromatics are determined by difference. Grosse and \Tackher ( 5 ) developed a method for aromatic determination based on specific dispersion. This procedure requires the determination of refractive index, density, and bromine number on a series of distillation fractions. The silica gel percolation technique has been developed by Conrad ( 4 ) ,in which a fluorescent additive is used to differentiate the aromatic-olefin portion from the saturates in a silica gel packed column. ?*lair ( 7 ) , Kurtz et al. ( 6 ) , and Spakoivski et al. (11) have used various combinations of distillation, silica gel percolation, acid absorption, and specific dispersion in developing procedures for aromatics determination. Each of these methods depends upon accurate determination of olefinic content, m hich in turn is dependent to some extent upon an estimate of the molecular weight of the olefins. Time requirements for the various procedures vary from 45 minutes to several hours, depending primarily on whether or not distillation is required. *4rapid method which has been in use in one or more forms in this laboratory for several years employs the mass spectrometer, The procedure will give a reliable value for total aromatics in boiling ranges up to about 350" F. and, in addition, %-illdistinguish and determine benzene, toluene, total CSaromatics, and total Cg aromatics. I n the mass spectral pattern of hydrocarbon mixtures containing paraffins, naphthenes, aromatics, and olefins there is no contribution to the masses used in aromatic analysis by the nonaromatic components. Thus it is possible to determine aromatics independently of other components in a complex mixture, provided the amounts of material charged to the mass spectrometer for analysis and for calibration are made equivalent. APPARATUS AND PROCEDURE
A description of the Consolidated Engineering Corp. mass spectrometer used in this study and a historical sketch on the use of the instrument have heen given by Washburn el al. ( 1 2 ) . The mass spectrometer has been widely used for analyses of gases and more recently for liquid hydrocarbons; an excellent survey of the application of this analytical tool to various theoreti-
cal and industrial problems has been recently made by Shepherd and Hipple (IO). B r o w et a/.( 3 )have applied the mass spectrometer to the analysis of normally liquid hydrocarbons in the Cs to Cg range; however, when analyses for individual naphthenes and paraffins are obtained, time-consuming separation and distillation techniques are required. Brown ( 2 ) has also developed a hydrocarbon-t>pe analysis which includes determination of aromatics. A procedure for the determination of aromatic molecular F\ eight distribution and aromatic content was developed by the authors several years ago. As pressure measurements could not be used as an index of the amount of liquid material charged to the instrument, the internal standard system was utilized. This system involved the blending of a constant amount of a standard compound (carbon tetrachloride and normal propyl sulfide were used) nith samples of calibration compounds in order to permit placing peak heights on a comparable basis. The time required to make the blends, the high cost of the rather large amount of pure compounds used in calibration, and the inherent inaccuracies associated with the internal standard system were major drawbacks to the method; these have been circumvented recently by adoption of the constant-volume pipet and introduction slstem designed by Purdy and Harris (9). Use of the constant-volume pipet has improved the accuracy of the aromatic molecular n eight determination and has made the determination of total aromatics competitive with the other standard methods for total aroniatic content. Although there is no interference from naphthenes, paraffine, and olefins in the aromatic determination, a small amount of interference in the parent mass position of the lower molecular T? eight aromatics by the heavler aromatics is observed. This interference is due primarily to isotopic peaks from fragments whose primary peaks occur one mass unit lower. Typical calihration data for the Ca to C9 aromatics, shown in Table I, indicate the extent of this interference to be relatively minor. Calibrations for benzene and toluene are made directly by introducing identical amounts of the high pdrity materials to the mass spectrometer, utilizing the constant-volume pipet, and making the runs a t standardized conditions. Weighted average data for the Cg to C9 aromatics may be obtained elther by aeighting the data from calibration runs on each of the pure compounds or by calibrating with two synthetic mixtures
1738
V O L U M E 23, NO. 12, D E C E M B E R 1 9 5 1
1739
containing the CS and the CS aromatics in their anticipated distribution. Isomeric distributions for the Cs and Cs aromatics used in this laboratory are also shorn-n in Table I. These are equilibrium concentrations calculated from thermodynamic data available in AkPIProject 44 reports (8) and check well with distributions determined in aromatic fractions of virgin naphthas. The calibration method involving runs on each pure compound is preferable, as in special cases, such as distillation fractions of aromatic concentrates, a particular isomer may be dominant in the sample. I n such cases, determination of total aromatic content is mole accurate if calibration data for that specific compound are applied. The ratios of the aromatic parent mass abundances of the sample t o those of the calibration compounds determine the aromatics in the various molecular weight groups. Total aromaticity i. determined by mmmation of the groups.
Table I.
Mass Spectrometer Calibration Data for Pure Aromatics 78
Compound
m l e (Parent Masses) !06 92 Dirisions/Pipeta
120-
1nn
Benzene 26.9 ... Toluene 0.03 16.4 Ethylbenzene (10X)b 1.93 1.95 o-Xylene (23% 1.43 1.36 m-Xylene ( 4 6 2 ) 1.21 1.08 p-Xylene (21%) 1.20 1.14 Weighted average Cs’s 1.33 1.24 I-Propylbenzene (3%) b 1.74 3.10 sopropylbenzene (1%) 1.26 0.12 o,81 o,60 -Methyl-2-ethylbenzene (8%) .-Methyl-3-ethylbenzene (19%) -Methyl-4-ethylbenzene (1 1%) ,2 3-Trimethylbenzene (10%) o 75 0.42 ,2:4-Trimethylbenzene (36%) 0.75 0.40 0.72 0.40 ,3,5-Trimethylbenzene (12%) Weighted average CP’S 0.81 0.63 a Volume of pipet approximately 0.001 ml. Isomeric distribution calculated from A P I 44 data.
E:ii !:A
M92
% toluene
Table 11. Analyses of Refinery Streams by Various Methods
Refinery Stream Hydroformer feed Light catalytic naphtha Hydroforms te Toluenecorcentrate Toluene corcn. C.P. toluene (I dded)
+
a
... ...
... ... ... ... .. .. .. ..,
7.75 10.6 9.15 9.36 9.39 0.11 2.C7 1,99
6.25 5.93 6.88
1.54 1.53 1.41 1-68
11.2 10.4 10.5 9.11
E::
;:$
The method of computation used is shown below by a simple illustrative equation Volume
data from the other tlvo methods. The standard deviations indicate that the mass spectrometer method is more reproducible than the specific dispersion method up to about the SOY0 concentration level. I t is more reproducible than the acid absorption method up to about the %70concentration level. Variation in type of stock is probably of equal o r greater significance nith regard to reproducibility than the aromatic content, hon ever. The specific dispersion method mas at a disadvantage in this study, as no distillation was used for separating the aromatic groups.
Aromatic Content, Volume Per Cent Xass hcid Specific Spectrometer Absorption Dispersion i s X s i 8 10.65 0 . 2 1 4 11648 0 . 8 5 3 9 . 9 2 0.641 14.10 0.378 12.73 1 . 1 8 0 41.10 0 , 4 7 2 4 1 . 1 3 0:607 41.62 1 . 1 7 4 54.77 0 . 5 5 3 55.67 0.435 55 0 5 1 . 3 3 2 80.63
0,874
81.27 0 , 5 4 1
78 2 3
0 359
Unable to obtain acid absorption values; olefin content too high.
The wkle range in total aromatic content and in molecular weight diiitribution that can be handled by this procedure is illustrated by the results shown in Table 111. A comparison of the mass spectromet,er method with the acid absorption and specific dizpersion methods for various aromatic distributions in pilot unit products is shoTvn. Fairly good agreement for total aromatics by different methods is observed for the randomly selected :.nalyses. Mass spectrometer analyses of t,wo synthetic nib tures containing Ce to Cg aromatics in aromatic-free base stock are shown in Table IV, and accuracies of about 4=1 volume for the molecular weight groups containing significant amount,s c f aromatics are noted, On the basis of the variation in sensit,i>ities of the Cg aromatics shown in Table I results for
=
%cc”sX A(92 - CS’s)- ‘c‘s’s -100 100 A(92 -toluene) ~
X A(92 - CS’s)
x
100
where ill = mixture peak heights, -4 = calibration peak heights, and A(92--toluene) = the A value a t mass 92 for toluene. The heaviest aromatic group is determined first,, then the next heaviest, proceeding to benzene.
Table 111. Analysis of Rliscellaneous Samples for Arolr atic hlolecular Weight Distribution by Mass Spectrometer Co npound Benzene Toluene
~f~~~:i!~
Total arom:itics (M,,S.) Total aromitics (acid abs.) Total arom&tics(sp.disp.)
81.6 2.2 0.2 0.0 84.0 84.0 85.4
Volume Per Cent 12.1 1.1 0 . 8 8.855.312.9 0.8 2 . 5 3 2 . 2 0.0 1.0 2.4 21.7 59.9 48.3 22.2 60.3 48.1 2 4 . 1 6 2 . 9 48.7
73.0 1.1 0.1 0.0 74.2 73.9 69.7
0.2 0.0 2.7 6.0 34.545.5 2.1 2.0 30.5 53.5 39.2 55.7 39.2 5 4 . 7
_ . _ _ _ _ - - _ . -
DISCUSS103 AND RESCLTS
I n assessing the value of a new method, comparison of data with those obt,ained on ident.ica1 samples by established procedures is desirable. I n Table I1 the mas8 spectrometer method is compared with an acid absorption method similar to that of the ASTM procedure, and with a specific dispersion method in which the sodium D and mercury G lines are employed. The comparison is made on five refinery streams covering an aromatic content range of about 10 to 80% aromatics. Six replicate determinations were obtained on each sample by each method. Average aromatic contents, 5 , and standard deviations, s, computed from these data are shown. The mass spectrometer values for total aromatic content are well within rtl’%of the acid absorption values and in each case are intermediate to the
Table IV.
Compound
Mass Spectrometer Analysis for -4romatics in Synthetic Mixtures Known Composition
Volume Per Cent
Average
Deviation of Average from Synthetic
Sample l a Benzene 3.9 ~ ; ~ ~ ~ a t i c s3l 59 ,. 39 2.0 CSaromatics __ Totalaromatics 61.1
3.7 3.8 38.3 38.2 16.8 16.9 2.1 2.0 60.9 60.9
3.5 3.9 38.8 38.9 17.6 17.0 2.1 2.1 62.0 6 1 . 9
3.8 3.8 3.76 38.2 3 8 . 3 38.45 l 5 , 9 1 6 . 6 16.80 2.0 2.0 2.05 59.9 60.7 6 1 . 0 5
-------
Sample 2 b Benzene 1.05 1.0 0.9 0.8 0.8 0.8 0.8 0.87 9.3 9.3 9.2 9.4 9.35 Toluene 10.41 9.6 9.3 Cs aromatics 36.46 3 7 . 8 3 6 . 8 3 7 . 9 3 7 . 3 3 7 . 0 3 7 . 3 37.35 2 . 2 2 . 2 2 . 3 2 . 3 2 . 3 2 . 3 2.27 2.C9 cs aromatics Totalaromatics 50.01 5 0 . 6 4 9 . 2 5 0 . 3 49.7 4 9 . 3 49.5 49.84 a Analyses ohtained over ?-day period. b Analyses obtained over 2-month period.
- -------
-0.15 -0.85 +090
70.05 -0.05
-0.18
-1.08 i 0 89
10.18 -0.18
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ANALYTICAL CHEMlSTRY
C Yaroniatics are probably a c c u i x t r 10 =tlyoonly if the amount of C9's is less than 10 o r 15%. ACKNOWLEDGMENT
Permission of the Humble Oil and Refining Co. to release the information contained herein is gratefully acknowledged. An evpression of thanks is also due C. T . Shen-ell for his assistance in making the statistical calculations. The kind assistance of C. R. Middleton in procuring and analyzing the aromatic mixtures is gratefully acknowledged. LITERATURE CITED
( l i .lm. SOC.Testing Materials, Tentative Alethod, Designation
(3) l31ow11, R. A , , Taylor, R. C.. Melpolder, F. W., and Toung, W.S., Ibid., 20,5 (1948). (4)Conrad. A. L.. Ibid.. 20. 725 (1948). (5) Grosse, A. V., and Wackher, R. C., Ibid., 19, 992 (1947). (6) Kurtr, S S., Jr., M~lls,I. R., Martin, C. C., Harvey, R. T., and Lipkin, M. R., Ibid., 19,175 (1947). (7) Mair, B. J., J . Research Natl. Bur. Standards, 35,435 (1945). ( 8 ) Natl. Bur. Standards, "Selected Values of Properties of Hydrccarbons," Washington, D. C., Government Printing Office, 1947. (9) Purdy, K.M., and Harris, R. J., ;INAL. CHEM.,22, 1337 (1950). (10) Shepherd, M.,and Hippie, J. A , , Ibid., 22,23 (1950). (11) Spakowski. A. E.,Evans, A , and Hibbard, R. R., Ibid., 22, 1419 (1950). (12) Washburn, H.W., Wiley, H. F., Rock, S. bf., and Berry, C. E., ISD. ENG.CHEM.,ANAL.ED.,17,74 (1945).
D 875-461'.
( 2 ) Brown, R. -4., ; L u . 4 ~ . CHEY.,23,430 (1951).
R E C E I V EApril D 19. 1951.
Ultravielet and Visible Absorption Spectra in Ethyl Alcohol Data f o r Certain Nitric. Esters, Nitramines, Nitroalkylbenzenes, and Derivatives of Phenol, Aniline, Urea, Carbamic Acid, Diphenylamine, Carbazole, and Triphenylamine W. A . SCHROEDEH, PIIILIP E. WILCOX', KENNETH N. TRUEBLOOD*, AND 4 L B E R T 0. DEKKER3 California Institute of Technology, Pasadena, Calif. Spectrophotonietric information for 135 organic compounds w-as gathered to aid in the identification of compounds that had been isolated chromatographically in other studies. Absolute ethyl alcohol was thesolvent usedfor thedetermination all the spectra. The data are presented in table and graph and for the most part of are only roughly quantitative-that is, their accuracy is estimated to be 2 to 5'7'; more precise values, however, have been determined and recorded for certain compounds. The spectra of most of the substances have not previously been reported in the literature. No attempt has been made to present a correlation between structure and observed spectrum.
I
S T H E course of chromatographic-sprctrophotonietricstudies of smokeless powder and of the derivatives that are formed from several stabilizers during accelerated aging of such powder ( I 7-19, 22), the ultraviolet and visible absorption spectra of ethanolic solutions of a large number of compounds were determined in order t o aid in the identification and estimation of compounds after isolation from the chromatographic column. Only a small portion of these data has been reported in the abovecited papers and, because there is in the literature little or no recent spectrophotometric information about most of the 135 compounds that were investigated, the present paper is a compilation of the spectra which were determined. EXPERIMENTAL
Spectrophotometer and Its Use. All absorption s ectra were measured by means of a Beckman quartz photoeyectric spectrophotometer, Model DU (S), which was equipped with quartz cells. No attempt was made t o control the slit width, but rather the "sensitivity', was set a t 3 to 4 turns from the counterclockwise limit and the slit width was varied as necessary to balance the instrument. The slit width is relatively unimportant unless the compound has very sharp maxima as, for example, carbazole. Measurements were made from the shortest attainable wave length (usually less than 215 mp) t o a wave length a t which absorption was inappreciable-for example, colorless compounds usually were not measured a t wave lengths longer than 300 or 350 mp. The frequency a t which reading4 of optical density were taken depended upon the intricacy of the
' Present address, Harvard Medical 3
School, Boston, Mass. Present address, University of California, Los Angeles 24. Calif. Present address, Aerojet Engineering C o r p . , I z u - a , Calif.
spectral curve; around relatively qharp maxima and minima, readings were taken a t intervals of 1 mp for about 5 mp on either side of the maximum or minimum, whereas in other regions of the curve, intervals of 2 to 5 m p ere used. Solvent. Unless specifically noted otherwise, the absorption spectrum of each compound was determined in absolute ethg 1 alcohol. Commercial absolute ethyl alcohol without drying or further purification was used. I n most instances, the compound was simply dissolved in ethyl alcohol and the spectrum of the solution was measured. HOMever, in the case of a few compounds whose spectra are especially sensitive to the presence of acid or base, the spectrum was measured after the addition of 1 or 2 drops of 6 S hydrochloric acid or 6 S sodium hydroxide to 100 ml. of solution.
It must not be inferred that only those spectra which were studied in acidic or basic solution are affected by the acidity or basicity of the solution; certainly, the spectra of many of the nitrated compounds would also be so affected. Roughly Quantitative Procedure. For many purposes, sufficient information for practical use, such aa qualitative identification, could be obtained by determining the shape of the curve and the spectrophotometric constants Rith less than utmost precision. In such instances the following roughly quantitative procedure was used. About 10 mg. of compound were carefully weighed and dissolved in ahsolute ethyl alcohol and the solution was diluted in a volumetric flask. h preliminary examination of the curve was then made and, if necessary, the solution was diluted quantitatively until the optical density of the principal maximum xas betneeii 0 4 and 1.0. The entire spectral curve was finally determined at this dilution, The accuracy of the roughly quantitative Ixocedure is discuwed belov .
