Retention Time-Boiling Point Cor relations during Programmed Temperature Capillary Column Analysis of C,-C Aromatic Compounds Donald E. Willis Hydrocarbons and Polymers Dicision, Monsanto Co., St. Louis, Mo. THE LINEAR RELATIONSHIP between retention time or temperature and boiling point with programmed temperature gas chromatographic analysis has been used for simulated distillation and for compound identification. For nonpolar liquid phases, the relationship is independent of compound type; for polar liquid phases, however, separate elution curves may be obtained for parafins, olefins, and aromatics. I n a recent paper Baumann and Csicsery ( I ) utilized the relationship for the identification of Cll aromatic compounds formed by the disproportionation of 1-methyl-2-ethylbenzene. Using a 300-foot by 0.02 inch i.d. Ucon LB 550X capillary column, a fairly smooth curve was obtained for C8-CI1 alkylbenzenes, indane, tetralin, and naphthalene. A smooth, though nonlinear curve was also obtained for C,-C14paraffins; because of the aromatic selectivity of the liquid phase, elution of the paraffins was accelerated relative to the aromatics. A similar relationship has been used in our laboratories for identification of components in the same boiling point region. The relationship is, however, specific within a given carbon number grouping of isomers. Distinctive elution curves which are obtained for each carbon number permit identification of components having identical boiling points. Knowledge of the existence of this relationship can aid materially in component identification.
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Figure 1. Retention temperature cs. boiling point and heat of vaporization Numbers refer to components in Table I. Conditions as given in text H GHio 0 C~HI? B CioHin
1324
ANALYTICAL CHEMISTRY
Figure 2. Retention time-boiling C8-C11 aromatic compounds
point correlations for
I3 CbHio
0 CyH,,-indan A CloH,d-tetralin 0 rr-Alkyl homologs
Resolution of the C8-Cla aromatic hydrocarbons is essentially identical to that obtained with a di-n-decylphtlialate capillary column as previously described (3). At least partial resolution is obtained for all C8 and Cg compounds. The 22 Cloaromatics are resolved into 15 singlet, 2 doublet, and 1
-
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RESULTS ;\ND DISCUSSION
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(1) F. Baumann and S. M. Csicsery, J . Chrornutog., 26, 262 (1967).
l f l
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A 200-foot by 0.02-inch i.d. stainless steel column was coated with ditridecylphthalate from a 10% solution in methylene chloride at 15-20 psig. The tubing used was 316 stainless steel, 10-32 microfinish and annealed to a bright
] ,
. __
finish from the Handy and Harman Tube Co. The column was initially conditioned overnight and had been used for several months to 200" C. The instrument used was a n F and M Model 810 which had been modified for on-column injection as previously described (2). The injection port and detector were operated a t 225" C. The electrometer was operated on range 10' at attenuations of X32-XI28 as required. The column flow was 6 ml per minute measured at atmospheric pressure with a soap film flowmeter. The column was programmed from 30" to 200" C a t a program rate of 4" per minute; there was no appreciable temperature lag during programming. The hydrogen and air flow rates were 30 and 300 ml per minute, respectively. The carrier gas was passed through a molecular sieve 5A column just ahead of the injection port. The recorder wasa I-mV Honeywell Model 15, with 1-second full-scale pen response. Blends of various compounds were prepared frotn pure compounds, many of which were obtained from the Chemical Samples Co., Columbus, Ohio. The samples were injected without split with a Hamilton 7101 microliter syringe.
EXPERIMENTAL
I
.,
(2) D. E. Willis and R . V . Engelbrecht, J . GNSC h r o m ~ ~ f o g5. , in press. (3) D. E. Willis and R . h4. Eiigelbrecht, J . G ~ r sChromrrog.. 5 , in press.
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Figure 3. Application of method to complex aromatic mixture triplet peaks. Ditridecylphthalate was used in the present study as the liquid phase permits operation to 200” compared to 150” for the di-n-decylphthalate liquid phase. Figure 1 is a plot of retention temperature us. boiling point and molar heat of vaporization of C8-C9 aromatics (4, 5). Within the limits of experimental uncertainty in the thermodynamic values, a linear relationship is obtained. The linear relationship is also observed for retention temperatureboiling point and distinctive curves are obtained for each carbon number. Although overlap occurs between carbon numbers 9 and 10, these are predicted from the elution curve. Because retention time is proportional to retention temperature in a linear temperature program, Figure 2 illustrates the existence of a linear retention time-boiling point relationship for the CS-Cll aromatic hydrocarbons. In this case the elution curves are plotted directly on the chromatogram, eliminating data abstraction and re-graphing. A simpler mixture has been chosen to include various isomer types to show the relationship is independent of structure. The curve for the C 9 aromatics includes all isomer types as well as indan. The curve for the Clo aromatics includes most isomer types as well as tetralin. A solid line has been drawn through the n-alkylbenzenes from ethylbenzene to n-hexylbenzene. Figure 3 illustrates the application of the technique to a complex mixture of CS-Cl1 aromatic compounds. The elution curves for Clo and Cll aromatics were established by superimposing the chromatogram in Figure 2 over this chromatogram. The retention times of components in this region were precisely reproducible under the conditions used. The boiling points were read directly from the curves and the components identified by comparison with known boiling points ( 4 ) . Because of the large number of Cll aromatics and overlap between Clo and Cll aromatics, only tentative identification can be made for many of the compounds without additional information as to which boiling point curve a particular compound belongs. These data coupled with (1) S. W. Ferris, “Handbook of Hydrocarbons,” Academic Press, New York, 1955. ( 5 ) F. D. Rossini, rt al., “Selected Values of Properties of Hydrocarbons,” U. S. Government Printing Office, Washington, D. C., November 1947.
RETENTION TIME , MINUTES Figure 4. Retention timeboiling point correlations for Cg alkenyl aromatic compounds CsHio
0 CeHl2-indan 0 CaHla-indene
rapid scan mass spectrometric identification can give fairly conclusive peak identification. Peak 19-20 was shown to contain both indan pa C,] and a Cl0 alkylbenzene. A Cl0 aromatic eluting at this retention time would be predicted to have a boiling point of 181 O , in close agreement with that of 1,3-diethylbenzene. Even though the two compounds are co-eluted, the boiling point of each is predicted from the respective curve. Peaks 40 and 41 were identified by mass analysis as methyl indans containing small amounts of Cll alkylbenzenes. The boiling points of the two components as read from the Clo aromatic curve are in good agreement with 4- and 5-methyl indan and as read from the Cll aromatic curve with the components listed in Table I. The identity of all Clo aromatics was subsequently confirmed by the addition of pure compounds which were not available at the beginning of the work. Because of the more limited flow and temperature operating ranges of the column used, no extensive investigation of the correlations with different flow or program rates was made. Lower flows and/or faster program rates have been observed to decrease resolution and d o not permit programmed operation to continue during elution of all components. Slower program rates have been used; it is necessary to use a higher initial temperature to minimize peak broadening. I n order to allow programmed operation to continue during elution of all components investigated, it was necessary to operate the column above the temperature considered a safe maximum operating limit (180’ C). The column used showed n o noticeable deterioration for operation to 200” over a period of several weeks, but longer column life would be expected with operation only to 170-180” C. Di-n-decylphthalate is generally considered to be a moderately polar gas chromatographic liquid phase. A moderate amount of polarity would also be expected for ditridecylVOL. 39, NO. 1 1, SEPTEMBER 1967
1325
phthalate. This polarity is evidenced by a retardation of alkenyl aromatics relative to alkyl aromatics of the same boiling point; styrene (b.p. 145.2") elutes just prior to isopropylbenzene (b.p. 152.4") and isopropenyl benzene (b.p. 165.38" C) elutes just prior to 1,2,4-trimethylbenzene (b.p. 169.35' C).
Table I. Identification of Components Peak Number 1 2 3
4 5
6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
52 53
54
1326
Component(s) Ethyl benzene p-Xylene m-Xylene 0-Xylene Isopropyl benzene n-Propyl benzene 3-Ethyltoluene CEthyltoluene 1,3,5-Trimethylbenzene 2-Ethyltoluene rert-Butyl benzene Isobutylbenzene sec-Butyl benzene 1,2,4-Trimethylbenzene 3-Isopropyltoluene CIsopropyltoluene 2-Isopropyltoluene 1,2,3-Trimethylbenzene 1,3-Diethylbenzene Indan 3-n-Propyltoluene n-Butylbenzene 4-n-propyltoluene 1,4-Diethylbenzene 1,2-Diethylbenzene 1,3-dimethy1-5-ethylbenzene 2-n-Propyltoluene 1,4-Dimethyl-2-ethylbenzene 1,3-Dimethyl-4-ethylbenzene 1 ,2-Dimethyl-4-ethylbenzene 1,3-Dimethy1-2-ethylbenzene 1,2-Dimethyl-3-ethylbenzene 1,2,4,5-Tetramethylbenzene 1,2,3,5-Tetramethylbenzene n-Pentylbenzene 1,2,3,CTetramethylbenzene Tetralin (tetrahydronaphthalene) n-Hexylbenzene Pentamethylbenzene Naphthalene n-Heptylbenzene
+
+
(l-Methyl-2,4-diethylbenzene) (l-Methyl-2,5-diethylbenzene) (1,2-Dimethyl-3-propylbenzene) (1,2,4-Trimethyl-5-ethyIbenzene and/or 1,2,4-tri-
met hyl-6-ethylbenzene) Styrene Allylbenzene Isopropenylbenzene cis-1-Propenylbenzene 3-Methylstyrene CMet hylst yrene 2-Methylstyrene trans-1 -Propenylbenzene Indene 2,6-Dimethylstyrene 2,CDimethylstyrene
ANALYTICAL CHEMISTRY
The boiling points for the seven C9Hloaromatics are not all accurately known. A linear relationship appears to exist for the Cgalkenyl aromatics and indene (C9HB) as these compounds define a curve intermediate between the CBand Cg alkylaromatics (Figure 4). Harris and Habgood (6) note that on theoretical grounds a linear relationship between retention temperature and boiling point is unlikely for a homologous series of polar solutes or polar solvents. Giddings (7) has concluded that such a relationship can occur only if the entropy of solution is constant and the heat of vaporization varies linearly with carbon number. That the heats of vaporization vary linearly with retention temperature is shown in Figure 2 . The displacement of the elution curves parallels the increase in the molar heats of vaporization. The similarity of the slopes of the curves for Cs and Cg aromatics (Figure 1) suggests that the heat of vaporization-retention temperature linearity should hold for the Cl0 aromatics also. For isomers containing the same number of atoms, it is not surprising to find a nearly constant entropy of solution. The only exceptions to elution in boiling point order observed to date are 1,2- and 1,4-diethylbenzene (b.p.'s 183.4 and 183.7' C , respectively). This elution order has been observed previously on polar substrates (8). The retention time-boiling point curve for the Clo aromatics is displaced 3" from the Cg aromatics curve; the C1l curve is displaced 3.5-4.5" from the Cia aromatics curve. Since the boiling points can be estimated to less than 1 ', a combination of mass spectrometric data and observed boiling point can give positive identification of components for which boiling point data is available. The characteristic of giving carbon number-specific elution curves is not limited to aromatic compounds. Some data are available to indicate the relationship also holds for paraffins and cycloparaffins. It is believed that the phenomenon described in this paper is not limited to the particular column chosen, but may be generally applicable to columns of low to moderate polarity. The UCON LB-550X column described by McTaggart and Mortimer (8) elutes the Cs-Clo aromatic compounds in the same order as the column used in this study and might be expected to exhibit the same phenomenon if programmed temperature operation is used. Its application to columns containing mixed liquid phases o r highly polar liquid phases is doubtful. Regularity in boiling p o i n t 4 u t i o n curves can be a valuable aid in component identification in gas chromatographic analysis, The deviations from regularity as described in this work can provide valuable information in component identification. RECEIVED for review May 8, 1967. Accepted June 16, 1967. ~
(6) W. E. Harris and H. W . Habgood. "Programmed Temperature Gas Chromatography," Wilep, New York, 1966, pp. 141-9. (7) J. C. Giddings. J . Chromatag., 4, 1 1 (1960). (8) N. G . McTaggart and J. V. Mortimer, J . O I J ~Perrol., . 50, No. 490, 255 (1964).
