Relation between Retention Indices and Boiling Points of Hydrocarbons Differing Slightly in Their Vapor Pressures Ladislav Sojak and Jan HrivGak Institute of Chemistry, Comenius University, Bratislava J6n Krup8k Institute of Analytical Chemistry, Slovak Technical University, Bratislava Jaroslav Janak Institute of Instrumental Analytical Chemistry, Czechoslovak Academy of Sciences, Brno, Czechoslovakia THESOLUTE VAPOR PRESSURE, which is related to the boiling point, is one of the properties of the chromatographed hydrocarbon, which substantially affect its position in the chromatogram. The dependence between the logarithm of absolute or relative retention quantities and the boiling points of compounds have been correlated (I, 2). The dependences of retention characteristics on boiling points for polar substances separated on polar and nonpolar liquid phases are complex functions of the structural properties of interacting compounds. On the other hand, the interactions between the hydrocarbons and the hydrocarbon type stationary phases enable substantial simplifications. The regularities in the relations between the retention indices and the molecular structure have been summarized by Kovhts into six rules (3). The second of these, 61
5 X 6Tb
86% -
(1)
gives an approximate relation between the retention indices and the boiling points of “closely similar chemical substances (isomers) separated on a hydrocarbon nonpolar liquid phase.” The Kovhts system of retention indices represents the most reproducible expression of the retention characteristics and is one of the best criteria for the gas chromatographic estimation of the quality of a substance. As the boiling points of the majority of hydrocarbon isomers are tabulated, Equation 1 can be used for their identification. According to Schomburg ( 4 ) it is impossible to obtain 61 from the differences of boiling points 6Tb if the examined substances are not members of a homologous series. He gives as examples the pairs 2methylpentane, 2,3-dimethylbutane (2-MP, 2,3-DMB) and 1-hexene, 2-methyl-1-pentene (1-H, 2-M-1-P) and assumes that these anomalies are caused by the symmetry differences of the interacting molecules (during their separation on a squalane column). We studied Equation 1 during the separation of all theoretically possible Ce through CII straight-chain alkenes on an efficient (200 m) open tubular capillary column with squalane (5). Substantial deviations from the rule mentioned (1) J. H. Purnell, “Gas Chromatography,” J. Wiley, London, 1962 pp 39 and 219. (2) D. H. Desty and B. H. F. Whyman, ANAL.CHEM.,39, 320 (1957). (3) E. KovBts, “Advances in Chromatography,” Vol. 1 , J. C. Giddings and R. A. Keller, Ed., Marcel Dekker, New York, N.Y., 1965, p 229. (4) G. Schomburg, “Advances in Chromatography,” Vol. 6 , J. C. Giddings and R. A. Keller.. Ed.,. Marcel Dekker. New York. N.Y., 1968,; 211. (5) L. Sojiik, J. Krupeik, K. Tesaiik, and J. Janiik, J . Chromatogr., 65, 93 (1972).
time
Figure 1. Separation of the pair cis-3- and trans-3undecene on squalane at 86 and 130 “C
were found even in the case of isomeric straight-chain alkenes differing only in the double bond position and spatial geometry. From the analyzed pairs (trans-2-/l-alkenes, trans3-/ 1 alkenes, cis-2-/1-alkenes, cis- 341-alkenes, trans-2-/transCalkenes, cis-2-/cis-4-alkenes, trans-2-/trans-3-alkenes,cis2-/cis-3-alkenes, cis-2-/trans-3-alkenes, trans-2-/cis-3-alkenes, trans-2-/cis-4-alkenes, cis-2-/trans-Calkenes, cis-2-/trans-3alkenes, cis-2-/trans-2-alkenes), it was found that the proportionality constant ( k , = 6Z/6Tb) changes in a broad range, e.g., from 1.8 to 13.9. In most cases the values k, increase with increasing the number of carbon atoms (by 0.5 for one carbon atom on the average). A decrease of k , was found in the case of the pairs cis-2-/trans-2-, and cis-2-/trans-3-alkenes. Further, larger irregularities were found in the case of alkene pairs having very close boiling points, e.g., cis-3- and trans-3-, cis-4- and 1-alkenes, etc. As an example, Figure 1 shows the gas chromatographic behavior of the pair of cis-3- and trans-3-undecene obtained with a 200-m open tubular squalane column at 86 and 130 “C. A relatively small change of temperature reverses the retention sequence of these compounds. As both the isomers
-
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Table I. Correlations of CsAlkanes and Alkenes alkanes 6T b 6Idet aIdet/aTb 2.2-DMB, /l-Cja 13.67 38.5 3.8 2,3-DMB, 11-c: 21.91 70.1 3.2 69.7 2.9 24.20 2-MP, H - C ~ 30.3 3.6 j~-Cg,"2-MP 8.47 II-C~, 2,3-DMB 10.75 29.9 2.8 61.5 3.2 ??-cg, 2,2-DMB 19.00 0.4 0.2 2-MP, 2,3-DMB 2.28 31.6 3.8 2,3-DMB, 2,2-DMB 8.25 31.2 3.0 2-MP, 2,2-DMB 10.53 c 6 alkenes 4-M-l-P, /PC; 17.81 50.4 2.9 80.5 2-M-l-P, 12-Cj 3.4 24.63 82.9 3.0 1-H, n-Cj 27.42 17.1 n-Cg, 1-H 5.25 3.3 19.5 2.4 T Z - C2-M-1-P ~, 8.04 49.6 3.3 N-C~, 4-M-1-P 14.86 0.9 1-H, 2-M-1-P 2.79 2.4 30.1 4.4 2-M-l-P, 4-M-1-P 6.82 3.4 32.5 1-H, 4-M-1-P 9.61 a n-C5is n-pentane; n-Cg, n-hexane. c 6
K(Eq. 2)
3.4 3.3 3.3 3.3 3.2 3.2 3.0 3.1 3.1
Contrib. 15 5 11 7 10 10 47 15 4 11 1 7 5 22
3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3
0
43 26 2
Table 11. Correlations of C7 Alkanes 2,2-DMP, n-Cs 2,4-DMP, n-Cg 2,3-DMP, n-Cg 3-MH, n-C6 n-Cp, 3-MH n-C,, 2,3-DMP n-CV, 2,4-DMP n-C,, 2,2-DMP 2,4-DMP, 2,2-DMP 3-MH, 2,3-DMP 2,4-DMP, 2,3-DMP 2,3-DMP, 2,2-DMP 3-MH, 2,4-DMP 3-MH, 2,2-DMP a n-C7 is n-heptane.
6Tb
6Idet
8IdetlbTb
10.46 11.76 21.04 23.11 6.58 8.65 17.93 19.23 1.30 2.07 9.28 10.58 11.35 12.65
27.5 30.6 74.1 77.1 22.9 25.9 69.4 72.5 3.1 3.0 43.5 46.6 46.5 49.6
2.6 2.6 3.5 3.3 3.5 3.0 3.9 3.8 2.4 1.4 4.7 4.4 4.1 3.9
have (6) about the same boiling point (193.4 and 193.5 "C, respectively), the value of k , varies, depending upon the column temperature, in the range from relatively highly positive to negative values. This behavior was found in several other cases of geometric isomers. The proportionality constant of the relation between retention indices and boiling points for straight-chain alkenes is a function of the number of carbon atoms, structure of the pairs of isomers, and column temperature. It follows that the mentioned rule does not express precisely enough the retention behavior of straight-chain alkenes separated on squalane because it does not take into account the differences of the activity coefficients and the temperature dependence of vapor pressures and activity coefficients of the compared isomers. On the basis of the results, the following relation can be derived (7) :
(6) F. Asinger, B. Fell, and G. Steffan, Chem. Ber., 97, 1555 (1964). (7) L. Sojhk, J. Kruprik, L. Barnoky, and J. Janhk, J. Chromatogr., to be published. 1702
0
2) 3.5 3.5 3.5 3.5 3.8 3.8 3.7 3.7 3.6 3.9 3.5 3.5 3.6 3.6
Contrib. 20 21 8 6
7 15 6 4 23 34 24 19 13 9
where 61 = Iz - I , is the difference of retention indices of the two hydrocarbons ( x and y ) ; K = -kz x IOO/log a'; kz is taken from the relation log P o 2 , z l P o z ,=y kz x ST,;ST, = T,,,- Tb,v; Pz' is the vapor pressure of the solute; yzm is the solute activity coefficient at infinite dilution in the liquid phase; CY' = t ' m , n + l / t ' E , n is the separation factor of the nalkanes between a pair of hydrocarbons eluted. All values are expressed at column temperature. The validity of Equation 2 was Confirmed by a comparison of (the left-hand part of Equation 2), as expressed from the retention indices published by Tourres (8, 9) as well as (the right-hand part of Equation 2) by calculation of 61ca~c using the data by Martire (10). Both sets of data were compared for the temperature 74.1 "C. For straight-chain and branched alkanes, straight chain and branched alkenes, cycloalkanes, and aromatics, we found better agreement of SIcalc and than one index unity (i-u.) (7). The validity of the correlation was confirmed also by comparing the pair of hydrocarbons not belonging in the same homologous (8) D. A. Tourres, J . Chromatogr., 30, 357 (1967). (9) J. C. Loewenguth and D. A. Tourres, Fresenius' 2. Anal. Chem., 236, 170 (1968). (10) D. E. Martire and L. Z. Pollara, J . Chem. Eng. Data, 10, 40 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
series, e.g., by comparison of different types of hydrocarbons with n-alkanes. The results of correlations for the pairs of Csalkanes and C6-alkenes are given in Table I. The compared pairs of hydrocarbons have been arranged in groups according to the increase of 6Tb. To express the effect of molecular size and structure, the individual isomers have been compared to each other, with the n-alkanes having the next lower and higher retention times. In Table I are listed the values Bidet, k,, K (from Equation 2) and the values of the contribution of the second term in percentage (compared with the first term of Equation 2). The second term significantly influences the value of 61. The contributions are in the range up to 50 %. This fact is significant for small differences in boiling points and great differences of structural feature of isomers. Thus, the anomalous behavior of the pairs 2-MP, 2,3-DMB, and 1-H, 2-M-1-P (having 6Tb lower than 3 “C), as quoted by Schomburg ( 4 ) , can be explained by a greater contribution of the second term of Equation 2, which have a walue comparable with that of the first term. From Martire’s data (IO), it follows that 2,3-DMB and 2-M-1-P have, in comparison with other isomers, the lowest values of activity coefficients. The lower value of the activity coefficient for 2,3-DMB compared with the values of n-hexane, 2-MP, 2,2-DMB is explained by the lower molar volume and a higher electron polarizability of 2,3-DMB. For 2-M-l-P, the lower value of yZrnwith regard to 4-M-1-P and 1-H is explained by a higher electron polarizability of 2-M-1-P (as a result of the adjacent position of the methyl group to the double bond), because all three compounds have approximately the same molar volume (11). From the values for the activity coefficients of the isomeric Ci-alkanes, the lowest value of y r n 2was stated for 2,3-DMP (10). In Table I1 are correlated the values of C? alkanes. The greatest contribution of the second term in Equa(11) D. E. Martire and L. Z. Pollara, “Advances in Chromatography,” Vol. 1, J . C. Giddings and R. A. Keller, Ed., Marcel Dekker, New York, N.Y., 1965, p 335.
tion 2 (347J to the GI-value was found for the pair 3-MH/ 2,3-DMP. From the C6-alkanes, C6-alkenes, and Ci-alkanes investigated, the pairs 2,3-DMB/2,2-DMB, 2-M-l-P/4-M-l-P, and 2,3-DMP/2,3-DMP have the most differing values of activity coefficients. For these pairs, the greatest deviations from the Kov6ts rule should be expected. But as can be seen by comparison of Equations 1 and 2, the value of 61 is affected not only by the ratio of the activity coefficients but also by the ratio of the vapor pressures and the column temperature (which affect the value of K and k2 in Equation 2). The last columns of Tables I and I1 express the relative contribution of the second term of Equation 2 in all cases. These values depend on the absolute value of the first term, K X ST,. That means, the lower the values of K X STb are, the greater is the effect of the differences in activity coefficients of the compared substances on the Glvalues. CONCLUSION
The simple empiric rule, suggested earlier ( 3 ) between the retention indices and the boiling points does not express precisely enough the behavior of the closely boiling isomeric hydrocarbons during their gas chromatographic separation on squalane. The influence of different structures of isomers on their retention times is greater, the smaller the difference between their boiling points. This fact, together with the different dependences of vapor pressures and activity coefficients on temperature may markedly appear in the separation of isomers in high-efficiency capillary columns. Thus, in some cases, even a reversed retention sequence of the isomers was obtained by chromatographing in a relatively small range of column temperature. RECEIVED for review October 12,1971. Accepted January 27, 1972. A substantial part of this paper was presented at the 3rd National Conference on Analytical Chemistry held under the auspices of the IUPAC in Brashow, Roumania, September 24,1971.
Species Identification in Visible-Ultraviolet Vapor Spectrometry David S. Alderdice and Brian R. Crawford Department of Physical Chemistry, The University of New South Wales, Kensington, N.S. W . 2033, Australia
A COMMON EXPERIMENTAL PROCEDURE in spectrometry of vapors is to measure spectral absorbance in an apparatus as shown in Figure 1. Provided that the side-arm temperature T, is lower than that of any other part of the system (not difficult to achieve for a relatively volatile species), then it (T,) controls the composition of the vapor phase. We have, in several instances, encountered technical difficulties in the routine application of this method, and have developed the following observational procedure, which carries the bonus of many additional advantages. Usually, the spectrum of a given volatile substance is run at several different pressures (i.e., several different side-arm control temperatures), to encompass the typical variation of molar absorptivity e with wavelength. We find it useful to measure the side-arm temperature to good precision, and then
to plot logloA against l/Tc where A is the absorbance and T, the absolute temperature of the condensed phase. A linear plot usually results. The variation of vapor pressure with temperature, derived from the Clapeyron-Clausius equation assuming ideal gas behavior of a low pressure system and a heat of phase transition (L)which is temperature independent, is L
or loglop = -
m TC
+c
(2)
where m is the slope and c the intercept of the linear plot of loglop against l/Tc. At any particular wavelength at which a
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