Standard Stationary Phases G. A. Huber and E. sz. Kovats Laboratoire de Chimie technique de /'€cole Polytechnique Federale. Lausanne, Switzerland
There is a marked dependence of gas chromatographic data on the molecular weight of the stationary phase. Retention indices plotted vs. the variable f ' , which is in a first approximation of the inverse of the molecular weight of the stationary liquid, lay on straight lines. With the data at hand, specifications can be given for a polymer to be used as stationary phase.
which the upper temperature limit of use is given, not by its volatility, but by its thermal stability.
EXPERIMENTAL Paraffin hydrocarbons of t h e general formula
R,
'R It is surprising that the reproducibility of gas chromatographic data on polymers as stationary phases is as bad as it is, compared with the reproducibility of those measured on pure substances ( I , 2). No difficulty whatever is encountered in reproducing the structure of a polymer. On the other hand, its molecular weight will vary from batch to batch necessarily implying a corresponding variation of the number of end groups per unit volume of polymer. We presumed therefore that one of these factors should have a marked influence on the elution behavior of volatile substances. The effect of the molecular size of the solute has been discussed by Altenburg (3, 4) and by Young (5, 6). Furthermore the results of Evans and Smith (6a) show that retention indices of a series of substances strongly depend on the degree of polymerization of polyethylene glycols with free OH end groups. Their experimental design, however, does not allow differentiation between the effect of the strongly interacting end groups and that of the molecular weight. In order to study the effect of the molecular weight only, we synthesized four pure paraffin hydrocarbons to be used as stationary liquids. The effect of the end groups, methyl groups, is certainly negligible in this series of substances. The present work shows that the effect of the molecular weight of the stationary phase on gas chromatographic data is appreciable for some classes of substances. We demonstrate also that the effects found can be interpreted and systematized on the basis of the theoretical considerations discussed in our preceding communication (7). Precise information of this kind shall then permit the specification of polymers for use as stationary phases. The specifications are less rigorous a t high molecular weights. Polymers having excessively high molecular weights, however, show numerous disadvantages for use as solvents, such as high viscosities and high melting points. The latter property severely restricts its lower temperature limit to be used as stationary phase. Therefore, with the substances a t hand, we also tried to evaluate the lowest molecular weight of an oligomer for
( 1 ) P. A. T . Swoboda, in "Gas Chromatography, Rome 1966." A . B. Llttlewood, E d . Institute of Petroleum, London, 1966, p 395. ( 2 ) J. C. Loewengut, in "Column Chromatography. Lausanne 1969," E. s z . Kovats, E d . , Sauerlander AG, Aarau, Switzerland, 1970. ( 3 ) K. Altenburg. in "Gas Chromatography, Berlin 1968," H. S. Struppe, Ed., Akademie Verlag, G m b H , Berlin, 1968, p 1 ( 4 ) K . Altenburg. in "Column Chromatography, Lausanne 1969." E. s z . Kovats. E d . , Sauerlander A G , Aarau. Switzerland. 1970. (5) A . J. B. Cruickshank, M . L. Windsor, and C. L. Young, Proc. Royal SOC..A 2 9 5 , 259 (19661 ( 6 ) C. L. Young, Trans Faraday Soc.. 6 4 , 337 (1968). (6a) M . 8.Evans and J. F. Smith, J. Chrornatogr., 36,489 (1968). ( 7 ) 0.Fritz and E . s z . Kovats, Anal. Chem.. 4 5 , in press.
HC-(CH,),-CH
/
R
R'
with R = hexyl, decyl, tetradecyl, a n d octadecyl, respectively, have been synthetized. Synthesis, properties, a n d purifications will be described in a subsequent communication. T h e purity is thought t o be better t h a n 99%. The hydrocarbons will be referred t o a s t h e C-30, C-46, C-62, a n d C-78 hydrocarbon, respectively. Their melting points are C-30: 14-15 "C; (2-46: 47.5-48.0 "C; C-62: 65.5-66.5 "C; C-78: 72-77 "C. T h e melting points of t h e C-30 and the C-46 hydrocarbons are sharp, whereas the two others show a melting range which is certainly not due to impurities. Silanized Chromosorb-G served as solid support. T h e retention volumes have been determined on column fillings with 12 and 18% liquid phase.(weight by total weight) for t h e hydrocarbons C-30, '2-46, a n d (2-62 and on three column fillings of 6, 12, and 18% liquid phase for C-78 hydrocarbon. Adsorptive effects were not observed on 12 a n d 18% fillings. However, specific retention volumes showed a slight (less t h a n 2%) systematic variation on the heavily loaded columns with the column loading, a n effect we cannot explain. D a t a presented in this work are mean values strictly valid for the 12% column only. The column were borosilicate glass tubes of 225-cm length and 0.4-cm i.d. T h e apparatus used has been described earlier (8) as well as t h e method of evaluation of d a t a (9) T h e temperature of the column could be reproduced better t h a n f 0 . 2 "C a n d was stable during a working day. T h e injected quantity per substance was about 0.3 mg. Retention volumes have not been corrected for zero sample size. Helium was used as carrier gas with a mean flow rate of about 30 ml min-1. Thermogravimetric measurements have been carried out with a Mettler Model: Thermoanalyzer 1 (Mettler A.G. GreifenseeZurich, Switzerland).
RESULTS AND DISCUSSION Molecular Weight of the Stationary Phase and Upper Temperature Limit. The thermal behavior of the four hydrocarbons has been characterized by thermogravimetry. The substances have been heated under standardized conditions in a crucible and the weight loss due to evaporation and/or pyrolitic destruction has been determined. The material of the crucible had a marked influence in this experiment, aluminum and platinum having a strong catalytic effect. The best results have been obtained in gold containers-catalysis seemed to be negligible even a t 350 "C. A constant current of helium provided for the continuous removal of substance vapors and for inert atmosphere. Figure 1 shows the weight loss of the four hydrocarbons as a function of the temperature. Plotted on a logarithmic scale, the experimental points lay on straight lines. The hydrocarbons (2-30, C-46, and C-62 evaporated without destruction, whereas the pyrolytic degradation made a substantial contribution to the weight loss of the C-78 paraffin. (8) P. Toth, E . Kugler. and E. sz. Kovats, Helv. Chfrn. Acta. 42, 2519 ( 1959). (9) A. Wehrli and E. sz. Kovats, Helv. Chlm. Acta. 42, 2709 (1959).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1155
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Weight loss of t h e hydrocarbons a s measured by thermogravimetry. Heating rate, 6 "C min-'; helium flow rate, 83 mi min-'; gold crucible
Figure 1.
Gas chromatographic experience shows that the upper temperature limit for use of squalane as stationary phase is about 100 "C in open tubular columns. The evaluation of our thermogravimetric measurements is based on this experience. We assumed that our C-30 hydrocarbon has the same volatility as squalane, a C-30 hydrocarbon also. Furthermore, we supposed that the straight lines of Figure 1 can be extrapolated down to the region of low mass losses. In Figure 2, those temperatures are plotted a t which our hydrocarbons are believed to have the same volatility as the C-30 hydrocarbon, squalane a t 80, 100, and 120 "C, respectively. The figures for the C-78 hydrocarbon are certainly too low for, a t lower temperature, it will resist pyrolytic degradation. From the data of Figure 2, we can conclude that the molecular weight of a hydrocarbon to be used as stationary phase should be higher than 1100. For such a hydrocarbon, the upper temperature limit of use will be about 300 "C, conditioned by its thermal stability only. Molecular Weight of Paraffin Stationary Phases and the Specific Retention Volume of n-Paraffins. Recently we suggested the use of the molal Henry's coefficient for gas chromatography (7). This characteristic value can be calculated from the specific retention volume as
If the molal Henry's coefficient on a hypothetical stationary phase of infinite molecular weight, mgl, and the corresponding activity coefficient, PI, is known, then the molal Henry's coefficient of a given substance on a homolog of finite molecular weight can be calculated as Obviously
(3) where "V,,l is the specific retention volume of the substance on the hypothetical homolog of infinite molecular weight. We have also shown that the p-activity coefficient of n-paraffins of the elementary formula CzH2z+Zon paraffin stationary phases can be approximated by 1156
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
T OC
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(4)
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RT In g ,
=
RT In "g, + RT In P I
(5)
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Table I. Temperature Range of the Specific Retention Volumes Determination of n-Paraffins on the Four Paraffin Stationary Phases Pentane Hexane Heptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane
C30H62
C46H94
C62H1.26
C78H158
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