Interaction Second Virial Coefficients of Some Hydrocarbon-Hydrocarbon Gas Mixtures from Gas-Liquid Chromatography R. L. Pecsok and M. L. Windsor’ Department of Chemistry, Uniuersity of California, Los Angeles, Calif. 90024 Gas-liquid chromatography at pressures up to 6 atmospheres has been used to measure interaction second virial coefficients (BI2) of sample vapor and carrier gas mixtures. Previously, due mainly to the difficulty of detecting the sample, the method has been restricted to permanent gases as the carrier gas. However, with a sensitive microthermal conductivity cell, it is possible to detect relatively small samples of low molecular weight hydrocarbons in methane and ethane carrier gases. Results for B~~ of 11gas mixtures are presented: n-pentane, ;so-pentane, n-hexane, 2-rnethylpentane, and 2,2-dimethylbutane in methane at two temperatures and n-pentane in ethane at one temperature. The experimental precision is about 1 2 5 cm3/mole. In the case of 5 of these mixtures, a direct comparison with previous static measurements is possible. While agreement between GLC and static results for methane is good, there is a large, but not unexpected, discrepancy in the ethane results. The accepted GLC theory takes no account of the solubility of the carrier gas in the stationary liquid and in the case of ethane this i s appreciable. A recent extension to the theory by Cruickshank, Gainey, and Young has been used to make an approximate correction for this solubility effect and the corrected GLC values are in good agreement with static data.
THATGAS-LIQUID CHROMATOGRAPHY (GLC) can be used to obtain thermodynamic information about mixtures is, by now, quite well established. In particular, determinations of activity coefficients at infinite dilution (n”)and interaction second virial coefficients (BIZ)by G L C agree well with statically measured data (1-5). (It is the activity coefficient of the solute or sample at infinite dilution in the stationary liquid, and the interaction second virial coefficient of the solute vapor and carrier gas mixture that is obtained.) The limitations of the method have, however, been those of chromatography. In the case of activity coefficients, measurements have been restricted to solutions of volatile solutes in nonvolatile liquids, and to conditions of infinite dilution of the solute. However, these restrictions are not fundamental and it is possible to work with volatile stationary liquids and at concentrations other than infinite dilution (6, 7). In this (1) D. H. Everett and C. T. H. Stoddart, Trans. Faraday SOC.,57, 746 (1961). (2) D. H. Desty, A. Goldup, G. R. Luckhurst, and W. T. Swanton, “Gas Chromatography,” Butterworths, London, 1962, p 67. (3) A. J. B. Cruickshank, M. L. Windsor, and C. L. Young, Proc. Roy. SOC.,295A, 271 (1966). (4) S. T. Sie, W. van Beersum, and G. W. A. Rijnders, Separation Sci., 1, 459 (1966). ( 5 ) E. M. Dantzler, C. M. Knobler, and M. L. Windsor, J . Chromatog., 32, 433 (1968). (6) A. Kwantes and G. W. A. Rijnders, “Gas Chromatography,” Butterworths, London, 1958, p 125. (7) J. H. Purnell, Endeacour, 23, 142 (1964). (See also C. N. Reilley, G. P. Hildebrand, and J. W. Ashley, Jr., ANAL.CHEM., 34, 1198 (1962), and C. F. Chueh and W. T. Ziegler, Am. Inst. Clzern. Engr. J., 11, 508 (1965).)
1238
ANALYTICAL CHEMISTRY
paper we are concerned with the interaction second virial coefficients. In order to carry out a GLC experiment at all, one must be able to detect a very small amount of solute vapor in a flowing stream of carrier gas. Obviously the properties of the solute vapor must differ in some marked way from those of the carrier gas. Consequently, results reported so far have been for gas mixtures where the two components are very different in nature. Thus Bl2values have been measured for mixtures of low molecular weight hydrocarbons with argon, nitrogen, hydrogen, and helium (although CO, COa, and 0 2 have been used) (8). In the present work we have extended the GLC method for measuring BIZto include mixtures of components which are more similar than those mentioned above, such as hydrocarbon-hydrocarbon gas mixtures. Data on these gas mixtures are probably of more interest practically and theoretically than hydrocarbon-permanent gas mixtures. But perhaps more important is the fact that the GLC theory is extended to its limit, since some of the assumptions made in devising the theory-e.g., that the carrier gas is insoluble in the stationary liquid, and that third virial coefficients may be neglected-are either not justifiable with these gases, or their validity is difficult to assess. Since static BIZvalues are available for hydrocarbon-hydrocarbon mixtures, a direct comparison can be made with the chromatographic results. Finally, a recent extension to the theory by Cruickshank, Gainey, and Young (9) takes account of carrier gas solubility and this may be examined. On a practical level, the operation of a gas chromatograph with a hydrocarbon carrier gas and a low molecular weight hydrocarbon sample has become possible due to the introduction of very sensitive katharometer detectors. We were able to detect very small amounts of the Cs and Cg hydrocarbon samples in the methane or ethane carrier gas stream. The experiments require accurate determinations of net retention volume over a range of carrier gas pressure. The apparatus differs somewhat from that used previously in such work. It has been partially described elsewhere (10) and here we will mention only the important features. EXPERIMENTAL
The apparatus was constructed to operate with carrier gas inlet pressures from about 1.5 to 10 atmospheres. Generally, however, the difficulty of detection with increasing carrier gas pressure limited the pressure to less than 6 atm. The 1 Present address, Ministry of Technology, Humber Laboratory, Hull, England
(8) D. H. Everett, B. W. Gainey, and C. L. Young, Trans. Faraday SOC.,in press (1968). (9) A. J . B. Cruickshank, B. W. Gainey, and C . L. Young, Trans. Faraday SOC.,64, 337 (1968).
loop
Figure 1. Injection system Normal carrier gas flow (b) Sample injection
(n)
1.o
2.0
3.0
4.0
5.0
P@J:
Figure 2. A typical plot of log V.V us. p0.J; major pressure drop occurs not across the column but across a high pressure needle valve at the column outlet. The pressure drop across the column is never greater than 1 atm and is generally about 0.2 atm-measured by a differential mercury manometer. Inlet pressure is measured by a '/z%. The detector is a microthermal gauge accurate to conductivity cell (Model 1000, Carle Instruments, Anaheim, Calif.) which proved to be leaktight at 10 atm and was set in the high pressure line immediately downstream of the column. The reference side of the detector is fed by a reference column similar to the column used for the samples. Both are supplied by the same inlet pressure so that the flowrate and pressure in the two sides of the cell are about the same, and any fluctuations originating in the inlet tend to cancel out. The detector, columns, and most of the flow system are contained in the same water thermostat with the temperature controlled to =tO.Ol "C. Samples are injected into the high pressure gas stream by means of a two-way rotary valve (Carle Instruments, Anaheim, Calif.) which has been modified by connecting one of the sample loops to a sample reservoir and vacuum pump as shown in Figure 1 . The carrier gas normally flows through one loop and into the column. The sample loop is evacuated and then opened to the sample reservoir so that sample vapor diffuses in. The valve to the sample loop is then closed and the injector actuated sending carrier gas through the sample loop and injecting a small amount of sample vapor onto the column. The amount of sample injected can be changed by varying the temperature of the reservoir and loop. The rotary valve is sealed by the precise lapping of a Teflon slider with the valve body face. It was leakproof at 7 atm pressure (25 "C and 50 "C) provided that sufficient loading was put onto the slider and a pure Teflon slider was used. The valve proved excellent in use with some 2000 injections at various carrier gas pressures. The exact size of sample injected was not known but it was the equivalent of about 1 p1 of liquid. The columns are 6 feet coiled copper tubing filled with 2 0 z by weight of squalane (about 2 g) on celite (60180 mesh). Methane was Matheson Co. "Ultra Pure," and ethane was "C. P. Linde." RESULTS AND DISCUSSION
In this type of work a flame ionization detector would normally be used; it is barely possible to use a flame ionization detector with methane as carrier gas (8),but it is almost certainly impossible to use it with ethane or any other hydrocarbon as carrier. Even with the thermal conductivity cell, one is working at the detection limit; methane and ethane have low thermal conductivities which are close to those of the samples which are being detected. The signal received at the
recorder is frequently very small and high base line stability is essential. The thermal conductivity cell also has the disadvantage that it is concentration-sensitive (unlike the flame ionization detector which is mass sensitive) so that the signal decreases as the carrier gas pressure is increased. It is generally necessary to restrict the mass flowrate to a fairly low value with these detectors so volumetric flowrates must be considerably reduced at elevated pressures. Depending on the flowrate, pressure, etc., peak inversion may also occur when carrier gases of such low thermal conductivity are used (10). The characteristics of this microdetector with regard to peak inversion have been reported in a previous paper (10). It is this increased difficulty of detection at higher pressures that has limited the pressure studied to a maximum of 6 atm; consequently, the errors are rather higher than when permanent gases are used as carriers in conjunction with a flame ionization detector. However, any error due to the neglect of third virial coefficients in the theory will be much less serious at lower pressures. The theory of this method (11) shows that the interaction second virial coefficient (B12) characterizing interactions between a pair of molecules-one of solute vapor and one of carrier gqs-can be obtained by measurement of the net retention volume of a solute over a range of carrier gas pressure. The simplest equation given by Cruickshank et al. is where /3 = (2B12- Dlm)/RT,ulm is the partial molal volume of the sample at infinite dilution in the stationary liquid, subscript (1) refers to the sample, ( 2 ) to the carrier gas, and (3) to the stationary liquid, and J ; is a function only of column inlet and outlet pressures, p i and po,
This indicates that a value for B I Zmay be obtained from the slope of a plot of lnV,>rDS. p,Jj. In an average system studied here with BI2 of about -200 cm3/mole, the net retention volume ( V N )will decrease by less than 2 per atmosphere increase in carrier gas pressure so that the precision of the measurements must be quite high in order to obtain a meaningful value for &. (10) R. L. Pecsok and M. L. Windsor, ANAL.CHEM., 40,92 (1968). (11) A. J. B. Cruickshank, M. L. Windsor, and C. L. Young, Proc. Roy. Soc., 295A, 259 (1966). VOL. 40, NO. 8, JULY 1968
1239
Interaction Second Virial Coefficients of Hydrocarbon Mixtures Temuerature = 25 OC Temperature = 50 "C (cms/mole)(cm3/mole) Biz (GLC) Biz (GLC) Gas mixture corrected Biz (GLC) Bl2 (static) B12 (GLC) Biz (static) corrected (1) (2) (3) (2) (3) (1) Methane n-pentane -182 (h20) -222 ( 2 ~ 1 3 ) - 204 ( f42) -128 (3Z20) -190(112) -138 (130) Methane n-hexane -271 ( 1 3 3 ) -261 (A60) -292 (3Z55) -270 (f44) -225 (130) -280 (zt54) Methane 2-methylpentane -297 (120) ... -317 (142) -134 (124) ... - 144 (134) Methane 2,2-dimethylbutane - 196 (+20) ... -216 (3Z42) - 144 (zll9) ... - 154 (129) Methane iso-pentane - 177 ( 1 2 1 ) ... -199 (zk43) - 113 (9Z26) ... -123 (3~36) Ethane n-pentane -261 (k18) -448 (3~16) -414 (1171) ... ... ... Values of B12for hydrocarbon-hydrocarbon mixtures obtained from gas-liquid chromatography are compared with static data (12, 13). Table I.
A
+ + + + + +
If the carrier gas is appreciably nonideal a better equation is (11):
X is defined by the expansion of x2 (mole fraction of carrier gas in the stationary phase) as a series in local carrier gas pressure.
(3) where b = BzZ/RT; BZ2is the second virial coefficient of the carrier gas, p is the mean column pressure defined by p = pJZ. Although methane and ethane are appreciably nonideal at 25 "C (their second virial coefficients are, respectively, -41 cm3/mole and -185 cms/mole), the pressure drops used in this work are so small that p and p , differ on an average by bp)/(l bp,) is therefore less than 0.1 atm. The factor ( 1 very close to unity and the change in the ethane data when plotted in this manner is only + 2 cm3/mole. For processing these data Equation 1 has been used throughout. A typical plot is shown in Figure 2. The Blz results obtained are given in Table I. Five gas mixtures containing methane were studied at two temperatures, 25 "C and 50 "C; the ethane mixture was studied only at 25 "C. The results given in column 1 of Table I are those obtained directly from the experimental data using Equation 1. The errors given are simply the precision of the data. They may be compared with results for the same mixtures obtained from static measurements (12,13)of the excess quantity (E), where
+
E
=
Biz
- 0.5(&1
+ Bn)
+
(4)
The error in the static Biz's given derives from the experimental error in the measurement of E but also from the values of B11 and Bzzused to calculate B12. Where a comparison is possible -i.e., in five of the eleven results-the agreement between static and GLC results is reasonably good with the notable exception of ethane. This discrepancy can almost certainly be attributed to the assumption in the simple theory that the carrier gas is insoluble in the stationary liquid. Although relatively unimportant where permanent gases are used as carrier gases, this assumption is certainly invalid where hydrocarbons are used with a hydrocarbon stationary liquid. In a recent extension to the theory Cruickshank, Gainey, and Young (9) have taken this solubility effect into account. The result is to change the value of in Equation 1 to p' where p '
=
2B12 - virn RT
5) + x [I - ("'15(} ]
(12) E. M. Dantzler, C . M. Knobler, and M. L. Windsor, J. Phys. Chem., 72,676 (1968). (13) Sh. D. Zaalishvili, Zhur. Fiz. Khirn., 30, 1891 (1956). 1240
ANALYTICAL CHEMISTRY
Equation 5 now takes into account the two effects resulting from the solubility of the carrier gas (i) an increase in the number of moles of stationary liquid. (ii) a change in the activity coefficient due to the fact that the nature of the stationary phase has changed-e.g., it is no longer squalane but squalane ethane. To make use of this equation, we need to know the pressure dependence of the solubility of the carrier gas in the stationary liquid (to get A), and the change in activity coefficient of the solute ( 1 ) with change in mole fraction of carrier gas (2) dissolved in the stationary liquid. For the systems used here and indeed for most of the systems one could conceivably use in GLC, these quantities are not known. A reasonable estimate of [ l - (dln~l"/bxz)]in systems where there are no specific interactions is that the value must lie between 0 and 1 . Thus the best estimate we can make at present is that [l (dylm/bxz)]is 0.5 10.5. We can also estimate on the basis that the dissolution of the carrier gas in the stationary liquid is ideal. With these two approximations, the correction to B I Z at 25 "C is -22 ( 1 2 2 ) cm3/mole for methane and -153 (9~153)cm3/mole for ethane. At 50 "C the correction for methane is -10 (*IO) cm3/mole. In column 3 of Table I the gas chromatographic Biz values, corrected as described above, are given. The results with one exception agree within experimental error and the large discrepancy in the ethane results is accounted for, although of course the correction term has only been very roughly estimated. This work shows a consistency of data and theory regarding the effect of dissolved carrier gas on B l z , and that it is certainly possible to use hydrocarbon gases and obtain data on a wider range of mixtures but that additional measurements of X and blnyl"/bxz are essential in order to obtain an accurate value for B12. Although it is direct, it is unlikely that GLC would be used as a standard method for measuring Biz, but it may be particularly effective for certain types of gas mixtures. Probably the main use of GLC will continue to be in the measurement of activity coefficients at infinite dilution. The hydrocarbon gases used here would obviously not be chosen as carriers where ylmmeasurements are to be made and we have not repeated 71- values here, (no precautions against steady loss of stationary phase were taken). Finally, we feel that for work in this branch of nonanalytical gas chromatography, where it is important to detect very small
+
sample sizes often in the presence of carrier gases which would not normally be used (CHI, C2H6,CO, ' 2 0 2 , etc.) it might be worthwhile in the future to use radioactively labelled cornpounds as samples. Extremely small samples could then be detected so that infinite dilution would be very closely approached, and the nature of the carrier gas would be immaterial, thus widening the scope of gas mixtures for which interaction second virial coefficients can be obtained.
ACKNOWLEDGMENT
We thank Don Carle for making available to us the Carle Instruments' rotary valve used for injection, and to Brian W. Gainey for helpful suggestions. RECEIVED for review March 20, 1968. Accepted April 30, 1968. Based on a paper presented at the 155th National Meeting, ACS, San Francisco, Calif., April 1968.
Gas Chromatographic Analysis of Insensitive Pesticides as Their HalomethyldimethyI siIyI Derivatives C. A. Bache, L. E. St. John, Jr., and D. J. Lisk Pesticide Residue Laboratory, Cornell Unicersity, Ithaca, N . Y. 14850 The bromo- and chloromethyldimethylsilyl derivatives of acidic and phenolic pesticide and herbicide compounds have been prepared which greatly enhance their responsewhen subsequently analyzed by electron affinity and emission spectrometric gas chromatography. The method is rapid and versatile permitting detection of these compounds in the range of 1 to 100 nanograms. The procedure has been applied to analysis of soil.
GASCHROMATOGRAPHIC ANALYSIS of pesticides using halogensensitive detectors is often limited by the absence or lack of sufficient halogens in the molecule to provide adequate response. Bromination of aromatic amine ( I ) and phenolic (2) agricultural chemicals has been successfully applied to greatly enhance their response to electron affinity detection. Other reactions have been used such as the Zeisel alkoxy1 reaction with organophosphorus insecticides to produce alkyl iodides (3),chloroacetylation of phenols (4) and amines (3,haloacetylation of sterols trifluoroacetylation of amines ( 5 ) and amino acids (3, and pentafluoropropionylation and heptafluorobutyrlation of various amines (5). Fishbein and Zielinski (8) prepared trimethylsilyl derivatives of pesticidal carbamates and ureas prior to gas chrornatographic analysis. Steroids have been chromatographed as their bromo- (9) and chloro- (IO, 11) methyldimethylsilyl ether derivatives. In the work reported, the feasibility of preparing and chromatographing the bromo- and chloromethyldimethylsilyl derivatives of halogen deficient pesticide com-
(a,
(1) W. H. Gutenmann and D. J. Lisk, J. Agr. Food Chem., 11, 468 (1963). (2) Ibid., 13, 48 (1965). (3) Ibid., 11, 470 (1963). (4) R. J. Argauer, ANAL.CHEM., 40, 122 (1968). (5) _ . . , D. D. Clarke. S . Wilk. and S. E. Gitlow. J . Gas Chromaton.. 310 (1966). (6) R. A. Landowne and S. R. Lipsky, ANAL.CHEM., 35, 532 (1963). ( 7 ) M. Stefanovic and B. L. Walker, ibid., 39, 710 (1967). (8) L. Fishbein and W. L. Zielinski, Jr., J . Chromatog., 20, 9 (1965). (9) C. Eaborn, D. R. M. Walton, and B. S. Thomas, Chem. Ind. (London), 1967, p 827. (10) W. J. A. VandenHeuvel, J. Chromatog., 27, 85 (1967). (11) B. S . Thomas, C. Eaborn, and D. R. M. Walton, Chem. Commun., 2, 408 (1966).
pounds and certain of their metabolites in various chemical classes has been studied. EXPERIMENTAL
Reagents. The bromo- and chloro-methyldimethylchlorosilanes were purchased from Pierce Chemical Co., Rockford, Ill., and were kept refrigerated. The diethylamine was Eastman White Label grade and was redistilled and stored in the dark over anhydrous sodium sulfate. N-hexane was shaken with concentrated sulfuric acid and washed with distilled water. It was then redistilled over potassium hydroxide pellets and stored over anhydrous sodium sulfate, Procedure. The derivatives were formed by an adaptation of the procedure of Eaborn et a l . (9) as follows: One milliliter of hexane, 0.075 ml of diethylamine, and 0.09 ml of bromo- or chloromethyldimethylchlorosilane were added to a 5-ml glass vial which was stoppered and shaken vigorously. The mixture was centrifuged at 1500 rpm for 15 minutes. A 0.4-ml portion of the supernate was transferred to an 8-ml glass-stoppered test tube containing 0.1 ml of ethyl acetate in which was dissolved up to 100 micrograms of the pesticide or metabolite to be reacted. A 10/30 standard-taper male ground joint (with the full 15-cm length of glass tubing attached) was placed in the top (as an air condenser) and the contents were refluxed at 65 "C for 30 minutes. The contents were immediately cooled and the condenser was rinsed with 0.5 ml of hexane. The solution was appropriately diluted with hexane and chrornatographed. If the tubes were stoppered and the contents kept dry, the derivatives were stable for several hours. Two chromatographs and detection systems were employed. The derivatives were examined using an electron affinity detector. This system comprised a Barber-Colman Model 10 gas chromatograph with a battery operated BarberColman Model No. A-4071, 6-cc detector containing 56 pc of radium-226. The detector was operated at 2 volts and the relative electrometer gain was 10,000. The recorder was a Wheelco, 0 to 50 mV equipped with 10-inch chart paper, running 10 inches per hour. The column was borosilicate glass, U-shaped, 5-mm i d . , and 2 feet long. The packing was 10% DC-200 on 100- to 120-mesh Gas-Chrom Q. Connections between the column and detector were made with Teflon tubing and nitrogen (60 cc per minute) was the carrier gas. The isothermal column temperatures used for the various compounds studied ranged from 110 to 200 "C. Response was also measured using a microwave-powered, low pressure, helium plasma emission detector (11) and VOL 40,
NO. 8, JULY 1968
1241
