ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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Chronzatographic Conditions. Using a Varian Aerograph Model 1400, conditions were: attenuation, 64 x lo-"; column temperature, HC1, 165 "C and H B r , 90 "C; detector, flame ionization; detector temperature, 200' isothermal; flow rate, 60 mL/min; carrier gas, nitrogen; and column, 14-ft 20% OV 275 SO/lOO Chromasorb W/AW.
permit ten determinations to be made. T h e reproducibility of peak height response was observed to be h570 and the linearity of response over the expected concentration range also fell within this limit.
PRECISION
RECEIVED for review July 27, 1977. Accepted November 9,
A sufficient amount of the process stream was collected to
1977.
Gas Chromatography of High Molecular Weight Hydrocarbons with an Inorganic Salt Eutectic Column Lloyd
R. Snowdon"
Institute of Sedimentary and Petroleum Geology, 3303-33rd Street N. W., Calgary, Alberta, Canada, T2L 2A7
Eric Peake Environmental Sciences Centre (Kananaskis), University of Calgary, Calgary, Alberta, Canada, T2N 7N4
Gas chromatographic analyses of isoprenoid and normal alkanes and of steranes and triterpanes often play a key role in environmental studies of oil spills (1-3) and in petroleum geochemistry (4-6). T h e C19 saturated isoprenoid hydrocarbon, pristane, and t h e (&saturated isoprenoid, phytane, are usually determined by gas chromatography in the presence of saturated normal alkanes. T h e ratio of these isoprenoids t o selected normal alkanes is used as a n indicator of biochemical alteration of crude oils ( 7 ) . Conventional packed gas chromatographic columns coated with such liquid phases as OV-101, SE-30,and Apiezon L often produce only partial resolution of these isoprenoid hydrocarbons from t h e adjacent C17 and Cl8 normal alkanes. Capillary columns and open tubular columns operated under ideal conditions (2) are capable of the desired resolutions but examples in the literature show that this frequently is not achieved. There is a need for a column which will completely resolve isoprenoid and normal alkanes, which is easy to operate and which will produce rapid analysis for the routine examination of large numbers of samples. T h e conventional packed columns described in this paper meet these requirements.
EXPERIMENTAL Column packing consisting of 30% by weight of eutectic salt (8,9 ) on 60/80 mesh Chromosorb W (Johns-Manville) was prepared by dissolving the salt, 54.55 wt 70KN03, 27.3 wt % LiN03 18.2 wt % NaN03 (Analabs) in water and adding this to the solid support which was pre-wet with methanol. The resulting slurry was reduced to near dryness by rotary evaporation and then dried in an open container overnight at 400 OC. A 3.35 m by 3.2 mm outside diameter stainless steel column and a 3.66 m by 2 mm inside diameter glass column were packed and stabilized by heating at 350-400 "C for several hours with a carrier gas flow of about 10 mL/min of helium. The choice of the steel and glass columns was made on the basis of availibility, gas chromatograph geometry, and standard operating procedure used in the authors' laboratories. Crude oil samples and saturated hydrocarbons extracted from sedimentary rocks were routinely analyzed in the stainless steel column in one laboratory using a Hewlett-Packard model 5710A gas chromatograph equipped with a flame ionization detector. 0003-2700/78/0350-0379$01 .OO/O
A helium carrier flow of about 30 mL/min and heating rate of 8 "C/min resulted in the optimum high-speed separation of the saturated hydrocarbons. A slower heating rate (4 OC/min) resulted in peak spreading and a faster rate (16 OC/min) apparently reduced the column/sample interaction. Lower carrier gas flow rates also reduced the resolution, while higher flow rates required very high pressures on the front of column. Hydrocarbons were extracted from crushed Green River oil shale with benzene, transferred to isooctane, and the normal alkanes removed with a 5 8, molecular sieve, leaving a mixture of isoprenoids, steranes, triterpanes, and lesser amounts of other branched and cyclic compounds. This was chromatographed in the other laboratory in the 3.66 m by 2 mm glass column in a Varian 2100 gas chromatograph equipped with flame ionization detectors. The helium carrier gas flow rate was initially 85 mL/min, dropping to 60 mL/min at the maximum oven temperature of 400 "C. A program rate of 10 OC/min was found to give adequate separation with minimum analysis time. A standard mixture containing 22 polycyclic aromatic hydrocarbons was also chromatographed in the 3.66-m glass column but a program rate of 4"/min was required to achieve optimum resolution.
RESULTS AND DISCUSSION T h e sodium, lithium, potassium nitrate eutectic salt mixture proved to be an effective gas chromatographic coating phase for t h e rapid separation of saturated isoprenoids and n-alkanes in the nC4 to nCm carbon range. Mixtures of saturated isoprenoid and normal alkanes, such as those found in crude oils and in sedimentary rocks, were readily resolved with complete separation of pristane and phytane away from t h e adjacent n-alkanes (Figures l a and Ib). Separation of these components by the salt mixture is often equal or superior to that obtained with capillary columns coated with Apiezon L ( I O ) , OV-101 ( 1 1 , 12), or SE-52 ( 2 ) ,primarily because t h e pristane and phytane isoprenoid peaks fall approximately halfway between the nC17 and nCls and the nCls and nClg normal alkanes, respectively. Other liquid phases gave retention times for pristane and phytane similar to those of nC17 and nCls, respectively. Chromatograms of the same sample run on both salt eutectic and a commercially prepared OV-101 SCOT column gave essentially identical quantitative results of the heavy n-alkanes indicating that losses due to adsorption 1978 American Chemical Society
380
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
3
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Figure 1. Gas chromatograms of (a) a whole crude oil from the Altamont Bluebell Field, Unita Basin, and (b) saturated hydrocarbons from a Canadian Arctic Islands sedimentary rock
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Figure 2. Gas chromatogram of branched and cyclic saturated hydrocarbons from the Green River oil shale
are insignificant. Rapid program rates, up to 10 OC/min, together with column stability to 400 O C allowed analyses of crude oils for major saturated hydrocarbons to be conducted in about 30 min. Samples for the analysis of minor cyclic and branched saturated components are usually prepared by removal of the normal saturated alkanes with molecular sieve (13). As shown in Figure 2, the branched-cyclic saturated hydrocarbon fraction of the Green River oil shale was readily resolved into isoprenoids and a group of compounds previously identified as C2, to Cs0steranes and triterpanes (14). Resolution of the sterane-triterpane components was comparable to that previously achieved with a 15.2-m SE-30 SCOT column (15). Tricyclic diterpanes, found in some extracts of sedimentary rocks, gave peaks in the vicinity of nCls whereas the retention times of the triterpanes and steranes fell in the range of the
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Flgure 3. Gas chromatogram of a standard mixture of polycyclic aromatic hydrocarbons (1) fluorene, (2) 9-methylfluorene, (3) phenanthrene, (4) anthracene, (5) 2-methylfluorene, (6) 1-methylfluorene, (7) 1-methylanthracene, (8) 2-methylanthracene, (9) pyrene, (10) fluoranthene, (11) 3,4-benzofluorene, (12 and 13) 1,2-benzoRuorene and 2,3-benzofluorene, (14) triphenylene, (15) chrysene, (16) benz[alanthracene, (17) benzo[e]pyrene, (18) benzo[a]pyrene, (19) benzo[ blfluoranthene, (20) benzo[ ghi]perylene, (21) o-phenylenepyrene, (22) 1,2,3,4-dibenzanthracene
nC2salkane. The versatility of the eutectic salt mixture as a stationary phase was demonstrated by its ability to resolve a mixture of polycyclic aromatic hydrocarbons as shown in Figure 3. Of particular interest is the resolution of benzo[a]pyrene, a well known carcinogen, and its isomer, benzo[e]pyrene, which frequently occur in atmospheric particulate samples. Loss of resolution resulted if moisture was adsorbed on the column either by exposure of the hydroscopic salt to the atmosphere or by the use of impure carrier gas; however,
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
resolution was readily regained by conditioning t h e column overnight at 400 "C with helium carrier gas flow.
LITERATURE CITED (1) M. Gruenfeid, Proceedings of Joint Conference on Prevention and Control of Oil Spills, Washington, D.C., March 13-15, 1973, pp 179-193. (2) E. B. Overton, J. Bracken, and J. L. Laseter, J . Chromatogr. Sci., 15, 169 (1977). (3) J. G. Pym, J. E. Ray, G. W. Smith, and E. V. Whitehead, Anal. Chem., 47, 1617 (1975). (4) J. G. Bendoraitis, 8. L. Brown, and L. S.Hepner, Anal. Chem., 34, 49 (1962). (5) I.R. Hills, G. W. Smith, and E. V. Whitehead, J . Inst. Pet., 56, 27 (1970). (6) J. D. Brooks, K. Goukl, and J. W. Smith, Nature(London),222, 257 (1969). (7) N. J. Bailey, A. M. Jobson, and M. A. Rogers, Chem. Geol., 11, 203 (1973). ( 8 ) W. W . Hanneman, C. F. Spencer, and J. F. Johnson, Anal. Chem., 32, 1386 (1960).
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(9) J. M. Hunt, in vonder Borch et al., "Initial Reports of the Deep Sea Drilling Project", Vol. 22, U.S. Government Printing Office, Washington, D. C., 1974, pp 673-675. (IO) J. Han and M. Calvin, Geochim. Cosmochim. Acta, 33, 733 (1969). (1 1) 8. W. Jackson, R. W. Judges, and J. L. Powell, Environ. Sci. Techno/.. 9, 656 (1975). (12) W. E. Reed, Geochim. Cosmochim. Acta, 41, 237 (1977). (13) M. T. Murphy in "Organic Geochemistry", G. Eglinton and M. T. Murphy, Ed., Springer-Verlag, Berlin, 1969. (14) E. J. Gallegos, Anal. Chem., 43, 1151 (1971). (15) D. E. Anders and W. E. Robinson, Geochim. Cosmochim. Acta, 35, 661 (1971).
RECEIVED for review July 29, 1977. Accepted October 24, 1977.
Drying Small Amounts of Solvent for Use in Nuclear Magnetic Resonance Spectrometry Joseph B. Alper Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-
Madison, Madison, Wisconsin 53 706
A major problem in t h e area of high field proton magnetic resonance (PMR) spectroscopy is the contamination of solvents with water (1). This problem is especially acute when using very small amounts of sample; the water signal can obscure sample peaks as well as cause computer-associated dynamic range problems when using time averaging techniques. We have developed a simple, fast, and efficient method of drying such solvents as dimethylsulfoxide (DMSO), acetone, and chloroform using a surplus gas chromatograph (GC), a broken 10-mL volumetric pipet, a 6-inch syringe needle, a Pyrex, Leur-lock syringe, Swagelok fittings and 48, molecular sieves.
EXPERIMENTAL The column was constructed as follows and the end result is shown diagrammatically in Figure 1. The ends of a 10-mL volumetric pipet were cut and bent in such a manner as to allow a good fit in the GC oven: the end to be connected to the GC inlet was fitted with Swagelok fittings and the outlet end was joined to the syringe barrel. The barrel of the pipet was then packed with 4-A molecular sieves obtained from Altex Corp. A small hole ('/,-inch) was made in the back of the GC oven to accommodate the syringe needle, which is attached to the Leur-lock fitting of the syringe. The use of a syringe needle as the exit port allowed the dry solvent to be collected directly into a dry sample containing a NMR tube fitted with a septum cap. A small gauge needle is inserted through the cap during collection to allow for pressure relief. The column was then heated at 240 "C for 12 h, with a slight stream of dry nitrogen flowing during this time, to activate the molecular sieves. This procedure is also carried out after a day's use; we have found that after 9 months of daily use and overnight baking, the molecular sieves should be replaced. Samples are prepared in our laboratory by dissolving between 0.2-4.0 mg of a peptide in a small amount of water, lyophilizing in the NMR tube to give a dry, white solid. The NMR tube is then fitted with a suitable septum cap (Aldrich) and dry solvent was collected after fitting the NMR to the exit needle of the GC column. The GC oven is set at 100 "C for Me2SO-d6and 50 "C for both CCD13 and acetone-&; an injector temperature of 150 "C for Me2SO-d6and 100 "C for CDC13 and acetone-d, is used. Dry nitrogen was used as the carrier gas and the flow was such that 15 min after injection all solvent had been eluted from the column. Up to 4.0 mL of solvent has been dried at one time. 0003-2700/78/0350-0381$01 OO/O
Figure 1. Diagram of the apparatus used to dry small amounts of
solvents as described in this paper
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Flgure 2. C, proton region of the NMR spectra of tocinamide in
MezSO-d6. T h e bottom spectrum was taken using Me,SO-d, taken from a sealed ampule. The top spectrum was taken using Me,SO-d, dried using t h e method outlined in this paper C 1978 American Chemical Society
