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Anal. Chem. 1983, 55. 974-975
are roughly constant for all four columns, this implies a linear relationship between AH and AS values. A similar observation has been reported for some homologous series of compounds with pyrocarbon-coated adsorbents (15). The difference is that our system is for one solute with four different columns. However, this AH-AS linear relationship is not unusual as it has been found for many simple organic and inorganic reactions (16). As a hard electrostatic bonding mode dominates for column-solute interactions, the A S factor tends to be favorable and the AH factor generally counteracts the reaction. On the other hand, a large decrease of AH is observed along with a counteracting A S during soft covalent modes of interaction.
ACKNOWLEDGMENT Thanks are due to K. H. Pannell and W. C. Herndon for helpful discussions. Registry No. [C~(en)~]Cl,, 13408-73-6; [Co(en)(edda)]Cl, 14784-34-0;fructose, 57-48-7;glucose, 50-99-7;sucrose, 57-50-1; propylamine, 107-10-8;propyldiamine, 78-90-0. LITERATURE CITED (1) Linden, J. C.; Lawhead, C. L. J. Chromatogr. 1975, 105, 125-133. (2) Palmer, J. K. Anal. Lett. 1975, 8, 215.
Verhaar, L. A. Th.; Kuster, B. F. M. J. Chromatogr. 1981, 220, 313-328. Rabei, F. M.; Caputo, A. G.; Butts, E. T. J. Chromatogr. 1978, 126, 731-740. Hettinger, J.; MaJors, R. E. Varlan Instruments Applicatlons 1978, 10, 6. Wong-Chong, J.; Martin, F. A. J. Agrk. Food. Chem. 1979, 2 7 , 972. Binder, H. J. Chromatogr. 1980, 189, 414-420. D'Amboise, M.; Noel, D.; Hanari, T. Car6ohyd. Res. 1980, 79, 1. Chang, C. A.; Tu, C.-F. Anal. Chem. 1982, 54, 1179-1182. Chang, C. A.; Huang, C. S.; Tu, C. F., submitted for publication in Anal. Chem. Chow, F. K.; Grushka, E. J. Chromatogr. 1979, 185,361-375. Broquaine, M. J. Chromatogr. 1979, 170, 43-52. Emerson, M. T.; Crunwald, E.; Kapian, H.; Kromhout, R. A. J. Am. Chem. SOC. 1980, 8 2 , 6307-6314. Krung, R. R.; Hunter, W. G.; Grieger, R. A. J. Phys. Chem. 1976, 80, 2335-2341, 2341-2351. Colin, H.; Diez-Masa, J. C.; Guiochon, G. J. Chromatogr. 1978, 167, 41-65. Ahrland, S . Helv. Chlm. Acta 1987, 50, 306-317.
RECEIVED for review May 3, 1982. Resubmitted August 9, 1982. Accepted January 3,1983. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The financial support of the Robert A. Welch Foundation of Houston, TX, and the Research Institute of the University of Texas a t El Paso is also acknowledged.
Capillary Inlet for Packed Column Gas Chromatographs Kenneth D. McMurtrey" and Thomas J. Knight Department of Chemistty, Box 8 190, University of Southern Mississippi, Hattiesburg, Mississippi 39406
Routine instrumentation for analysis of complex substances by gas chromatography with capillary columns has advanced markedly in the last few years. Practically every chromatographic manufacturer now supplies instruments with requisite inlet systems, etc., configured for capillary columns. The advent of commercially available fused silica capillary columns has allowed relatively untrained personnel to use open tubular columns without many of the problems which fragile glass capillary columns display. However, for laboratories equipped only with packed column gas chromatographs, the capital necessary for acquisition of capillary chromatographs may act as a deterrent to change to the more efficient systems. Commercial user-installed inlet systems also are available for many older packed column instruments, but these inlet systems are relatively expensive. We wish to communicate the details of a capillary inlet system which we have used for over a year. The inlet system is easily added to many instruments configured for on-column injection using 1 / 4 in. 0.d. packed columns. The system is very inexpensive and requires little installation time, and the only fabrication steps necessary are cutting of glass and metal tubing and assembly of pressure fittings-steps which are routine for all laboratories using gas chromatography. In addition, the inlet splitter is easy to remove and the instrument may be reconfigured rapidly for packed column analyses.
EXPERIMENTAL SECTION Gas chromatographs used in this study include a Sigma 1 (Perkin-Elmer, Norwalk, CT) with a packed on-column injection port for use with 1/4 in. 0.d. columns, a commercial all-glass capillary splitter, a combined NPD/FID detector, and a 63Ni electron capture detector and also a MT-220 isothermal chromatograph (Tracor Instruments, Austin, TX) with on-column injection pork for 1/4 in. 0.d. columns and a 63Nielectron capture detector. Both glass and fused silica capillary columns have been used
with the splitter. Capillary columns were typically 0.24 mm i.d. X 25 m coated with methylsilicone or methylphenylsiliconestationary phases. Construction. The heart of the inlet system (Figure 1)is a stainless steel fitting designed for postcolumn derivatization of high-performance liquid chromatography effluents (Valco Instruments, Houston, TX, Part No TCFE 250/062/062). The fitting (Figure 1B) consists of a machined block with a 1/4 in. and two '/le in. female tube connections. The female in. threads match the male threads of the packed column inlets (Figure 1A) of many gas chromatographs. As illustrated, a 1 / 4 in. 0.d. glass tube (1,Figure 1)of sufficient length to approach within 2-5 mm of the septum is placed in the packed column inlet with two graphite ferrules (2, Figure 1). The analytical column (3, Figure 1)is secured with a in. graphite ferrule and pressure fitting. A 2-ft length of '/I6 in. 0.d. X 1.2 mm i.d. stainless steel tubing (5,Figure 1)is fastened to the remaining '/le in. outlet and a length of capillary tubing (7, Figure 1)of the same internal diameter as the analytical column is sealed to the end of the metal tubing with a stainless steel '/I6 in. to in. connector (6, Figure 1). The metal tubing can be bent to form a hanger to support the analytical column.
RESULTS AND DISCUSSION The split ratio of the inlet is equivalent to the inverse ratio of the length of the analytical column to the restrictor capillary length if the analytical column and the restrictor capillary have the same internal diameter. That is, for a capillary analytical column of 25 m a restrictor capillary of 1m will give a measured gas flow ratio of 1:25 for the analytical column and restrictor. This equivalency was found to hold for split ratios of k12.5, 1:25, 1:50, and 1:lOO a t ambient temperature and a t 250 O C with inlet pressures of approximately 50 psi and a total inlet flow of 60 mL/min helium carrier gas. Column flow varied from 4.1 mL/min to 0.5 mL/min for split ratios of 1:12.5 and 1:100, respectively. Thus, for a particular analytical column, split ratios may be changed rapidly and re-
0003-2700/83/0355-0974$01.50/00 1983 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
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Schematic of capillary inlet: (A) existing packed column inlet port, (B) postcolumn derlvatization fitting for llquid chromatography; (1) in. glass tube, ( 2 ) '/., in. graphite ferrules, (3) analytical caplllary column, (4) ' / l e in. graphite ferrule, (5)stainless steel tubing, (6) '/le in. to in. connector, (7) restrictor capillary. Flgure 1.
Table I. Instrumental Response to an Alkane Standard Mixture at Low Split Ratios and Varying Temperatures inlet temp, "C
250 260 270 280
split ratiob 1:25 1:510 1:llOO 1:25
1:513 1:lOO 1:25 1:50 1:lOO 1:2!3 1:50 1:lOO
re1 response (area)a
C14
C15
C16
34.1 i 0.1 33.4 i 0.2 32.5 i 0.4 34.2 i. 0.1 33.9 i 1.4 32.7 i. 0.7 34.1 t 0.2 33.2 i. 0.2 32.9 t 0.1 33.8 i. 0.5 33.0 i. 0.3 33.0 i. 0.1
32.8 i 0.1 33.1 i. 0.2 33.0 i. 0.3 32.7 * 0.2 32.8 t 1.8 32.9 i 0.1 32.8 i 0.1 33.0 i 0.2 33.2 i 0.2 32.6 i 0.4 33.4 i 0.3 33.0 i 0.2
33.0 i 0.1 33.1 t 0.4 33.3 + 0.6 32.7 k 0.1 30.7 i. 2.8 32.8 i 0.8 33.0 i. 0.2 33.2 i 0.2 33.5 i 0.2 33.5 A: 1.0 33.0 t 0.5 33.0 i. 0.5
Mean i saniple standard deviation for five measurements. Measured at a constant total flow rate of 100 mL/min. a
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producibly by changing the restrictor capillary length. The metal t,ubing connecting the Valco fitting and the restrictor capillary serves as a buffer volume which diminishes pressure surges when relatively large volumes of volatile solvents are injected. The geometry of the inlet splitter and the intimate metal-to-metal contact of the packed column inlet port and the Vdco fitting ensure that the temperature in the region where splitting of the gas stream occurs is at the inlet port temperature. This combination of characteristics may account for the apparent lack of discrimination between analytes of differing boiling points found when a series of hydrocarbons (tetra-, penta-, and hexadecane, 0.1% in hexane) were chromatographed with the inlet splitter temperature varied over a range of 250-280 OC, Table I. As may be seen, instrument response to an equimolar mixture of these hydrocarbons did not vary appreciably over this temperature range while the split ratio was changed from 1:25 to 1 5 0 to 1:lOO. Other splitters of similar design described in the literature show considerable discrimination over this temperature range a t split ratios of 1:lOO or less (1). Performance of the splitter in the analysis of complex mixtures is represented by portions of chromatograms of
Chromatographic profiles of the alkane fraction from (A) South Louisiana and (B) Kuwait crude oils: methylsilicone fused silica capillary (0.24 mm X 25 m), oven programmed from 100 to 250 O C at 10 OC/min, split ratio 1:100, helium carrier gas. Flgure 2.
alkane fractions of two crude oils presented in Figure 2. Chromatographic efficiency is excellent; normal alkanes are well separated from each other with no appreciable tailing. When tested on the same instrument using the same analytical column againfit a commercial all-glass capillary inlet splitter, the inlet system described in this paper showed at least equivalent performance with respect to chromatographic efficiency. In fact, for these mixtures of alkanes the commercial splitting routinely gave slightly lower (ca. 10%) plate counts for normal hydrocarbons because of slight tailing. In addition, the commercial splitter is approximately 20 times more expensive than the described system, requires several hours to install, and cannot easily be converted back to packed oncolumn injection. Several modifications to the system are easily visualized. For example, the capillary restrictor can be replaced by a micro needle valve which would allow the split ratio to be varied continuously over a wide range. As described, the inlet splitter is very easy to install, split ratios are determined by the ratio of analytical column to restrictor capillary length (if the same material is used for each), and the splitter does not display discrimination between analytes of differing boiling point. It allows chromatographs equipped for packed columns to be used with highly efficient capillary columns with very little outlay in installation time or funds.
LITERATURE CITED (1) Jennings, W. "Gas Chromatography with Glass Capillary Columns", 2nd ed.; Academic Press: New York, 1960; pp 63-77.
RECEIVED for review October 22,1982. Accepted January 20, 1983.
