AIDS FOR ANALYTICAL CHEMISTS Preparatory Trapping of Gas-Chromatographic Effluents with a Fritted Filter E. C. Schluter, Jr. Union Oil Company of California, P.O. Box 76, Brea, Calif. 92621 COLDTRAPS are adequate for the efficient collection of volatile effluents from a gas chromatograph. Low volatility solutes tend to form aerosols, however, and are difficult to trap efficiently. Common approaches to aerosol collection are the use of thermal gradient traps ( I , 2); of a cold trap and electrostatic precipitation (3, 4 ) ; of total trapping of effluent and carrier gas (5, 6 ) ; of co-condensation and solvent entrainment (7,8); and of filter methods (9-11). While published devices are undoubtedly effective for their intended application, they have not been generally applicable to our needs. We required a simply constructed, inexpensive, efficient,nonhazardous, and easily used trap. The trap shown in Figure 1 fills these requirements and is principally a filter type trap. EXPERIMENTAL The trap consists of a central tube made from borosilicate glass tubing and an extra coarse gas dispersion tube; an outer jacket, either a test tube or a 1 5 4 centrifuge tube, to isolate the filter from the coolant, and, a rubber stopper with a vent hole to hold the filter in the jacket. The lip on the jacket supported the trap in a hole in the Dewar cover, and the liquid nitrogen coolant level was kept about 2 cm above the fritted filter. The proximity of liquid nitrogen kept the rubber stopper cold and reduced heating from connection to the transfer line to the chromatograph. The trap was connected to a preparatory chromatograph with Teflon (Du Pont) ferrules and a Swagelok union. The transfer line, including the union, was heated to within 1 cm of the rubber stopper. Connection to and from the chromatograph could be done in less than 10 seconds. RESULTS AND DISCUSSION The effectiveness of the trap was evaluated gravimetrically by trapping a series of low volatility petroleum samples, both neat and from solution. A weighed sample was injected and the recovered sample weighed back. Recoveries were 95 to 97 % for a volatility range of n-hexane to n-doctriacontane and trapping times of 2 to 90 minutes. Sample sizes ranged from 0.2 to 2 grams from single injections. Sample fractionation was carried out with a 4-foot X a/4-inch0.d. stainless steel column packed with 10% SE-30 on 30/60 silanized Chromosorb P. Helium flow was 2 l/min. At 270 “C column “bleed” was less than 1 mg in 90 min. Further evidence of trapping effectiveness was a mass spectrometer analysis of sample composition before and after trapping. No change in composition was apparent within the precision and accuracy of the mass spectrometer method for hydrocarbon and nonhydrocarbon compound types from petroleum samples. (1) R . Teranishi, J. W. Corse, J. C. Day, and W. G. Jennings, J. Chromatogr., 9, 244 (1962). (2) R . K. Stevens and J. D. Mold, ibid., 10, 398 (1963). (3) A. E. Thompson, ibid.,6, 454 (1961). (4) D. W. Fish and D. G. Crosby, ibid.,37, 307 (1968). (5) P. A. T. Swaboda, Nature, 199, 31 (1963). (6) I. Hornstein and P. Crowe, ANAL.CHEM.,37, 170 (1965). (7) J. H. Jones and C. D. Ritchie, J. Assoc. Oflc.Agr. Chemists, 41, 753 (1958). (8) R . Hardy and J. N. Keay, J. Chromatogr., 17, 177 (1965). (9) H. W. Leggon, ANAL.CHEM.,33, 1295 (1961). (10) A. K. Hajra and N. S . Radin, J . Lipid Res., 3 (l), 131 (1962). (1 1) K. Witte and 0. Dissinger, Z. Alia/. Cliem., 236, 119 (1968). 1360
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ANALYTICAL CHEMISTRY
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\ Figure 1. Filter trap Sample accumulation inside the frit causes a pressure increase and connections must be adequate to withstand a back pressure of about 30 psi. Waxy samples partly collect in the nonfritted part of the trap and tend to cause greater backpressure possibly because there are less crystal fracture channels for gas flow. The maximum back pressure measured in evaluating this trap was 15 psi and no change in flow was detected. In addition to mechanical strength, the fritted glass filter is inert both to sample type and to most solvents used for removing trapped material from the frit. Some sample can be removed with a narrow probe but if it is necessary to recover a larger amount of solvent-free sample, scale-up of the trap and a different geometry should be no problem. The fritted filter is used mostly to trap the aerosol and the normally condensed and usually larger fraction of sample could be collected at some other point of such a trap. Conversely, scale-down, to trap from analytical gas chromatographs, should be equally valid since the principle of effective aerosol trapping on a solvent-free fritted filter at cryogenic temperature still holds. Smaller surface fritted filters are available. The use of dry ice to cool the trap was less effective than the use of liquid nitrogen. Recoveries of 84 to 90% were measured. This observation agrees with that by Witte and Dissinger ( I I ) . RECEIVED for review April 24, 1969. Accepted June 11, 1969.
