Matrix effect in the cryotrapping of gas samples in the ppbv range

A gas mixture containing ppbv concentrations of volatile chlorinated and ar omatic hydrocarbons was prepared in helium, nitrogen, or argon in a plastic (Nalophan) bag, in which it was found the mixture could be safely stored f or several days. Samples of the mixture were subsequently analyzed by gas c hromatography after direct cryotrapping in an empty metal U-tube. The cooli ng medium used was liquid nitrogen, liquid argon, or a dry ice-acetone slur ry (197 K). The efficiency of cryotrapping in liquid nitrogen was over 90% when the aromatic hydrocarbon mixture was prepared in the helium matrix, bu t between 50 and 70% when it was prepared in nitrogen or argon, the recover y from the argon matrix being somewhat higher. The poor recovery in nitroge n at 77 K and argon at 87 K was explained by the reduced rate of diffusion to the wall owing to mist or aerosol formation. At 197 K condensation was n egligible if the partial pressures were lower than the saturated vapor pres sures.