On-Line Multibed Sorption Trap and Injector for the GC Analysis of Organic Vapors in Large-Volume Air Samples

Abstract

A capillary-dimension on-line sorption trap is used to preconcentrate organic vapors from large-volume air samples and inject the organic compounds into the separation column as a relatively narrow vapor plug. The multibed trap is made from a Co-Ni alloy for resistive heating during sample desorption and uses four different carbon-based adsorption materials that are graded from weakest to strongest in the direction of the sample gas flow during sample preconcentration. The flow direction then is reversed for sample injection. The multibed design and the flow direction reversal during thermal desorption prevents the higher-boiling-point compounds in the sample from reaching the strongest adsorbing material, from which they would be difficult to desorb as a sufficiently narrow vapor plug. A relatively high current pulse is used to rapidly achieve trap temperatures in the 200-400 degrees C temperature range, and a lower current is used to maintain the maximum temperature for several seconds in order to ensure injection of the entire trapped sample. A temperature of 350 degrees C is reached after degrees 1.5 s, and injection plug widths are typically in the range of 0.6-1.3 s. Plots of peak area versus sample collection time show excellent linearity and shot-to-shot relatively standard deviations of about +/- 5%. Performance data are presented for a mixture of 42 volatile compounds spanning a volatility range from n-C5 to n-C12. Data are presented for injection plug width and shape for both polar and nonpolar compounds. Decomposition of thermally labile compounds is observed for injection temperatures above 300 degrees C.