Evaluating porous-layer open-tubular capillaries as vapor preconcentratorsin a microanalytical system

Measuring environmental concentrations of organic vapors with microfabricat ed chemical sensors or sensor arrays often requires a means to enrich colle cted samples prior to detection. With such an application in mind, a prelim inary evaluation is described of two porous-layer open tubular (PLOT) capil lary traps as vapor preconcentrators for a series of vapors. Short (l-cm) s ections of commercial PLOT-Q and PLOT-S capillary having wall coatings of s tyrene-divinylbenzene copolymer and vinylpyridine-divinylbenzene copolymer, respectively, are fitted with a metal sleeve for rapid thermal desorption of preconcentrated vapor samples, and tested using a downstream 97-MHz poly isobutylene-coated surface acoustic wave (SAW) sensor. Calibrated responses to vapors of 2-butanone (MEK), trichloroethylene (TCE), toluene, and in-xy lene are collected with and without preconcentration. Dimethylmethylphospho nate could not be efficiently desorbed from either PLOT trap. For the remai ning vapors, increases in sensitivity of 3-9-fold are achieved by preconcen trating and analyzing just 1 ml of sample air. Calculated limits of detecti on (LOD) range from 1-8 ppm. Differences in sensitivities are observed betw een the PLOT-Q and PLOT-S sampling trains for MEK and TCE. A theoretical mo del of penetration yields limiting values of flow rate and trap dimensions. Measured 10%-breakthrough times at 1 ml/min ranged from similar to 1 to 6 min and, for PLOT-Q, are greater than or equal to modeled values obtained u sing the modified Wheeler equation. The implications of the results for the design and operation of microanalytical systems for vapor analytes are dis cussed. (C) 2000 Elsevier Science S.A. All rights reserved.