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

Abstract Measuring environmental concentrations of organic vapors with microfabricated chemical sensors or sensor arrays often requires a means to enrich collected samples prior to detection. With such an application in mind, a preliminary evaluation is described of two porous-layer open tubular (PLOT) capillary traps as vapor preconcentrators for a series of vapors. Short (1-cm) sections of commercial PLOT-Q and PLOT-S capillary having wall coatings of styrene–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 polyisobutylene-coated surface acoustic wave (SAW) sensor. Calibrated responses to vapors of 2-butanone (MEK), trichloroethylene (TCE), toluene, and m -xylene are collected with and without preconcentration. Dimethylmethylphosphonate could not be efficiently desorbed from either PLOT trap. For the remaining vapors, increases in sensitivity of 3–9-fold are achieved by preconcentrating and analyzing just 1 ml of sample air. Calculated limits of detection (LOD) range from 1–8 ppm. Differences in sensitivities are observed between the PLOT-Q and PLOT-S sampling trains for MEK and TCE. A theoretical model of penetration yields limiting values of flow rate and trap dimensions. Measured 10%-breakthrough times at 1 ml/min ranged from ∼1 to 6 min and, for PLOT-Q, are ≥ modeled values obtained using the modified Wheeler equation. The implications of the results for the design and operation of microanalytical systems for vapor analytes are discussed.