A dual-adsorbent preconcentrator for a portable indoor-VOC microsensor system

The development and testing of a miniature dual-adsorbent preconcentrator f or a microsensor-based analytical system designed to determine complex vola tile organic chemical (VOC) mixtures encountered in indoor working environm ents at low part-per-billion levels is described. Candidate adsorbents were screened for thermal-desorption bandwidth and breakthrough volume against 20 volatile organic vapors and subsets thereof as a function of several rel evant variables. A glass capillary (1.1 mm i.d.) packed with 3.4 mg of Carb opack X and 1.2 mg of Carboxen 1000 provides sufficient capacity for a l-L dry-air sample containing all 20 vapors at concentrations of 100 ppb as wel l as providing a composite half-height peak width of < 3 s at:a desorption temperature of 300 degreesC and a now rate of 4 mL/min. Required adsorbent masses increase to 7.0 and 1.5 mg, respectively, for the same mixture at co mponent concentrations of 1 ppm. Vapor breakthrough volumes for the Carbopa ck X are unaffected by humidity from 0 to 100% RH, but those for the Carbox en 1000 are significantly reduced, requiring an additional 0.9 mg of the la tter to avoid premature breakthrough at the 100 ppb level. Good peak shapes , efficient chromatographic separation of preconcentrated sample mixture co mponents, and detection limits in the low-parts-per-billion range using an integrated surface-acoustic-wave (SAW) sensor are achieved. Preconcentrator design and operating parameters are considered in terms of the vapor bed-r esidence times and breakthrough volumes in the context of the modified Whee ler equation.