Analyzing organic vapors in exhaled breath using a surface acoustic wave sensor array with preconcentration: Selection and characterization of the preconcentrator adsorbent

Abstract

The analysis of organic vapors in exhaled breath can provide information about chemical exposures and health status. This article describes work aimed at developing a small prototype instrument that employs an array of four polymer-coated surface acoustic wave (SAW) sensors and a thermally desorbable adsorbent preconcentrator for rapid breath analysis. The adsorbent used in the preconcentrator is critical to achieving adequate sensitivity and compensating for the high background of water vapor. Eight granular adsorbents packed into narrow bore glass tubes wrapped with NiCr wire were evaluated individually and in selected dual-bed configurations with respect to the pressure drop of the packed bed, retention of water vapor, and adsorption/desorption efficiency of each of several organic solvent vapors. Although adsorbents of Tenax GR ® and Carbotrap ® performed well, a highly porous styrene-divinylbenzene resin demonstrated superior overall performance and was selected for further testing. Solvents ranging in vapor pressure from 8 mm of Hg (m-xylene) to 420 mm of Hg (dichloromethane) were efficiently trapped from 0.25-l spiked breath samples and efficiently desorbed at 170°C. Incorporating an intermediate dry-air purge step prior to thermal desorption of samples selectively removed co-adsorbed water and reduced the limits of detection (LOD) by an order of magnitude. Results of detailed breakthrough studies were considered in the context of the modified Wheeler and Langmuir adsorption models and used to determine the minimum quantity of adsorbent required to prevent saturation of the adsorbent bed for each test vapor. Measurement of vapors at concentrations ranging from sub-ppm to 200 ppm was demonstrated.