Evaluating the dynamic retention capacities of microfabricated vapor preconcentrators as a function of flow rate

Adsorbent-packed preconcentrators are essential elements of most microanalytical systems designed for monitoring airborne volatile organic compounds (VOC) at low concentrations. These devices also serve as thermally desorbed injectors that transfer focused bands of captured VOCs to downstream separation and/or detection modules. Despite their importance, the factors affecting the capture efficiency of such devices have not been extensively or systematically studied. In this study, the dynamic retention capacities of four deep-reactive-ion-etched Si micropreconcentrator-focusers (μPCF) packed with a commercial graphitized carbon, Carbopack X (C-X), were characterized for several VOCs and compared to those of a reference capillary preconcentrator-focuser (cPCF). Devices were challenged with ~100 parts-per-billion of benzene, 2-butanone, toluene, or n-heptane in dry N2 over a range of volumetric flow rates, Q. The relationships between the bed residence time, τ, and the 10% breakthrough volume and breakthrough time (Vb−10 and tb−10, respectively) were evaluated in the context of the modified Wheeler Model. Both Vb−10 and tb−10 decrease monotonically with decreasing τ, in accordance with the model. The performance of the largest μPCF, packed with 2.3 mg of C-X, was comparable to that of the reference cPCF packed with a similar quantity of C-X. The critical flow rates, Qc−10, corresponding to immediate breakthrough, ranged from 70 to 290 mL/min and varied directly with the affinity of the vapor for the adsorbent. As a practical operating guideline, it is recommended that flow rates be limited to <0.4Qc for reliable performance of any μPCF used for quantitative analysis. Estimated preconcentration factors range from 730 to 39,000. Challenges to predicting device performance via the modified Wheeler Model are illustrated.