Performance evaluation of a sorbent tube sampling method using short path thermal desorption for volatile organic compounds
Air sampling, using sorbents, thermal desorption and gas chromatography, is a versatile method for identifying and quantifying trace levels of volatile organic compounds (VOCs). Thermal desorption can provide high sensitivity, appropropriate choices of sorbents and method parameters can accommodate a wide range of compounds and high humidity, and automated short-path systems can minimize artifacts, losses and carry-over effects. This study evaluates the performance of a short-path thermal desorption method for 77 VOCs using laboratory and field tests and a dual sorbent system (Tenax GR, Carbosieve SIII). Laboratory tests showed that the method requirements for ambient air sampling were easily achieved for most compounds, e.g., using the average and standard deviation across target compounds, blank emissions were ≤0.3 ng per sorbent tube for all target compounds except benzene, toluene and phenol; the method detection limit was 0.05 ± 0.08 ppb, reproducibility was 12 ± 6%, linearity, as the relative standard deviation of relative response factors, was 16 ± 9%, desorption efficiency was 99 ± 28%, samples stored for 1–6 weeks had recoveries of 87 ± 9%, and high humidity samples had recoveries of 102 ± 12%. Due to sorbent, column and detector characteristics, performance was somewhat poorer for phenol groups, ketones, and nitrogen containing compounds. The laboratory results were confirmed in an analysis of replicate samples collected in two field studies that sampled ambient air along roadways and indoor air in a large office building. Replicates collected under field conditions demonstrated good agreement except for very low concentrations or large (>4 l volume) samples of high humidity air. Overall, the method provides excellent performance and satisfactory throughput for many applications.