Optimization and evaluation of multi-bed adsorbent tube method in collection of volatile organic compounds

Abstract

The feasibility of using adsorbent tubes to collect volatile organic compounds (VOCs) has been demonstrated since the 1990's and standardized as Compendium Method TO-17 by the U.S. Environmental Protection Agency (U.S EPA). This paper investigates sampling and analytical variables on concentrations of 57 ozone (O3) precursors (C2-C12 aliphatic and aromatic VOCs) specified for the Photochemical Assessment Monitoring Station (PAMS). Laboratory and field tests examined multi-bed adsorbent tubes containing a sorbate combination of Tenax TA, Carbograph 1 TD, and Carboxen 1003. Analyte stabilities were influenced by both collection tube temperature and ambient O3 concentrations. Analytes degraded during storage, while blank levels were elevated by passive adsorption. Adsorbent tube storage under cold temperatures (−10°C) in a preservation container filled with solid silica gel and anhydrous calcium sulfate (CaSO4) ensured sample integrity. A high efficiency (>99%) O3 scrubber (i.e., copper coil tube filled with saturated potassium iodide [KI]) removed O3 (i.e., <200 ppbv) from the air stream with a sampling capacity of 30h. Water vapor scrubbers interfered with VOC measurements. The optimal thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) desorption time of 8min was found at 330°C. Good linearity (R2>0.995) was achieved for individual analyte calibrations (with the exception of acetylene) for mixing ratios of 0.08–1.96 ppbv. The method detection limits (MDLs) were below 0.055 ppbv for a 3L sample volume. Replicate analyses showed relative standard deviations (RSDs) of <10%, with the majority of the analytes within <5%.