Development of a method for determination of VOCs (including methylsiloxanes) in biogas by TD-GC/MS analysis using Supel™ Inert Film bags and multisorbent bed tubes

An analytical method based on TD-GC/MS was developed and validated for the determination of volatile organic compounds (VOCs), including linear and cyclic volatile methylsiloxanes (VMSs), in biogas. Biogas was first sampled in Supel™ Inert Film bags and subsequently dynamically sampled onto multisorbent bed tubes (Carbotrap, Carbopack X and Carboxen 569) using portable pump equipment. Two sample volumes, 100 and 250 ml, were evaluated. Desorption efficiency values for both volumes are in the range of 99-100% for almost all studied compounds while breakthrough values (%VOC on sample tube back section) are below 1% for most evaluated VOCs. However, acetaldehyde, carbon disulphide, ethanol and 1,3-butadiene have breakthrough values higher than 5%. Method detection limits were in the range of 0.01-0.8 ng per sample. The most abundant VOCs in biogas were terpenes with concentrations between 500 and 700 mg m −3 . Other important families were ketones, aromatic hydrocarbons and alkanes, with concentrations in the range of 36-46 , 20-35 and 14-16 mg m −3 , respectively. VMSs presented average concentrations of 4.9 ± 0.4 mg m −3 . Additionally, the Supel™ Inert Film bags were evaluated for stability for 4 days at room temperature. Although several VOC families' concentrations in the bag increased or decreased significantly (t-test; p ≤ 0.01, n = 5) 2 days after collection, recoveries were around 70-130% for most studied VOCs. The results shown demonstrate that the presented methodology is reliable and satisfactory for the evaluation of VOCs in biogas and presents an alternative to the currently existing biogas analytical techniques.