Analysis of low concentration reduced sulfur compounds (RSCs) in air: storage issues and measurement by gas chromatography with sulfur chemiluminescence detection

Reduced sulfur compounds (RSCs) were measured at low concentrations in small volume air samples using a cryo-trapping inlet system and gas chromatograph outfitted with a sulfur chemiluminescence detector (GC-SCD). The relative sensitivity of the system to the RSCs follows the sequence H2S < CH3SH < OCS ∼ DMS < CS2. The analytical system achieves a detection limit of 120 ppt in a 100 mL air sample, which is suitable for measuring reactive RSCs (e.g., H2S and CH3SH) at ambient or near ambient atmospheric concentrations. The inlet system allows for replicate sampling from a stored air sample (sub-sampling), thereby improving estimates of instrumental precision and demonstrating the reproducibility of the analytical method. Although the SCD theoretically provides linear responses equivalent to the sulfur mass injected, we found that the response properties for each RSC differed. At concentrations below 2 ppb, the compounds H2S and CH3SH have diminished responses, leading to larger measurement uncertainties. Two generations of commercially available SilcoCan canisters were tested to evaluate the relative RSC loss due to storage in the canister and loss of inertness because of coating age. The older generation canister (>6 years from initial coating) saw significant loss of H2S and CH3SH within 2 days, while the more recent generation canister (<1 year from initial coating) yielded percent recoveries of RSCs in the range of 85% (H2S and CH3SH) to 95% (OCS, DMS and CS2) after 7 days of storage, suggesting that these canisters may be suitable for the short-term storage of low level RSCs. The development of this low concentration, low sample volume method is well suited for measuring RSC gas fluxes from natural soils in laboratory incubations and in field flux chamber studies.