A dual-mode GC analysis of reduced sulphur compounds in air over a wide concentration range

In this study, the general requirements of the GC/PFPD analysis have been investigated in the measurements of reduced sulphur compounds (RSC) (hydrogen sulphide (H₂S), methyl mercaptan (CH₃SH), dimethylsulphide (DMS), and dimethyldisulphide (DMDS)) in air over a wide concentration range. To cover samples collected under various environmental conditions, a dual-mode analytical system was developed for both low (i.e. combination of a Peltier cooling (PC) and thermal desorption (TD) method) and high concentration detection settings (i.e. the direct loop injection (LI) method). They were combined to measure both lower- (ambient air samples in ppt level) and upper-bound concentrations (source-affected samples in ppb or ppm level) without the modulation of samples (e.g. dilution of samples). Their relative performance was evaluated in terms of differences in the analytical sensitivity by comparing both the calibration slope ratios and detection limits. According to this comparison, the result from the high mode setting exhibited a generally enhanced sensitivity relative to the low mode setting; such a difference can be explained in part in that the two analytical modes are calibrated under each respective set-up. Nonetheless, their relative detection characteristics were found to be highly consistent in various respects. The sensitivity of different S compounds tends to increase on the order of H₂S, CH₃SH, DMS, and DMDS (with two S atoms), regardless of the selected analytical mode. Based on the comparative analysis of the two GC settings, it is concluded that the versatile application of the GC/PFPD technique can be used effectively for the accurate quantification of S gases in various environmental samples.