Optimization and Calibration of a GC-PFPD-Cryogenic Trapping System for the Analysis of Volatile Sulfur Compounds at Trace Levels in Gases

摘要

The two main challenges for the measurement of sulfur compounds in ambient air are very low concentrations (low ppb down to ppt levels) and losses due to the high reactivity of reduced sulfur species. This paper describes the optimization and calibration of a trace level sulfur analyzer consisting of a gas chromatograph equipped with a pulsed flame photometric detector (PFPD) and interfaced with a cryogenic trap for pre-concentration of ambient samples. The system was optimized with respect to 1) the split flow ratio at which the analytes are transferred from the cryogenic trap to the chromatographic column, 2) the split flow sequence, and 3) the column flow rate. Calibration curves were obtained for hydrogen sulfide (H{sub}2S), sulfur dioxide (SO{sub}2), methyl mercaptan (CH{sub}3SH), dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) using permeation tubes to generate calibration gases of known compositions. Calibration runs carried out by varying the trap loading time and the composition of the calibration gas show that the chromatographic peak area for each sulfur compound is not only a function of the mass of compound loaded in the cryogenic trap, but also of the concentration of the compound in the calibration gas. The dependence of calibration parameters on analyte concentration poses a serious practical problem since the ultimate objective is to analyze air samples of unknown composition. Therefore, it is recommended to carry out the calibration by varying the concentration of sulfur compounds in the calibration gas while keeping the trap loading time constant at the same value used for the analysis of unknown samples. Since calibration parameters also depend on the molecular structure of sulfur compounds, individual calibration curves should be obtained for each analyte.