The use of a custom mode electron capture detector to determine mixing ratios of environmental tracers: Sulfur hexafluoride, chlorotrifluoromethane and bromotrifluoromethane

摘要

Atmospheric concentrations of anthropogenic trace gases, such as sulfur hexafluoride, SF_6, chlorotrifluoromethane, CF_3Cl, and bromotrifluoromethane, CF_3Br, are increasing. Their long lifetimes and limited chemical reactivity make them attractive environmental tracers for hydrology and oceanography. While ambient SF_6 concentrations can be readily measured using GC-ECD, the simultaneous analysis of CF_3Cl and CF_3Br is hampered due to their low ECD sensitivity. The response of a commercial ECD for those gases was enhanced using the resonance detection mode which is based on shifting the mean energy of electrons in the ECD detector towards the region where the electron-capture reaction reveals a distinct maximum. A custom electronic system enabled operation of a commercial ECD in the resonance detection mode. An approximately 50-fold amplification of the ECD signal was obtained for CF_3Cl by application of high-frequency electric field (amplitude of 50V and frequency of 40MHz). For CF_3Br, a 3.5-fold increase of the ECD signal was obtained, with a lower HF field (20-30V). In the case of SF_6 the application of the HF field reduces the magnitude of ECD signal by a factor of 40. The electron-capture coefficients for SF_6, CF_3Cl and CF_3Br were determined from 453 to 633K in the standard and the resonance modes. The electron-capture coefficients for CF_3Cl and CF_3Br increase with increasing temperature for both modes, while that for SF_6 decreases slightly with increasing temperature. The application of the resonance detection mode to a commercial ECD provides an attractive and cost-effective alternative to GCMS for high-quality quantitative analyses of CF_3Cl and CF_3Br as environmental tracers.