TEMPERATURE-DEPENDENT ELECTRON-CAPTURE DETECTOR RESPONSE TO COMMON ALTERNATIVE FLUOROCARBONS

The relative electron capture detector (ECD) response to alternative f luorocarbons (AFCs) using gas chromatography are found to be at least 1 order of magnitude lower than that for CFC-12. Detection limits for the chlorofluorocarbons CFC-12, HCFC-22, HCFC-123, and HCFC-124 are fo und to be 2.5, 90, 30, and 90 pg, respectively. Those for the hydroflu orocarbons are significantly poorer; 14 and 45 ng for HFC-125 and HFC- 134a, respectively. HFC-152a was not detected using ECD. Since atmosph eric concentrations of these compounds are in the low part-per-trillio n level, GC-ECD is apparently not sensitive enough to be used for AFC analysis without substantial preconcentration. Two columns are evaluat ed for the AFC separation. The Poraplot Q WPLOT column showed good sep aration ability, though column bleed limits detection performance. A C arboxen 1004 packed column exhibits much lower interference. But separ ations are time consuming and peak broadening adversely affects limits of detection. Mechanisms for the ECD response are proposed based on t hermodynamics and temperature-dependent ECD responses. CFC-12, HCFC-12 3, and HFC-125 apparently undergo ion-forming dissociative electron ca pture. The electron capture process for HCFC-22 and HFC-134a appear to form molecular ions. Both mechanisms appear to be operative for HCFC- 124 electron capture. Dissociative electron capture rate constants for HCFC-123, HCFC-124, and HFC-125 are estimated to be 3.5 x 10(-10), 1. 0 x 10(-10), and 5.6 x 10(-13) cm(3) s(-1), respectively at 300 degree s C.