The application of a dual-sector high-resolution mass spectrometer (HRMS) to the detection of several CF3-containing compounds in Earth's atmosphere is described. By this technique, the contents of air samples collected in remote areas surrounding Bozeman, Montana, are introduced to the HRMS by gas chromatography (GC) while the HRMS is tuned to the exact masses of the ions expected in the electron impact ionization of each compound. By the analysis procedure developed here, the mass scale of the instrument is continuously recalibrated during the analysis so that any level of mass resolution that is required for the complete separation of hydrocarbon-based from halocarbon-based ions of the same nominal mass can be selected. Owing to the unusually large ion optic system of the mass spectrometer, high ion throughput to the detector is maintained even when higher levels of mass resolution are selected. By this technique, the sensitive detection of CF4 (CFC-14), CF3Cl (CFC-13), CHF3 (HFC-23), CF3CF2Cl (CFC-115), CF3Br (halon-1301), and CF3CH2F (HFC-134a) in background air samples is demonstrated using essentially all of the major ions that appear in the electron impact mass spectrum of each compound. Most of these compounds are thought to be present in background air at the low parts-per-trillion by volume level. This is one of only two reports, to date, of CHF3 presence in background air. A potential source of this compound is suggested by additional measurements provided here. ¿ 1998 American Geophysical Union