A Continuous Fast-Response Dual-Tracer Analyzer for Halogenated Atmospheric Tracer Studies

Abstract

An apparatus for the simultaneous measurement of two tracers, sulfur hexafluoride (SF6) and a perfluorocarbon compound, is introduced. The new instrument is a modification of a commercially available fast-response, continuous analyzer for single halogenated atmospheric tracer studies. A two-channel flow system was implemented consisting of an alumina cartridge in one channel and a glass beads cartridge of equal flow resistance in the second channel. The alumina passes only sulfur hexafluoride, while the glass beads pass both SF6 and the perfluorocarbon tracer. The SF6 is quantified directly from the electron capture detector (ECD) signal in the alumina channel, and the perfluorocarbon concentration is obtained from the difference of the ECD responses in the two channels. The dual-tracer analyzer is field portable for mobile operations or fixed-location monitoring, has a response time of 1.2 s, and has limits of detection of about 15 pptv for SF6 and 10 pptv for perfluoromethylcyclohexane, which was the principal perfluorocarbon tracer used in this study. The present instrument configuration, which requires periodic purging of the adsorbent trap, can obtain continuous measurements for a 10?15-min segment in every half hour of operation. Dual-tracer data from a field demonstration test are presented.