Dual-detector system for the shipboard analysis of halocarbons in sea-water and air for oceanographic tracer studies

Abstract

The measurement of chlorofluorocarbons (CFCs) has become well established as a technique for tracing oceanographic processes, providing time-dependent information about the mixing and transport of water masses. Until recently, sea-water analyses have been confined to CFC-11 and CFC-12, using a packed column electron capture detector–GC system. Recent changes in CFC use, together with the desire to extend the timescale covered by this technique, has necessitated the broadening of the range of compounds measured to include CFC-113 and tetrachloromethane. This has involved the development of a new chromatographic strategy based on a DB-624 megabore column together with a redesigned sample trapping/injection system. In addition, a dual-detector configuration, permitting increased sample throughput, has been designed. The new instrument also includes a more sophisticated standard/air injection system, providing the ability to cover a wider range of concentrations, but with a greater number of calibration points if required. Increased automation helps to ensure analytical quality whilst enhancing the instrument's usability. Results from the first field trials of this instrument, on RRS Discovery cruise 213, are reported. These indicate that the system has detection limits of 0.05, 0.005, 0.015 and 0.015 mol l^–1 for CFC-12, CFC-11, CFC-113 and tetrachloromethane, respectively.