Simultaneous determination of sulfur hexafluoride and three chlorofluorocarbons in water and air

We have developed an analytical technique for the simultaneous measurement of the four trace gases sulfur hexafluoride (SF(6)) and the chlorofluorocarbons CCl(2)F(2) (CFC-12), CCl(3)F (CFC-11) and CCl(2)FCClF(2) (CFC-113) in water and air. Water samples are flame sealed into 350-ml glass ampoules which allow storage and sampling in locations where field measurements are not practical. For analysis, these ampoules are stripped of dissolved gases after their stems are cracked in an enclosed chamber such that the headspace fraction in the ampoule is included in the measurement. The extracted gases are then trapped cryogenically and are separated on packed columns. CFC-11 and CFC-113 are measured on one electron capture detector (ECD), while SF(6) and CFC-12 are cryofocussed on a second trap and analyzed on a second ECD. Detection limits for seawater samples are about 0.015 fmol kg(-1) for SF(6), 0.010 pmol kg(-1) for CFC-12, 0.014 pmol kg(-1) for CFC-11, and 0.024 pmol kg(-1) for CFC-113. This analytical technique also allows for analysis of air samples with low concentrations or at low pressures. Results from a profile in the northeastern Pacific Ocean show that SF6 partial pressure ages are consistent with those of CFC-11 and CFC-12 over the age range covered by this profile. From this, we infer that SF6 is useful for the dating of recently ventilated waters, thus complementing the dating of older waters using CFCs. Earlier reports of the degradation of CFC-113 in oxygenated water are supported by our results for samples stored in ampoules. (C) 2002 Elsevier Science B.V All rights reserved.