Headspace extraction method for simultaneous determination of SF6, CCl3F, CCl2F2 and CCl2FCCIF2 in water

Recently, sulfur hexafluroride (SF6), freon-11 (CCl3F), freon-12 (CCl2F) and freon-113 (CCl2FCClF2) have become popular as environmental tracers for hydrogeological purposes. In this paper a chromatographic method for simultaneous determination of these compounds in water samples has been presented. The analytical system comprised three chromatographic columns operating in a back-flush mode, and two electron capture detectors. A constant frequency voltage was applied to the detectors. As the concentrations of SF6 and freons differed by three orders of magnitude, the frequencies of the voltage pulses were appropriately chosen to provide optimum determination conditions. Water samples were stored in special glass bottles utilised for the headspace extraction of gases. Next, the extracted gas was placed in a special trap under low pressure. The trap was filled with glass balls, which were, in turn, immersed in the liquid nitrogen. The latter was used in order to remove Ar and CO2 passing through the trap. After that, the sample was desorbed at the temperature of 100degreesC, and injected on the first column. This column was connected to the second and third one for the particular time interval. The duration of these connections decided, which of the sample constituents were separated. The first detector was connected to the second column, on which SF6 was separated (or SF6 and F-12). To this detector a pulsed voltage with the repetition time of 1920 mus was applied to assure the best detection level. Freons-11 and 113 (or also F-12) were separated on the third column, and determined using the second detector. To this detector, in turn, a pulsed voltage with the repetition time of 80 mus was imposed to obtain a sufficiently wide range of linear response. The following detection levels were obtained: 0.0054 fg cm(H2O)(-3) (0.04 fmol L-1) for SF6, 0.35 fg cm(H2O)(-3) (2.5 fmol L-1) for F-11, 0.22 fg cm(H2O)(-3) (1.8 fmol L-1) for F-12 and 0.55 fg cm(H2O)(-3) (2.9 fmol L-1) for F-113.