SF6 Lab: Groundwater and Soil Air
The SF6 system consists of a Shimadzu GC-8A Gas Chromatograph equipped with an electron capture detector. SF6 is separated from other gases by a molecular sieve 5A column at room temperature. Currently, water samples are collected using glass syringes. After equilibration of the gases dissolved in the water with a head space in the syringe, the gas samples are injected directly into the GC system via the sample loop method. With this method, the minimum and maximum concentration typically measured are 8 fM and 8E4 fM, respectively (1 fM = 1 femtomoles = 10 E-15 mol/L). The typical measurement precision is around ±2% including contributions from the detector as well as uncertainties in volume determinations. The SF6 elutes in approximately 1 minute and the column is then backflushed for 2 minutes to purge the residual gases. This rapid turnaround time (3 minutes) allows us to utilize the GC in the field and make near real-time measurements of SF6.