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 = 10E-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.
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