Purging of halocarbons from the water

The purge gas flow rate is kept constant at = 80 mL/min by a metal bellows flow controller (Porter Instruments, VCD-1000, see Fig. 23-1). A toggle valve downstream of the flow controller is used to stop the gas flow for maintenance of the system or for determination of extraction efficiencies, etc. The purity of the purge gas is checked by running the analytical programme without injection of seawater. 

If the sample water contains particles, e.g., planktonic algae in surface waters or other solids, there is a risk of scratching valve rotors or blocking the narrow passages. Such particles may be removed with glass fibre filters (Whatman GF/F) of 2.4 cm diameter, provided they are cleaned properly. The filters are inserted between the syringe and valve 1. 

Most of the 100 mL of seawater in the sample syringe (about 2/3) is used to flush the sample volumetric loop, before valve 1 is turned. The purge gas backflushes the water out of the loop and via valve 2 to the purge chamber. 

Valve 2 is an internal loop valve used for the injection of liquid standards it may be disconnected when the system is used for gaseous halocarbons only. Valve 2 can also serve as a device for introducing internal standards into the sample stream which is especially useful for quantifying late peaks in the chromatogram. A suitable internal standard is bromotri-chloromethane (CBrCb) dissolved in methanol. 

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