Drying the purge gas

Drying the gas before it enters the heated zone is essential for three reasons. First, chromatographic columns are liable to deterioration, if the carrier gas contains traces of water vapour. Second, all droplets contain seasalt which, when the water evaporates, forms salt 

Tbere are several ways to remove water from the gas downstream of the purge chamber. When its upper part is cooled, most of the water condenses on the surface and flows back into the bulk liquid. To this end, either a metal block is mounted around the top, kept cold by water from a cooling bath, or it is more efficient to lead the cooling water through a few coils of glass tube inside the upper part of the chamber. 

After the purge chamber, the gas stream passes through 1 m of a Nafion tube (1/16 in o.d., Perma Pure Inc., USA). Nafion is a semipermeable membrane through which water vapour and polar compounds pass freely, but none of the halocarbons. The Nafion is coiled in a Perspex (Plexiglass) tube ( 20cm long, o.d.30 mm) through which a counterflow of dry gas is led, e.g., the effluent from valve 4 (see Fig. 23-1). 

Hydrogen sulphide, present in anoxic waters, co-elutes with the halocarbons in the first part of the chromatogram it has a moderate BCD response, but high concentrations make any measurements of methyl halides and the earliest CFCs impossible. The Ascarite tube shown in Fig. 23-1 (dimensions about the same as the desiccant tube) is an optional device for removing hydrogen sulphide it is used only for analyses of anoxic/suboxic waters. Ascarite is sodium hydroxide coated silica, which binds acidic gases such as hydrogen sulphide and carbon dioxide. 

---