Column performance and stability for high-speed vacuum-outlet GC of volatile organic compounds using atmospheric pressure air as carrier gas

The development of lightweight, portable GC instrumentation is handicapped by the need for compressed carrier gas to drive the separation. The use of air as carrier gas eliminates the need for compressed gas tanks. If a vacuu m pump is used to pull carrier gas and injected samples through the column, atmospheric pressure air can be used as carrier gas. Vacuum outlet operati on also improves performance for high-speed separations by reducing detecto r dead time and by shifting optimal carrier gas velocity to higher values. Under vacuum outlet conditions using atmospheric pressure air as carrier ga s, a 6-m-long, 0.25-mm-i.d. capillary column can generate similar to 12 500 theoretical plates, and a 12-m-long column can generate similar to 44 000 plates but with a 3-4-fold increase in separation time. The principal issue s in column selection for highspeed GC with air as a carrier gas are effici ency and stability. Several bonded and nonbonded stationary phases were eva luated for use with air as carrier gas in the analysis of volatile organic compounds of interest in air-monitoring applications. These include dimethy lpolysiloxane, 50% phenyl-50% methyl polysiloxane, 50% cycanopropylphenyl-5 0% methyl polysiloxane, trifluropropyl polysiloxane, poly(ethylene glycol), and dicyanoallyl polysiloxane (nonbonded). The dimethyl polysiloxane and t he trifluropropyl polysiloxane columns showed good efficiency and no signif icant deterioration after 5 days of continuous operation with air as carrie r gas. The 50% phenyl-50% methyl polysiloxane and the 50% cycanopropylpheny l-50% methyl polysiloxane columns showed poorer efficiency, and the poly(et hylene glycol) and dicyanoallyl polysiloxane columns showed excessive deter ioration in air.