High-Speed, Vacuum-Outlet GC Using Atmospheric-Pressure Air as Carrier Gas

A model is developed to explore operating conditions and performance tradeoffs for high-speed GC using atmospheric-pressure air as carrier gas and a vacuum pump to draw the carrier gas and injected samples through the system. The model is based on the rate theory for open tubular columns and conventional equations for gas flow in capillary tubes. The model predicts the effects of column outlet pressure, column length, column diameter, and detector dead time on the number of theoretical plates generated in a 30-s analysis spanning a retention factor range from 0 to 5. The outlet (detector) pressure range considered is 1−100 kPa (0.01−1.0 atm). A 0.1-mm-i.d. column is found to generate more plates than either larger or smaller diameter columns because of the constraint of using atmospheric pressure at the column inlet. About 25 000 plates are generated with a 2.5-m-long column for outlet pressures less than ∼20 kPa. The model is validated with a high-speed GC instrument using a cryofocusing inlet system and a photoionization detector. The number of theoretical plates measured for o -xylene agrees very well with the model predictions for the lowest pressure case. System performance degrades at higher outlet pressures and with smaller diameter columns because of increased dead time of the detector. Results are considered in the context of designing portable GC instruments for ambient VOC analysis.