High speed, vacuum-outlet GC using atmospheric-pressure air as carrier gas

A model is developed to explore operating conditions and performance tradeo ffs for high-speed GC using atmospheric-pressure air as carrier gas and a v acuum 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 conven tional equations for gas flow in capillary tubes. The model predicts the ef fects of column outlet pressure, column length, column diameter, and detect or dead time on the number of theoretical plates generated in a 30-s analys is spanning a retention factor range from 0 to 5. The outlet (detector) pre ssure 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 colum ns because of the constraint of using atmospheric pressure at the column in let. About 25 000 plates are generated with a 2.5-m-long column for outlet pressures less than similar to 20 kPa, The model is validated with a high-s peed GC instrument using a cryofocusing inlet system and a photoionization detector. The number of theoretical plates measured for o-xylene agrees ver y well with the model predictions for the lowest pressure case, System perf ormance degrades at higher outlet pressures and with smaller diameter colum ns because of increased dead time of the detector. Results are considered i n the context of designing portable GC instruments for ambient VOC analysis .