Effect of vacuum on the performance of the flame ionization detector used for vacuum-outlet gas chromatography

Vacuum-outlet operation of short fused-silica open tubular columns for gas chromatography provides benefits to analysis speed by increasing the optimu m velocity while minimizing the loss in resolution. Vacuum-outlet operation of a column with a gas chromatographic detector necessitates that the dete ctor also be under vacuum. Simple modifications were made to a gas chromato graph-flame ionization detector (GC-FID) for vacuum-inlet and vacuum-outlet operation. The vacuum-inlet system was operated in the splitless mode to a llow for efficient sample loading prior to operation at reduced pressures. The goal of FID operation at 100 torr or less was achieved by using oxygen in place of air and optimizing the gas flows under vacuum to maintain a sta ble flame at pressures as low as 46 torr. An outlet pressure of 85 torr wit h optimized gas flows allowed for routine operation of the FID without solv ent flame-out. It was discovered that the sensitivity is enhanced compared to atmospheric operation over a range of outlet pressures from approximatel y 200 to 400 torr; however, operation of the FID at the lowest possible pre ssures decreases the analytical sensitivity due to both the outlet pressure and the absence of helium makeup gas, (C) 2000 John Wiley & Sons, Inc.