A portable, high-speed, vacuum-outlet GC vapor analyzer employing air as carrier gas and surface acoustic wave detection

Vacuum-outlet GC with atmospheric-pressure air as the carrier gas is implem ented at outlet pressures up to 0.8 atm using a low-dead-volume polymer-coa ted surface acoustic wave (SAW) detector. Increases in the system outlet pr essure from 0.1 to 0.8 atm lead to proportional increases in detector sensi tivity and significant increases in column efficiency. The latter effect ar ises from the fact that optimal carrier gas velocities are lower in air tha n in more conventional carrier gases such as helium or hydrogen due to the smaller binary diffusion coefficients of vapors in air. A 12-m-long, 0.25-m m-i.d. tandem column ensemble consisting of 4.5-m dimethyl polysiloxane and 7.5-m trifluoropropylmethyl polysiloxane operated at an outlet pressure of 0.5 atm provides up to 4 x 10(4) theoretical plates and a peak capacity of 65 (resolution, 1.5) for a 3-min isothermal analysis. At 30 degreesC, mixt ures of vapors ranging in vapor pressure from 8.6 to 76 Torr are separated in this time frame. The SAW detector cell has an internal volume of <2 muL, which allows the use of higher column outlet pressures with minimal dead t ime. The sensor response is linear with solute mass over at least 2-3 decad es and provides detection limits of 20-50 ng for the vapors tested. The com bination of atmospheric-pressure air as carrier gas, modest operating press ures, and SAW sensor detection is well-suited for field instrumentation sin ce it eliminates the need for support gases, permits smaller, low-power pum ps to be used, and provides sensitivity to a wide range of vapor analytes.