Detection of petroleum hydrocarbons at low ppb levels using quartz resonator sensors and instrumentation of a smart environmental monitoring system

Petroleum hydrocarbon vapors at low ppb levels can be detected using a thic kness shear mode resonator (TSMR) coated with a chemical-sensing overlayer, prepared by radiofrequency sputtering of porous sintered-polyethylene (PS- PE). The sensing capabilities of PS-PE sensors were profoundly affected by the sputtering methods; they were enhanced by the photo-excitation effect, and were reduced by carbonization and water treatment. The photo-assisted P S-PE sensor was extremely sensitive and could detect linear hydrocarbon (> C-12) vapors below the ppb level. The time constant of the sorption curve, however, was large, indicating a slow sensing speed. Toward creating instru mentation for a smart environmental monitoring system, the TSMR sensors wer e arrayed on a circuit board equipped with a serial interface and signal pr ocessing chips of the oscillation drive and frequency counter. Co-sorption with water vapor at a relative humidity of about 10% has almost no effect o n the sensing ability of PS-PE sensors for 1,2,4-trimethylbenzene. Converse ly, it enhances the sensitivity of the TSMR sensor coated with a D-phenylal anine film. Upward shifts in the baseline are evident with elapsed time. Ho wever, a rigorous ten-cycle iteration test for 100 ppm toluene vapor demons trated good reproducibility of the sensor's signals.