Comparison of Tenax and OV-1 Thin-Films for Microfabricated Preconcentrators

Gas chromatography is a common technique for the separation and analysis of mixtures of volatile organic compounds (VOCs). In many applications, such as breath analysis and room air sampling, the concentration of analytes is so low that preconcentration is necessary to selectively trap VOCs over a longer period of time, before analysis. The goal of this project is to develop an energy efficient, compact vapor preconcentrator for air sampling. We present thin-film preconcentrators (PCF) with integrated heaters and temperature sensors that were coated with polydimethylsiloxane (OV-1), poly(2,6-diphenylphenylene oxide) (Tenax-TA) and a mixture of OV-1 and Tenax-TA. The polymer thin-films were fabricated using a dynamic coating method with 20mg/ml OV-1 in a 1:1 mixture of DCM and pentane, 10mg/ml of OV-1 and 5mg/ml of Tenax-TA in chloroform, or 20 mg/ml of Tenax-TA in chloroform. The OV-1 and OV-1/ Tenax-TA PCFs were initially tested by connecting them to the inlet and FID of a gas chromatograph with 1 m of 50um guard column. A volume of 0.2 µl of an alkane was injected via the heated inlet 175 oC at 1 psi, after any breakthrough had passed, the GC was pressurized to 8 psi and the PCFs were heated using an integrated platinum film heater. The GC was kept at a constant temperature of 40°C during these tests while the PCF was independently heated outside of the GC oven. The adsorption ratio of these experiments is summarized in table 1. Alkanes above decane such as dodecane proved difficult to desorb, even with the integrated heater reaching temperatures of approximately 260°C. The air sampling experiments for each PCF were performed for decane and benzene using a dynamic headspace sampling method with a constant flow of nitrogen induced by a mass flow controller at a rate of 0.3 ml/min. at a pressure of 5psi. A six-way valve connected the PCF to the GC for desorption at a pressure of 15psi, after a set period of time. Analysis with Agilent 5890GC using a 20 m Rxi-1ms column, temperature ramp from 40°C to 180°C at a rate of 20°C/min, and FID detector. The results for the headspace sampling are summarized in figure 1. For benzene the thin-films with Tenax-TA saturate much more quickly than the OV-1 film and the pure Tenax-TA film which has the lowest capacity. For decane, the thin films with Tenax-TA still saturate faster but the largest capacity was observed for the mix of OV-1 and Tenax-TA over this limited range of sampling times. The authors would like to thank the Georgia Tech Research Institute for funding this research out of Independent Research and Development (IRAD) funds. Figure 1