Low Power Microfabricated Preconcentrator with Integrated Heater

Abstract

Introduction Micro GC systems based upon MEMS fabrication technology have been developed for portable analysis systems to enable detection and quantification of volatile organic compounds over the past twenty years (1). In particular, work at Sandia National Laboratories, has pioneered development of these miniature systems (2). Novel designs for pre-concentrators have also been developed to assist with sample injection into these low flow rate systems (3). We have studied micro-GC columns made by silicon to silicon direct bonding (4), and integrated a micro-GC column 6m length, with an ion trap mass spectrometer for this purpose (5). To inject the sample into the column, our initial method was to use a syringe injection into a heated septum with Agilent 6890. However, for automated operation and method of sampling, pre-concentration of the volatile organic compounds is required, followed by rapid heating for sample injection for analysis. A microfabricated pre-concentrator can fulfill this function by providing a high surface area and a compact platform with reduced thermal mass, compared to commercial desorption tubes. Method We designed a pre-concentrator (MPC) with an integrated platinum heater. The MPC is fabricated using standard photolithography, deep reactive ion etching and wafer bonding. The layout of the geometry in the preconcentrator is shown in Figure 1. Pillars 50 um in diameter are located in the channel to provide a high surface area. The silicon surface is liquid coated with Tenax TA (Tenax 60-80 mesh from Supelco) and PDMS (OV-1 from Ohio Valley Specialty) in an organic solvent, dichloromethane, typically a dozen times to build up a layer of approximate thickness less than a micrometer. Results Sample collection from air flow was modelled with COMSOL and indicates the pressure drop for an air flow of 600sccm a pressure drop of less than 3 psi at room temperature. Figure 3 shows results of the model with a channel depth of 350 um. To evaluate the performance, the pre-concentrator it is mounted inside a commercial Agilent 6890 GC system. A 2m length of Guard Column, 50 um ID was selected to minimize the dead-volume in the fluidic connections between the sample injection port, MPC and the flame ionization detector. Liquid injection of decane at volume of 0.02uL, using a syringe into Agilent 6890 heated injector at 275°C, with a 100:1 split, at a helium flow rate of 0.005 ml/min. Typical results, shown in Figure 4A for heating profile, indicate complete absorption, and no break-through, followed by rapid desorption with heating Figure 4B. The flow rate could be increased at the outlet during the desorption cycle using electrical current of 130 mA to the heater approx.180 ohms, provided rapid heating to 180°C, based upon resistance temperature calibration of the heater and temperature sensor, located on the lid of the MPC. Conclusions This miniature pre-concentrator provided the opportunity to build a small thermal mass designs using silicon microfabrication. With an integrated platinum heater, rapid heating at low power consumption of 5W was achieved, capable of desorption of the captured analyte to detection with FID in GC system.