Thermal Management of Micro Gas Chromatographs

Gas chromatography is an analytical technique for separating and analyzing individual components in a gas mixture. Temperature programming is a form of gas chromatography in which the column temperature is controlled to follow a specific heating rate, dwell time and cooling rate during the course of the analysis. Temperature programming enables separation of a wider range of components, when compared to isothermal analysis, in less time. Thermal modeling was used to determine the necessary power required to achieve a given heating ramp rate for a LIGA fabricated nickel GC column. A lumped parameter analysis was carried out to understand the Joule heating in the heater layout. The heater will be fabricated using UV lithography. Polyimide is going to be used as the insulating layer between the heating coil and the GC column. Nickel, which will be used as the heating element is going to be electroplated onto the polyimide surface. INTRODUCTION Gas chromatography is a technique widely used for separation and analysis of gas samples. Applications of gas chromatographs (GCs) include chemical analysis, environmental maintenance, green house gases monitoring, and forensic analysis when used in conjunction with mass spectrometry. Typical GCs are bulky, because they consist of a carrier gas supply, a sample injection system, a separation column, temperature programmable oven, an output detector and data processing unit. Miniaturization of GCs has tremendous potential because it offers capabilities like portability, fast response times and low power consumption and low cost [1]. Temperature programming is a mode of gas chromatography in which the column temperature is raised progressively during the course of the analysis. It is used to analyze complex mixtures containing compounds with wide range of vapor pressures (boiling points). When samples contain components with a wide range of vapor pressures, it is impossible to determine a suitable temperature for an isothermal run, so overall performance is compromised since the column temperature favors one or more analytes. Temperature programming facilitates reduced analysis times and better output resolution at the detector [2]. BACKGROUND Advances in micromachining technology over the past 20 years have helped in developing micro fluidic devices in silicon, glass and plastics. Since the original idea of fabricating an integrated GC on silicon wafer was developed at Stanford [Terry, 1979], researchers at Lawrence Livermore National Laboratory [3], Sandia National Laboratories [Hudson, 1998], Georgia Institute of Technology [1] and other groups have worked on making miniaturized GC columns. At Louisiana State University, Bhushan [4] fabricated 50μm wide, 200 500μm tall gas chromatograph columns using the LIGA process. Pasupaleti [5] made Ni-Cr heaters on silicon wafers and was able to integrate it with the LSU microGC. METHODS Thermal Modeling Thermal modeling was used to determine the necessary power required to achieve a given heating ramp rate for a LIGA fabricated nickel GC column. Dynamic response of the model was used to define the characteristics of the heat source for heating the GC column. A two lump heat distribution model corresponding to the insulating layer and the nickel chip was developed and solved using an energy-based approach (Fig. 1). A lumped parameter analysis of the heater layout was also carried out to study its behavior for the given application (Fig. 3). The effect of using different materials and dimensions for the heating element, insulating layer, and heat sink was investigated. Microfabrication The heater will be fabricated using UV lithography process. Polyimide is going to be used as the insulating layer between the heating coil and the GC column. Liquid polyimide (PI5878G, HD Microsystems, Parlin, NJ) will first be spun on a nickel GC column and cured at 300°C for 2 hours in a nitrogen environment. Next a thin conductive