Miniaturized Analytical Sensor System Gas Chromatograph and Its Application in Environmental Monitoring

This research focuses on designing, fabricating, and integrating components for a miniature gas chromatograph sensor. A gas chromatograph (GC) is an analytical tool widely used to separate and analyze mixtures of gases. The primary working principle behind a GC is that the different mobilities (rate of diffusion) of different compounds in a separating medium, called the stationary phase, cause them to separate from one another and elute from the column at different times. The instrument consists of four main parts: (1) injector – where is sample is collected, concentrated, and injected into the column; (2) column – where the components are separated from one another; (3) detector – where the separated components are detected; and (4) electronics – for pneumatic and thermal control and the detector electronics. Since the column is the primary component of the sensor, a major emphasis of this work was on designing and fabricating the column. The motivation for this research is to build field deployable sensors which can analyze chemicals in under 10 seconds and consume very little power. Such a sensor will be useful in industrial process monitoring, environmental analysis/monitoring, detection of chemical warfare agents (on the field), and human health monitoring via odor analysis. The use of a GC for these applications currently is limited by the size and ease of use of the available instruments. The industry leader in the GC business is Agilent Technologies. The portable GC’s from Agilent and Varian Inc., while being small in size, are seriously limited since they can analyze only volatile organic compounds. The microFast GC, developed at LSU is a shoebox size instrument built from COTS components, which matches the analytical power of the benchtop GC’s (analyzing both volatiles and semi-volatiles), and performs the analysis in 30 s – 5 min. The current research is aimed towards reducing the instrument size in half and reducing the analysis time by an order of magnitude. The performance of the injector is measured by its preconcentration factor (higher is better) and the injection plug width (narrower is better as it increases peak capacity and resolution). A column’s performance is measured by the peak capacity (roughly the number of compounds that can be separated in one analysis), the resolution, and the analysis time (short is better). The lowest concentration of analytes and the speed at which eluting peaks can be identified are important metrics for the detector. Apart from these factors, the efficiency of the overall system depends on other factors, most of which can be bunched into extracolumn effects. These include the band broadening due to fluidic connections between the injector – column – detector, internal volume of the injector and the detector, and presence of cold/hot spots. While these parameters are mundane in the benchtop systems, they can reduce the peak capacity for a micro instrument dealing with small volumes! The column resolution can be improved by reducing the column diameter. But doing so requires a higher column head pressure and reduces the column sample capacity. By using a rectangular cross-section column, the width and height can be independently tailored to provide the desired resolution (governed by the column width) and the sample carrying capacity (governed by the column height) [1]. Such rectangular cross-section columns are realizable by the MEMS technology and are incorporated in at least one commercially available sensor [2]. Since the 1970’s when Terry originated the idea of fabricating an integrated GC consisting of an injection valve, a long column, and a detector [3], different groups have realized miniature GC columns in silicon and polymers. These columns, typically 1-3 m long, with an aspect ratio ranging from 1-4, have some limitations [4]. Simulations have shown that very parallel, vertical walls are important for obtaining a good resolution. Moreover, nickel columns have better thermal characteristics and are better suited for temperature programming.