s9_Multiple-Stage Microfabricated Preconcentrator-Focuser for Gas Chromatography System

Multiple-Stage Microfabricated Preconcentrator-Focuser for Gas Chromatography System

S. W. Pang, W. C. Tian, C. J. Lu, H. K. L. Chan, and E. T. Zellers

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

The design, fabrication, and testing of a preconcentrator–focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph (µGC). The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 µm(h) x 50 µm(w) x 3000 µm(l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, ~200-µm in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly.

A multiple-stage Si microfabricated preconcentrator-focuser (µPCF) for µGC system that can provide real-time quantification and identification of complex organic vapor mixtures is developed. The µPCF consists of a Si microheater loaded with Carbopack B, Carbopack X, and Carboxen 1000 carbon adsorbent granules, and a Si micromachined cover plate. Deep reactive ion etching is utilized to produce mechanically robust fluidic interconnection adapters hermetically sealed to fused silica capillary tubing for connection to the other components in the µGC. This three-stage device is designed to capture compounds spanning up to 4 orders of magnitude in volatility. The dead volume, thermal mass, heating efficiency, and pressure drop of the three-stage µPCF are improved significantly over its single-stage µPCF predecessor. We demonstrate the successful capture, desorption, and high-resolution chromatographic separation of a mixture of 30 common organic vapors using our three-stage µPCF in a conventional gas chromatography (GC) system. The peak width at half height is <2.05 s for all compounds after elution from the GC column.

Figure 1 shows the PCF packed with the Carbopack X before sealing. The adsorbent material, Carbopack X, is a graphitized carbon having a specific surface area of 250 m²/g.

Figure 1. Microheater with Carbopack X adsorbent granules loaded between the heating elements.

The relationship between the stop-flow time, peak width at half height (PWHH), and preconcentration factor is plotted in Figure 2(a) where the PWHH of benzene is decreased from 3.7 to 0.8 s as the stop-flow time is increased from 0 to 25 s. Figure 2(b) shows the dependence on the desorption flow rate of the PWHH as well as the signal-to-noise (S/N) ratio.

Figure 2. (a) Dependence of PWHH and preconcentration factor on stop-flow period showing preconcentration factor of 5600 and PWHH of 0.8 s for a 25-s stop-flow period; and (b) dependence of PWHH and signal-to-noise ratio on desorption flow rate (following a 15-s stop-flow).

The top view of the microheater is shown in the upper center of Figure 3. The microheater is divided into three stages, one for each type of adsorbent. The size of each stage is dictated by the adsorbent mass of the particular adsorbent necessary for the given vapor pressure range of compounds.

Figure 3. Three-stage microfabricated preconcentrator-focuser using thick microheater (upper center) packed with three carbon adsorbents to cover a wide range of compound volatilities.

Better performance was obtained using the three-stage µPCF than with its single-stage µPCF predecessor as shown in Table I.

Table 1. Comparisons of single-stage and three-stage microfabricated preconcentrator-focuser.

The extension of volatility range is demonstrated by the composite of four chromatograms showing the desorption of 30 organic compounds by the three-stage µPCF as shown in Figure 4.

Figure 4. A chromatogram showing successful separation of 30 organic vapors using three-stage microfabricated preconcentrator-focuser with a conventional gas chromatography system.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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