An Easy to Manufacture Micro Gas Preconcentrator for Chemical Sensing Applications

Abstract

We have developed a simple-to-manufacture microfabricated gas preconcentrator for MEMS-based chemical sensing applications. Cavities and microfluidic channels were created using a wet etch process with hydrofluoric acid, portions of which can be performed outside of a cleanroom, instead of the more common deep reactive ion etch process. The integrated heater and resistance temperature detectors (RTDs) were created with a photolithography-free technique enabled by laser etching. With only 28 V DC (0.1 A), a maximum heating rate of 17.6 °C/s was observed. Adsorption and desorption flow parameters were optimized to be 90 SCCM and 25 SCCM, respectively, for a multicomponent gas mixture. Under testing conditions using Tenax TA sorbent, the device was capable of measuring analytes down to 22 ppb with only a 2 min sample loading time using a gas chromatograph with a flame ionization detector. Two separate devices were compared by measuring the same chemical mixture; both devices yielded similar peak areas and widths (fwhm: 0.032-0.033 min), suggesting reproducibility between devices.

Keywords: chemical sensor; detectors; gas preconcentrator; microelectromechanical systems (MEMS); sorbent.

Fig 1.

The micro gas preconcentrator μPC). A) Images of both faces of the μPC B) Heater and RTD details C) Three-dimensional view of layers108×139mm (300 × 300 DPI)

Fig 2.

μPC manufacturing process. The depth of each layer is provided in the legend 99×55mm (300 × 300 DPI)

Fig 3.

Schematic of sample loading (adsorption) and GC-FID chemical analysis (desorption). A) The sample (VOC mix) was diluted with helium (He) using two mass flow controllers (MFC) for a total flow of 90 SCCM through the μPC B) Analytes are carried to the GC-FID using a flow of helium at 25 SCCM 60×43mm (300 × 300 DPI)

Fig 4.

Model of heat transfer. The power required to achieve a temperature of 260 °C at the sorbent cavity was calculated to be 2.5 W 70×59mm (300 × 300 DPI)

Fig 5.

Comparison of RTD 1 resistance measurements from the μPC and temperature measurements from an externally applied thermocouple. Each cycle consisted of 28 V applied across the heater for 15 min (heating) followed by no voltage for 15 min (cooling). A) Close up of one cycle B) Twelve consecutive cycles. A maximum heating rate of 17.6 °C/s was observed 106×132mm (300 × 300 DPI)

Fig 6.

Effect of sample adsorption flow through the μPC on detector response (GC-FID data). Each point is the average of n=2 samples. Ultimately, an adsorption flow of 90 SCCM was chosen for this sample matrix (acceptable peak areas with minimal sample volume) 45×24mm (300 × 300 DPI)

Fig 7.

Calibration curve of 4-ethyltoluene, benzyl chloride and 2-hexanone measured with the μPC-GC-FID. Each point is the average of n=5 measurements; one standard deviation is shown 50×30mm (300 × 300 DPI)

Fig 8.

GC-FID response of two μPCs (n = 3 per μPC). A slight retention time shift existed between the two devices (approx. 0.028 min) but both yielded similar data quality, measured by peak area and peak width, of the three compounds 48×27mm (300 × 300 DPI)