Design and Testing of a Micro Thermal Conductivity Detector (TCD) System

Abstract

This work describes the design, simulation, fabrication and characterization of a microfabricated thermal conductivity detector to be used as an extension of the µChemLab™. The device geometry was optimized by simulating the heat transfer in the device, utilizing a boundary element algorithm. In particular it is shown that within microfabrication constraints, a micro-TCD optimized for sensitivity can be readily calculated. Two flow patterns were proposed and were subsequently fabricated into ninepromising geometries. The microfabricated detector consists of a slender metal film, supported by a suspended thin dielectric film over a pyramidal or trapezoidal silicon channel. It was demonstrated that the perpendicular flow, where the gas directly impinges on the membrane, creates a device that is 3 times more sensitive than the parallel flow, where the gas passes over the membrane. This resulted in validation of the functionality of a microfabricated TCD as a trace-level detector, utilizing low power. The detector shows a consistent linear response to concentration and we are easily able to detect 100-ppm levels of CO in He. Comparison of noise levels for this analysis indicates that sub part per million (ppm) levels are achievable with the selection of the right set of conditions for the detector to operate under. This detector was originally proposed as part of a high-speed detection system for the petrochemical gas industry. This system was to be utilized as a process monitor to detect reactor "upset" conditions before a run away condition could occur (faster than current full-scale monitoring systems were able to achieve). Further outlining of requirements indicted that the detection levels likely achievable with a TCD detector would not be sufficient to meet the process condition needs. Therefore the designed and fabricated detector was integrated into a detection system to showcase some technologies that could further the development of components for the current gas phase µChemLab as well as future modifications for process monitoring work such as: pressurized connections, gas sampling procedures, packed columns. Component integration of a microfabricated planar pre-concentrator, gas-chromatograph column and TCD in the separation/detection of hydrocarbons, such as benzene, toluene and xylene (BTX) was also demonstrated with this system.

Characterization of a micro-helium discharge detector for gas chromatography

“…Miniaturization offers unique advantages such as light-weight, low power consumption, less reagent usage and innovative architectures apart from lower cost when batch fabricated [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Stereotypical miniaturization utilizes components fabricated in silicon/glass.…”