Control and Operation Schemes for Micro-Thermal Conductivity Detectors in Gas Chromatography
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1222-DD05-20
Control and Operation Schemes for Micro-Thermal Conductivity Detectors in Gas Chromatography Bradley C. Kaanta1, Hua Chen2, Xin Zhang1 Department of Mechanical Engineering, Boston University, 110 Cummington St, Boston, MA 02215, U.S.A. 2 Schlumberger-Doll Research, 1 Hampshire St. Cambridge, MA 02139, U.S.A. 1
ABSTRACT As the use of sensor networks has expanded, the demand for robust detectors able to operate in a variety of environments has grown. We present the sensitivity testing of a micro thermal conductivity detector (µTCD) operating in two different modes. The microfabricated device we have designed and tested is composed of a resistive heating element suspended in a microchannel, which creates excellent thermal isolation and a high heat transfer coefficient between the element and the fluid. The sensitivity of a µTCD integrated into a micro-gas chromatography (GC) system, can be increased by a factor of 10 simply by switching between operation in constant temperature and constant voltage modes. This result agrees with the analytical models and testing data previously reported for macro systems and devices. INTRODUCTION Gas chromatography systems use a detector to identify and quantify gases in a sample. Such systems can be utilized in the real-time testing of gases inside industrial equipment, improving efficiency and reducing pollution. A thermal conductivity detector is an excellent choice for miniaturization because as the dimensions of a GC system are decreased the amount of sample being processed is reduced exponentially. For any detector which is mass sensitive, such as a flame ionization detector or a mass spectrometer, the output signal is reduced in proportion to sample volume. In contrast, a TCD is sensitive only to the concentrations of substances within a mixture. Therefore, volume reduction has no effect on the output. [1].
Figure 1: (Left) GC system block diagram. (Right) Fabricated high sensitivity µTCD used for testing. Structure is ~1000µm in length along flow channel. TCDs measure the physical properties of a fluid as it flows over an electrically heated resistive element [2, 3]; the fluid’s properties can be determined by using one of several different operating modes [4, 5]. In one the most common operating schemes, constant voltage (CV) mode, the output of the system is determined by the temperature change of the resistive element. When a less thermally conductive fluid is exposed to the active device, less heat is transported away, increasing the filament temperature. In another common operating mode, constant temperature (CT), the filament is maintained at a fixed operating temperature, using a feedback
system. Any change in current required to maintain this fixed temperature is measured. Physical testing and theoretical models have shown that CT µTCD operation is ~10 times more sensitive, as well as having a broader linear operational region than in a CV control mode [4, 5]. However, common mode electrical noise is easier to eliminate in CV control, which lowers a de
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