Temperature distribution on thermal conductivity detectors for flow rate insensitivity

A thermal conductivity detector (TCD) design that is minimally affected by changing flow rates in the microchannel is presented. In gas chromatography systems, pressure fluctuations can result in false peaks and an unstable baseline, reducing the limit of detection. Theories of TCD operation have suggested that the center region of a detector is minimally affected by changing carrier gas flow rates. However, these theories assume that the detector elements are maintained at a constant temperature along their entire length, which is incorrect. We have performed testing and modeling to determine the accuracy of this assumption and its effects on overall TCD performance. We developed a model of multiple resistively heated TCD elements, which calculated that flow invariance can still be achieved even without perfectly constant temperature elements. A fabricated multiple element TCD was tested to show how the development of a flow rate invariant detector can be used to reduce the complexity and increase the portability of gas chromatography systems.