Development and Application of a Microfabricated Conductivity Detector for use in Photothermal Absorbance Detection

The photothermal project has been an on-going project in the Jorgenson group for the last several years, first using a capillary system and now using microfluidics. The majority of analytical systems involving small volume samples generally make use of laser induced fluorescence and electrochemical detection, both of which are limited to use with a relative few species of interest without further modification. While absorption techniques would provide more universal responses, their dependence on path length precludes their use in small volume applications. The process of photothermal absorbance detection, which is an indirect absorbance detection method using conductivity detection to identify the temperature change caused by light absorbance, has been shown to be interesting alternative method for use in capillary-based systems. Due to limitations in fabricating the capillary-based contactless conductivity detector, the laser excitation region is extremely small in comparison to total detection region, therefore drastically reducing the detected signal. A contact conductivity detector using metal film electrodes incorporated into a microfluidic chip has been shown to circumvent this deficiency. A number of studies have been performed in order to optimizing photothermal response including variation of electrode and channel dimensions, laser power and modulation, and conductivity excitation parameters. All studies, to date, were carried out using dabsyl-tagged analytes excited by the 488-nm line of an argon ion laser, or with a 266-nm frequency doubled solid state laser using a number of native analytes. Successful implementation of this detector in the UV will open up high-speed microchip-based separation devices to a broad spectrum of small molecule applications.