Contaminant Detection, Identification, and Quantification Using a Microchip Laser Fluorescence Sensor

This paper describes a series of laboratory tests conducted to assess the performance of a novel fluorescence-based in situ sensor for environmental contaminants. The sensor, which can be deployed downhole in a monitoring well, or incorporated into the shaft of a cone penetrometer, is less than $4\mathrm{cm}$ in diameter and uses a miniature microchip laser that produces $\sim 200\mathrm{ps}$ pulses of ultraviolet radiation at a high repetition rate $(\sim 10\mathrm{kHz})$ to excite fluorescence in a wide range of compounds. Results from laser induced fluorescence tests on single compound aqueous solutions of benzene, toluene, and o-xylene (BTX) demonstrate the sensor’s ability to perform contaminant analyses on compounds with fluorescence lifetimes on the order of $1\mathrm{ns}$ . A linear relationship between contaminant concentration and fluorescence intensity was observed for concentrations over several orders of magnitude from the sensor’s detection limit ( $<1\mathrm{ppm}$ for o-xylene) to solutions of pure BTX compounds at aqueous solubility. Owing to the microchip laser’s short pulse length, fluorescence lifetimes were obtained directly from measurements without the need for spectral deconvolution. Analysis of data from these tests highlights the importance of differentiating a sensor’s ability to detect, identify, and quantify compounds of interest—performance thresholds that ultimately define potential applications for the device.