ULTRALOW DETECTION LIMIT OF A NEAR-INFRARED DYE BY DIODE-LASER-INDUCED FLUORESCENCE IN A FLOWING STREAM

The rather powerful diode lasers are very useful in laser-induced fluorescence (LIF) detection of very small amounts of red and near-infrared fluorophors. To our knowledge, the lowest detection limit obtained of a fluorescent species with the use of diode laser excitation is 46,000 molecules in a 56-nL volume of the laser dye IR-140 flowing in a liquid jet emanating from a capillary. The detection limit of 3000 molecules obtained in this experiment is the lowest LOD reached by diode laser spectrochemical methods yet reported, to our knowledge. Despite the unfavorable initial appearance of the diode laser experiments, note that: (1) the diode laser irradiance is inferior to the Ar+ laser by over 3 orders of magnitude; (2) the fluorescence collection efficiency of this work is less by a factor of 10; (3) the residence time is 4 times less; and (4) fluorescence in the near-infrared entails lower PMT photocathode and fluorescent dye quantum efficiencies, and still the LODs of the rudimentary, economical diode laser investigations are only 3 orders of magnitude greater than the more demanding and costly Ar+ laser investigations using a sheath flow cuvette.