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Ultralow Detection Limit of a Near-Infrared Dye by Diode-Laser-Induced Fluorescence in a Flowing Stream

Diode lasers are compact, highly efficient, temperature-tunable, long-lived, low-noise, simple-to-operate, and relatively inexpensive solid-state lasers that are rapidly becoming more useful in spectrochemical studies. Recently, Imasaka and Ishibashi reviewed the uses of diode lasers in trace chemical analyses. Due to the virtual elimination of background fluorescence from sample impurities, and the large wavelength shift of the Raman scatter from the solvent at red and near-infrared excitation wavelengths, 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. With some simple improvements upon that system, the detection limit has been lowered to 3000 molecules of IR-140 in a 250-pL volume.