[Ultrasensitive analysis of R6G and R110 in solution]

This paper describes the ultrasensitive detection of R6G and R110 in a flowing aqueous sample, based on laser-induced fluorescence (LIF) combined with mode-locked pulsed excitation and time-correlated single-photon counting. A high repetition rate mode-locked argon ion laser operated at 514.5 nm was used as the excitation source. The laser beam was found tightly into a thin wall capillary flow cell. The probe volume was about 12.6 x 10(-12)L. Fluorescence was collected at 90 degrees using a x 40 microscope objective, and was detected by a fast response PMT detector. A time-gate was used to discriminate Raman scattering light background. Using the criterion of 3 standard deviations above background, the detection limits obtained for R6G and R110 were 3.8 x 10(-14) mol/L and 1 x 10(-13) mol/L respectively, and the average probability was lower than 1 in the presence of a R6G or a R110 molecule in the detection volume.