A wavelet transform assisted laser-induced fluorescence detector with high sensitivity was developed and evaluated. An unconventional microscope objective with a numerical aperture (NA) of 0.5 and a working distance (WD) of 4.0 mm was employed, which improved fluorescence collection efficiency, increasing SNR to about 5 times. An aspherical lens was used in the collection optical path to reduce the diameter of the fluorescence imaging size, increasing the SNR to about 1.3 times. The "inverted suction injection" method of flow injection analysis (FIA) was used for the first time in LIF evaluation to reduce contamination errors and improve the accuracy of ultra-trace analysis. The wavelet transform method was proposed for data processing of LIF signals, which can reduce noise from 0.009 mV to 0.002 mV without changing the peak height and peak width. The limit of detection (LOD, 3σ method) for sodium fluorescein was 9.7 × 10-14 M or 3.9 fluorescein molecules in 68 pL detection volume, which was the lowest level of LIFs evaluated by FIA mode. The LIF combined with capillary electrophoresis (CE) achieved an LOD of 4.9 × 10-15 M for fluorescein sodium, which is the best level that has been reported.
Keywords: Good reproducibility; High sensitivity; Inverted suction injection; Laser-induced fluorescence; Wavelet transform.
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