Laser-induce fluorescence (LIF) is a sensitive and widely used detection method for capillary electrophoresis (CE).Laser diode is an ideal light source for miniaturized LIF detector due to its small size,low power consumption and high reliability.Purple laser diodes emitting at 405 nm have been used to construct handheld CE analyzers[1,2].However,the emitting wavelength of 405 nm of this type of laser diode is relatively far from the exciting wavelength (λmax = 495 nm) of currently,widely used fluorescence labeling reagent,fluorescein isothiocyanate (FITC) in CE separation,which results in poor exciting efficiency and low detection sensitivity.Although 470-nm light emitting diodes (LEDs) can be used as an alternative light source[3],the detection sensitivity was still relatively poor due to the limited light density of LEDs.This work reports a miniaturized LIF detector based on 445-nm laser diode for CE separation.The structure of the LIF detector is as shown in Figure 1.The LIF detector adopted a confocal optical arrangement and employed a 445-nm laser diode as excitation source.The laser beam was reflected by a dichroic mirror and then focused at the channel center of a capillary via a focusing lens.The fluorescence was collected by the same focusing lens and went through directly.After passing through the dichroic mirror and a 525 nm band-pass filter,and reflected by a mirror,the fluorescence was detected by a photomultiplier tube detector.Taking advantages of the small size laser diode and compact optical structure,the size of the LIF detector was miniaturized to 20 cm × 20 cm × 12 cm.The performance of the LIF detector was evaluated using sodium fluorescein as model sample.A series of standard solutions of sodium fluorescein were introduced into a capillary (75-μm i.d.,375-μm o.d.) and their fluorescence signals were recorded (Figure 2).The limit of detection was 1.8 nM (S/N=3).The sensitivity was comparable to those in LIF detection systems using diode-pumped solid-state lasers which have bigger sizes and higher power consumptions.The present detector was applied in the separation of a mixture of 1 μM FITC-labeled amino acids.High separation efficiencies ranging from 0.90 to 1.32 μm plate heights were obtained with a separation length of 20 mm.A typical result is shown in Figure 3.