In this work,we dedicated to achieve high sensitive laser induced fluorescence (LIF) detection for round fused-silica capillaries with a confocal optical arrangement,because round capillaries are widely used in microscale separation systems and a confocal LIF system has a relatively compact optical structure.The main challenge in the building of a confocal LIF system for round capillaries is the significant reflected and scattered laser light from the inner and outer walls of the capillary due to the large refractive index difference between air and capillary.This problem can be solved by using a high numerical aperture objective,and immersing the objective and capillary in an index-matched oil [1].The detection region of the capillary was also required to be pulled to 0.5–8 μm to match the diffraction-limited focus of laser beam.However,the fabrication and operation of the ultra-thin pulled capillary require expensive equipment and special attention,respectively,which may limit its broad application in routine analysis.We proposed a simple and efficient method to significantly reduce the reflected and scattered laser light intensities from the capillary surface to improve the sensitivity of confocal LIF detection for round capillaries (Figure 1).This method was developed on the basis of a novel phenomenon,namely the fluorescence beam and the refracted laser beam are spontaneously separated at a pinhole by laterally shifting the laser focus point from the center of the capillary channel to the sides.Using this phenomenon,the reflected laser light can be easily eliminated before it reaches the detector by using a pinhole to allow fluorescence beam pass through and prevent the laser light,which is particularly favorable for achieving high sensitive LIF detection.Comprehensive study on the phenomenon and optimization of the shift distance were carried out using both experimental and simulation methods.A best shift distance of ±20 μm was obtained (Figure 2),with which background intensity could be significantly reduced by 98.9%,while fluorescence intensity was only reduced by 25.7%,resulting in an improvement of signal-to-noise ratio of 8.3 times,compared with that at a shift distance of 0 μm usually used in most of confocal LIF systems for round capillaries.A limit of detection of 66 fM was obtained for sodium fluorescein (Figure 3).To demonstrate its potential as an on-column sensitive detector for microscale separation systems,the present system was coupled with a capillary electrophoresis system for separation of four fluorescein isothiocyanate labeled amino acids with concentrations of 100 pM.