Abstract: Objective To establish a method for simultaneous determination of Hg and Be,Al,V,Cr,Mn,Co,Ni,Cu,As,Se,Mo,Ag,Ba,Sb,Cd,Tl,Pb,Th,U in ambient PM_2.5 by ICP/MS with L-cysteine direct addition after ultrasonic mixed acid extraction. Methods The sampling quartz membrane filters of PM_2.5 were extracted at 70 ℃ by ultrasonic cleaner for 3 hours with a mixed acid consisted of 2% hydrochloric acid and 5% nitric acid with 0.025% L-Cysteine. Mercury and other 19 elements were prepared into a mixed calibration series with a mixed acid consisted of 0.8% hydrochloric acid and 2% nitric acid with 0.01% L-Cysteine. The mercury and other 19 elements of the samples were determined in helium collision mode by ICP/MS simultaneously. Results The most suitable concentrations of L-cysteine and hydrochloric acid in the sample extract were 0.025% and 2% respectively. When the sampling volume was 1 056 m³,the method detection limits of 20 elements were within 0.004-2.152 ng/m³ and the correlation coefficients were all greater than 0.999 4. The recoveries of spiked quartz membrane filters at different concentration levels were between 89.6% and 105.3%,with the relative standard deviation（RSD） all less than 3.2%. Conclusions Adding L-cysteine directly to standards and samples eliminates the memory effect of mercury effectively. Adding hydrochloric acid to the extract improves the stability of antimony and silver. The application of Kinetic energy discrimination（KED） mode eliminates the interference of polyatomic ions introduced by hydrochloric acid,and the mixing of mercury and other elements to formulate the calibration series improves the working efficiency. This method is suitable for simultaneous determination of mercury and multiple elements in PM_2.5 samples.

Key words: L-Cysteine, Ultrasonic extraction, Inductively coupled plasma mass spectrometry, Fine particulate matter