When a small volume (<50 μL) of liquid is sampling, it is easy to produce large sampling error. In order to reduce the uncertainty of analytical results, the quantification based on the sample volume is usually replaced by the quantification based on the sample mass. The conventional standard addition method (SAM), in which the quantification is based on the sample volume, cannot be applied when the quantification is based on the sample mass. Therefore, a modified SAM with mass-based quantification is proposed. As a demonstration, the modified SAM has been applied to determine low or trace elements Hg, Mo and Rh in complex solution with ICP-OES. The aim of the SAM is to correct the matrix effect, while the matrix effect includes two kinds of interferences: constant interference and proportional interference. Only the proportional interference can be corrected with the modified SAM, which can characterize the extent of the proportional interference with a quantitative index k: k=1 means there is no proportional interference; while the larger difference between k and 1 means greater proportional interference existing in the sample. As for constant interference, it can be reduced or eliminated by applying the background correction function of the ICP-OES instrument. The uncertainty of analytical results mainly comes from background correction, and depends on the signal-to-background ratio of the analytical line selected. The lower the signal-to-background ratio is, the greater the uncertainty of background correction is. Therefore, in the actual analysis, we should try to choose the analytical line which has as high signal-to-background ratio as possible. Otherwise, the final analytical result maybe contains very large uncertainty, even though the background correction has been made in advance.