According to the PAR of 71 days, 31 days and freshly prepared IS stability samples, it can be seen that is has a significant reduction. It may fail to continue to repeat the stability test of 71 days. The solution stability is a comparative experiment. The dilution steps and acquisition method for all samples are completely the same. If there is the inference that the decomposition of the IS could result in mass spec fragments that are the same as the ions selected for the analyte, they also exist at the same time There is no effect on the stability result. In addition, the stability data of IS working solution for 31 days is enough for the sample analysis. Before each sample analysis, we will confirm whether the IS working solution is expired.