Abstract: A method is described to improve the sensitivity of biological samples in LC-MS which contains trifluoroacetic acid (TFA) in the mobile phase by adding a suppressor (CSRS Ultra Ⅱ) after the column. The method is developed on dual-gradient liquid chromatographic (DGLC) systems. And one peptide sample hydrolyzed by cytochrome C was analyzed on the left pump by the reversed phase model where the mobile phase contained 0.1 % TFA and the flow rate was 0.25 mL/min. 0.05 mol/L ammonium bicarbonate solution was delivered by the right pump with a flow rate of 1.00 mL/min. The two mobile phases converged in the post-column suppressor after which the concentration of TFA was reduced. As a result, the signal of peptide analyzed by MS improved about 1-fold to 16-fold. Using this method, the qualitative analysis of impurities in etimicin sulfate drug was perfectly achieved without any additional operations between LC and MS.