A new stable isotope labeling (SIL) reagent pair, 10-methyl-acridone-2-sulfonohydrazide (MASH) and its deuterated counterpart d3-MASH was synthesized and successfully applied to the analysis of perfluorinated carboxylic acids (PFCAs) in serum samples. The limits of detection (LODs) were in the range of 0.07-0.42μg/L, and the limits of quantitation (LOQs) were in the range of 0.25-1.38μg/L. Besides ionization enhancing effect, MASH also showed excellent fluorescence property. Therefore, the mass spectrometer operation cost was greatly lowered by carrying out parameter optimization experiments on HPLC which is easier to operate and maintain. The SIL strategy was confirmed to be effective in reducing matrix effect. The developed multiple-reaction monitoring (MRM) condition of PFCAs was also suitable for other carboxylic acid due to the introduction of MASH which is more prone to fragmentation than the analytes. With the MRM conditions obtained from PFCAs, fatty acids were also found in serum samples. This feature made the proposed method show powerful potential in the identification of acidic compounds in complex samples in the absence of corresponding standard.