Variation in concentration of perfluorooctanoic acid in methanol solutions during storage

•Perfluorooctanoate in methanol solutions decreased with length of storage.•Methyl perfluorooctanoate and methyl formate increased with length of storage.•Methyl perfluorooctanoate increased with increase in methyl formate.•Methyl perfluorooctanoate increased when perfluorooctanoate decreased.•Variation in the solutions stored around 5°C was smaller than that at room temp. Perfluoroalkyl carboxylic acids (PFCAs) including perfluorooctanoic acid (PFOA) have been widely recognized as persistent environmental contaminants. For accurate quantification of PFCAs, their stability in calibration solutions is important because they are criteria of quantification. To examine stability of PFCAs in methanol, we monitored PFOA and its related compounds around 4years. Interestingly, perfluorooctanoate was varied randomly, and methyl perfluorooctanoate (MePFOA) and methyl formate were observed when perfluorooctanoate decreased. Moreover, no detection of both methyl esters was in methanol solutions immediately after preparation. In each of prepared methanol solution of perfluorohexanoic, perfluoroheptanoic, and perfluorononanoic acids, their corresponding methyl esters and methyl formate were observed. Furthermore, MePFOA was observed even in the solutions stored around 4months and thereafter MePFOA increased with increase in methyl formate. Therefore, PFCAs including PFOA should be used immediately after preparation when methanol is used as a solvent.