The composition of impurities in perfluorooctanoic acid (PFOA) was measured and determined to be notably different between lots. Since the purity of PFOA varied, the accuracy of prepared calibration solution concentrations was reduced. In this study, factors influencing the purity were investigated using a Karl Fischer titrator and a LC/MS to support the development of reference materials. The results showed variances of water and isomer/homologue contents under different ambient conditions during sample handling. Moreover, these variances, especially for water, greatly affected the purity of PFOA (0.95–0.99 kg kg−1). Therefore, PFOA of reliable purity should be used for the preparation of calibration solutions.