This International Standard specifies a method for the determination of the linear isomers of perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) in unfiltered samples of drinking water, ground water and surface water (fresh water and sea water) using high-performance liquid chromatographytandem mass spectrometry (HPLC-MS/MS). Other isomers may be reported separately as non-linear isomers and qualified as such. The analytes specified in Table 1 can be determined by this method. The method is applicable to a concentration range of 2,0 ng/l to 10 000 ng/l for PFOS and 10 ng/l to 10 000 ng/l for PFOA. Depending on the matrix, the method may also be applicable to higher concentrations ranging from 100 ng/l to 200 000 ng/l after suitable dilution of the sample or reduction in sample size. The user should be aware that particular problems could require the specification of additional conditions.