Ion-paring extraction method and solid-phase extraction method using four commercially available cartridges; namely, Sep-Pak^®tC18, Oasis^®HLB, Oasis^®MCX, Oasis^®WAX were tested for their capacity for the trace analysis of poly- and perfluoroalkyl substances (PFASs) including PFOS in open-ocean seawater. Six perfluoroalkyl sulfonic acids, twelve perfluoroalkyl carboxylic acid, three perfluoroalkyl sulfonamides, one fluorotelomer unsaturated carboxylic acid and one fluorotelomer carboxylic acid were investigated for each experiment. The former three cartridges and ion-paring extraction method failed to obtain acceptable recoveries of PFASs using open-ocean seawater. Although Oasis^®WAX showed an acceptable recovery for most PFASs tested, several limitations to the use for open-ocean water remained. Hence, a newly manufactured type of weak anion-exchange cartridge, "WAXsea", with 30 μm of pore size and 500 mg of adsorbent was optimized for this purpose. "WAXsea" was validated using 1 L of open-ocean seawater collected from the Pacific Ocean and successfully detected PFOS and PFOA at 0.05 pg L⁻¹ and 0.1 pg L⁻¹, respectively.