Methods of determining the presence or absence of fluorocarbon(s) on a substrate using X-ray Photoelectron Spectroscopy (XPS). A method may be used to determine the presence or absence of per- or polyfluoroalkyl substances PFASs. A method may use a porous polymer substrate. A method may use solid-phase extraction (SPE). A method may be used to determine the presence or absence of fluorocarbons in an aqueous sample. An aqueous sample may be a groundwater sample, wastewater sample, potable water sample, drinking water sample, or surface water sample. The limit of detection of fluorine in a method may be 0.05% F or less (for XPS analysis) and/or 20 ng or less on a substrate.
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