The invention concerns a method for capturing a chemical compound, typically an impurity, and for accumulating said chemical compound on a capture mass CAPT included in at least one removable adsorbant cartridge MC over a long period exceeding 2 days, with a view to the external chemical analysis thereof.The invention also concerns an apparatus ST suitable for capturing at least one chemical compound in a fluid flow for its detection and/or quantification comprising, downstream of said cartridge or cartridges, at least one means for measuring the cumulative flow which has traversed said cartridge or cartridges over a set time period. In a preferred variation of the invention, the capture mass CAPT is fragmented into a plurality of elementary masses CAPTi which allowing the spread of an irreversible adsorption front or the displacement of a chromatographic zone to be determined.