Evaluating the\nExtraction and Quantification of Marine\nSurfactants from Seawater through Solid Phase Extraction and Subsequent\nColorimetric Analyses

Surfactants are amphiphilic molecules that adsorb to\ninterfaces\nand affect the interfacial tension. Surfactants in seawater can impact\ngas-exchange, surface properties, and the composition and fate of\nsea spray aerosol. The accurate quantification of surfactants and\ntheir classes is crucial to constraining the effect of surfactants\nin seawater and their role in air–sea exchanges. Here, we evaluate\nand optimize a solid phase extraction (SPE) method paired with colorimetry\nand UV–vis spectroscopy to quantify the concentrations of anionic,\ncationic, and nonionic surfactants in seawater. We compare tandem\nSPE with two-step SPE and different elution volumes and evaluate the\nimpact of different interferents. Improved extraction efficiencies\nwere obtained with an 8 mL acetonitrile elution and with separate\nENVI-18 and ENVI-Carb extractions, instead of tandem. With complex\nsurfactant mixtures, the presence of anionic surfactants interfered\nwith the quantification of cationic surfactants and caused underestimations\nof up to 83%. Using a two-step extraction and analyzing each seawater\nSPE extract separately during colorimetric quantification help avoid\nthe effects of interferents and ensure more representative quantification\nof surfactants. With this method, average seawater surfactant concentrations\nranged from 0.04 to 0.06 μM. At the highest concentrations,\nthe class composition comprised 23% anionic, 21% cationic, and 56%\nnonionic surfactants.