In preliminary experiments, the dependence of tr for anionic PFAS on the concentration of ammonium acetate ([NH4CH3COO]) was examined. For each compound, the value of tr increased slightly with increasing [NH4CH3COO] from 1 to 50 mM (Figure ). Similar results were reported by Hammer et al. Retention was substantially weaker when no electrolyte was added to the eluent, suggesting that control of ionic strength is important to obtain stable results. Based on these results, we chose [NH4CH3COO] of 10 mM for further measurements. Although the exact retention mechanism is unknown, we surmise that anionic surfactants sorb primarily to the interface between the C18 phase and the mobile phase, with the anionic head group oriented toward the mobile phase and the hydrophobic tail in the C18 phase. Higher ionic strength in water reduces the repulsive interactions between the negative charges of the silica particles and the anionic analytes. Ion-pair formation is unlikely to be the main retention mechanism because of the relatively small influence of [NH4CH3COO] on tr.