*3.2. Chromatographic Conditions*

The mobile phase consisted of acetonitrile:water (62:38 *v*/*v*) containing 25 mM of TBA-Br bu ffer. After mixing, mobile phases were filtered through a 0.45-μm filter and degassed before use by an ultrasonic cleaner (Sonorex Digitec, Bandelin, Berlin, Germany). Chromatographic conditions were set at a constant flow rate of 1 mL/min in isocratic mode, the injection volume was 20 μL, the column was maintained at room temperature (20 ± 2), and the detection wavelength was set a 236 nm. Dissolved snSW GAP was pre-treated before injection, with a dilution of 60 mM of TBA-Br in a proportion of 1:3, allowing us to achieve ion pairing (IP) with a desirable k. This IP-RP-HPLC method was properly developed and validated according the ICH Guidance for Industry Q2 (R1) through several parameters, namely specificity/selectivity, linearity, precision, accuracy, range, limits of detection (LOD) and quantification (LOQ) (Tables S1 and S2) [40]. Concerning the leaching water samples, a di fferent proportion of the mobile phase was used (50:50 *v*/*v*), in order to better visualize the signal of GAP without interference.
