As illustrated in Figure , results for HLB and PSA+C18 were comparable resulting in "good" trapping efficiency for 90% of the analytes (analyte loss < 10% during loading and washing), whereas RE was "good" for 59% of the analytes when using HLB, compared to 39% of compounds when using mixed-mode sorbent (PSA+C18). Analyte losses >5% were observed for six analytes in the LogP range of 1.7-3.2 when using the PSA+C18 for SPE (Figure (B) and (D). In contrast, analyte losses >5% were only observed for a single compound when using the HLB sorbent (Figure ) and (C)). In general, most analytes demonstrated recoveries within the range of 50% to 150% either using HLB or the mixed-mode sorbent Figure (E), 15 analytes had RE values <50% using the mixed-mode sorbent whereas only 6 analytes with RE<50% were observed when using the HLB sorbent. Consequently, the HLB was selected as the sorbent material for further workflow optimization. Buffer Selection for Sample Before Loading. As shown in Figure , >95% of all analytes showed analyte losses <10% after dilution of samples with PBS buffer, and which was classified as "good" performance in terms of analytes loss. When using NH4Ac and H2O as dilution solvents, the percentages of "good" performing compounds dropped to 80% and 87%, respectively. Consequently, PBS was chosen as the sample buffer for subsequent optimization steps.