Analyte loss was compared for both SPE sorbents and HLB and PSA+C18 showed comparable performance (see Figure ). "Good" performance (analyte loss <10% during loading and washing) was achieved for 90% of all analytes, whereas "good" RE was achieved for 59% of all analytes using HLB compared to 39% using the mixed-mode sorbent PSA+C18. Analyte loss >5% was observed for six analytes with LogP values between 1.7-3.2 when using the PSA+C18 for SPE, whereas analyte loss >5% was only observed for a single compound when using the HLB sorbent (see Figure ). Most analytes showed sufficient RE (50%-150%) using either of the two sorbents but RE<50% were observed for 15 analytes using the mixed-mode sorbent, whereas only six analytes with RE<50% were observed using the HLB sorbent. Consequently, the HLB was selected as the sorbent material for further workflow optimization. Buffer Selection for Sample Before Loading. Subsequently, the buffer system used for sample loading was optimized. As shown in Figure , >95% of all analytes showed analyte losses <10% after diluting samples with PBS buffer. This fraction dropped to 80% and 87% of the analytes when using NH4Ac and H2O as dilution solvents. Consequently, PBS was chosen as the sample buffer for further process optimization.