DL-lactic acid exhibits minor but unsatisfactory improvements, while other quench buffers, i.e. solvent A (0.23% formic acid) and DHB, do not lead to any performance gain. The quench buffer of 500 mM glycine-HCl appears to prevent unspecific interaction sites in the ZrO 2 matrix. The average loss of peptide intensity is with roughly one order of magnitude (90%) still high, yet, more importantly, almost 100% of the peptides could be recovered through a sufficient signal-to-noise ratio (Figure ). Quite unexpected though, with an increasing number of technical replicates performed, we observed that the issue of unspecific interactions between protein and ZrO 2 beads became less severe, as evidenced by the obtained chromatograms (Figure ). This phenomenon could be explained by the protein increasingly 'blocking' unspecific binding sites of the beads. To test whether unspecific protein adsorption could be reduced by protein bead blocking, we prepared a phospholipid ZrO 2 bead trap column blocked with 3% bovine serum albumin (BSA) solubilized in solvent A (0.23% formic acid). BSA is not only a common blocking agent in immunoassays to prevent unspecific protein binding , but has also been applied in combination with ZrO 2 beads to obtain robust and reliable results in cell lysis assays . We then injected PhosB over the blocked and unblocked ZrO 2 phospholipid trap columns using 500 mM glycine-HCl (pH 2.35) as quench buffer.