Figure 3. Affect of formic acid in TiO₂ loading buffer.

One pmol of a tryptic digest of α- and β-casein was loaded onto TiO₂ using the MDLC and analyzed as in Fig. 1, except that each mixture was loaded in 20% acetonitrile and varying concentrations of formic acid (FA). Bound peptides were washed with loading buffer and eluted with 200 mM NH₄HCO₃ pH 9. A comparison of 0.1-4 % formic acid (FA) used in both loading and wash buffers is presented. Spectra counts are normalized to the results observed for 0.1% FA. Non-normalized data of all identified phosphopeptides is included in Supplemental Information (S-Table 9).