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MALDI mass spectra of β-casein tryptic digest treated with (A) IMAC-Ni, (B) IMAC-Ga, and (C) ZrO2 micro tips. Left 700–2000 m/z and right 2000–3400 m/z ranges. Phosphopeptides are denoted by vertical arrows. Non-phosphopeptides with acidic and basic characters are marked by filled and empty circles, respectively.
