Hollow fiber supported TiO2 monolithic microextraction combined with capillary HPLC-ICP-MS for sensitive absolute quantification of phosphopeptides

Protein phosphorylation analysis is important for understanding cell regulations. Mass spectrometry (MS) is a powerful technique for peptide phosphorylation analysis. However, quantification of low abundance phosphoproteins in a complex mixture of proteins is still a challenge. Here we report the development of hollow fiber (HF) supported TiO2 monolithic microextraction combined with capillary high performance liquid chromatography (capHPLC)-inductively coupled plasma-collision reaction cell (ICP-CRC)-MS for the absolute quantification of phosphopeptides. The membrane pores (∼200 nm) of the HF effectively carry more TiO2 on its surface to enhance the adherence of the TiO2 monolith with the HF. By using β-casein as a model phosphorylated protein, we optimized the HF-supported TiO2 monolithic microextraction towards phosphopeptides. Under the optimal conditions, a 100-fold enrichment factor was obtained and the monoliths can be reused 40 times. By monitoring 31P16O with O2 as a reaction gas in the CRC, polyatomic mass interference at m/z 31 is avoided and a low detection limit (2.9 nM) of phosphorus is achieved. The method of HF-supported TiO2 monolithic microextraction-capHPLC-ICP-MS provided the detection limit for phosphopeptides at the nM level. The proposed method was applied to the quantification of phosphopeptides in milk and milk powder samples.