A novel tantalum-based sol–gel packed microextraction syringe for highly specific enrichment of phosphopeptides in MALDI-MS applications
A new tantalum-based sol–gel material was synthesized using a unique sol–gel synthesis pathway by PEG incorporation into the sol–gel structure without performing a calcination step. This improved its chemical and physical properties for the high capacity and selective enrichment of phosphopeptides from protein digests in complex biological media. The specificity of the tantalum-based sol–gel material for phosphopeptides was evaluated and compared with tantalum(V) oxide (Ta2O5) in different phosphopeptide enrichment applications. The tantalum-based sol–gel and tantalum(V) oxide were characterized in detail using FT-IR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM), and also using a surface area and pore size analyzer. In the characterization studies, the surface morphology, pore volume, crystallinity of the materials and PEG incorporation into the sol–gel structure to produce a more hydrophilic material were successfully demonstrated. The X-ray diffractograms of the two different materials were compared and it was noted that the broad signals of the tantalum-based sol–gel clearly represented the amorphous structure of the sol–gel material, which was more likely to create enough surface area and to provide more accessible tantalum atoms for phosphopeptides to be easily adsorbed when compared with the neat and more crystalline structure of Ta2O5. Therefore, the phosphopeptide enrichment performance of the tantalum-based sol–gels was found to be remarkably higher than the more crystalline Ta2O5 in our studies. Phosphopeptides at femtomole levels could be selectively enriched using the tantalum-based sol–gel and detected with a higher signal-to-noise ratio by matrix-assisted laser desorption/ionization-mass spectrometer (MALDI-MS). Moreover, phosphopeptides in a tryptic digest of non-fat bovine milk as a complex real-world biological sample were retained with higher yield using a tantalum-based sol–gel. Additionally, the sol–gel material was packed into a standard syringe (0.5 mL) to enhance the ease of use of the sol–gel material and for the elimination of additional mixing and separation procedures during the adsorption, washing and elution steps of the enrichment procedure. It was found that up to 28 phosphopeptides in milk digest were easily detectable by MALDI-MS at femtomole levels (around 20 fmol) using the microextraction syringe within less than one minute.