Electric-field driven sample clean-up strategies for mass spectrometry applications

Sodium dodecyl sulfate (SDS) is commonly used for protein solubilization prior to tryptic digestion, while inorganic salts, with SDS, are typically used during the extraction of macromolecules from biological samples in bottom-up proteomics. Above a certain threshold, the presence of inorganic salts and/or SDS interferes with electrospray ionizationmass spectrometric (ESI-MS) analysis of peptides and digested proteins. Sample preparation strategies have been developed to circumvent this issue. This work describes electric-field driven techniques for the removal of inorganic anions and SDS from relatively high conductivity and micellar sample solutions in microliter volumes of peptides and digested protein samples. The one-step inorganic anion removal process is based on controlling the apparent electrophoretic velocities of anions at a boundary that separated the sample and the acidic extraction solution (ES) in a fused-silica capillary. Removal of >80% inorganic anions and >80% analyte signal restoration were achieved. The applicability of the method was also extended to the removal of chloride ions prior to the analysis of low levels of arsenic and vanadium using inductively coupled plasma MS. The dodecyl sulfate (DS-) as sodium salts were electrokinetically removed offline using an acidic ES containing acetonitrile in a fused silica capillary dipped into the sample. The high amount of organic solvent collapses the micelle-peptide complex, releasing the peptide and thus is amenable to efficient removal of DS- from the sample solution. Percent MS signal intensity restoration of 74-83% and 89-95% percent MS signal intensity reduction of DS- were attained. Lastly, an online DS- removal system directly connected to the MS instrument was developed. The removal of DS-, and the retention and subsequent analysis of peptides were performed using an acidic ES with 40% acetonitrile. The mechanism was similar to the offline strategy and resulted in the high improvement of ESI-MS signals of peptides.