One-step selective electrokinetic removal of inorganic anions from small volumes and its application as sample clean-up for mass spectrometric techniques

The presence of inorganic anions in a sample interferes with mass spectrometric (MS) analysis. Here, a simple method to remove these ions from a liquid sample in one-step is described. The inorganic anions present in a 50 μL sample were extracted into a low pH solution inside a 200 μm i.d. × 33 cm long capillary by the use of an electric field. The selective removal of unwanted anions and retention of target analytes was accomplished by control of the apparent electrophoretic velocities of anions and analytes at a boundary that separated the sample and extraction solution. No physical barrier (e.g., membrane) was required and with the boundary situated at the tip of the capillary, efficient removal of inorganic anions (e.g., >80% removal) and good recovery of target analytes (e.g., >80% recovery) were achieved. The time required for removal of the inorganic anions was found to depend on their initial concentrations. The removal process was investigated using different concentrations of bromide and nitrate (as potassium salts) and negatively chargeable drugs as target analytes. This micro-sample clean-up technique used no organic solvents and little consumables and was studied to the determination of 0.6 μg/L arsenic and 8.3 μg/L vanadium in 500 mg/L sodium chloride using inductively coupled plasma MS and 50 μM angiotensin I in 1000 mg/L sodium chloride using electrospray ionisation MS. Micro-sample clean-up was performed for 45 min at 3 kV in both demonstrations. The calculated recoveries for the metals at trace levels were 110–130%, and for the peptide was 103.8%.