An LC/MS Method Providing Improved Sensitivity: Electrospray Ionization Inlet.

Electrospray ionization inlet (ESII) combines positive aspects of electrospray ionization (ESI) and solvent-assisted ionization (SAI). Similar to SAI, the analyte solution is directly introduced into a heated inlet tube linking atmospheric pressure and the initial vacuum stage of the mass spectrometer. However, unlike SAI, in ESII a voltage is applied to the solution through a metal union linking two sections of fused silica tubing through which solution flows into the inlet. Here, we demonstrate liquid chromatography (LC) ESII/MS on two different mass spectrometers using a mixture of drugs, a peptide standard mixture, and protein digests. This LC-ESII/MS approach has little dead volume and thus provides excellent chromatographic resolution at mobile phase flow rates from 1 to 55 µL min-1. Significant improvement in ion abundance and less chemical background ions were observed relative to ESI for all drugs and peptides tested at flow rates from 15 to 55 µL min-1. At a low inlet tube temperature, ESII has an ionization selectivity similar to that of ESI but, at higher inlet temperatures, appears to have the attributes of both ESI and SAI.