Influence of pressure in the first pumping stage on analyte desolvation and fragmentation in nano-ESI MS

In ESI MS, some classes of biomolecules are detected only with low signal intensities due to difficulties in achieving efficient analyte desolvation, either because an analyte tends to fragment already at gentle desolvation conditions (i.e., noncovalent protein complexes or nucleotides) or because an analyte requires very strong activation in order to remove solvent molecules (i.e., carbohydrates). Even though the pressure in the first pumping stage of the ESI instrument is known to have an influence on the desolvation conditions, it has never been the focus of a detailed investigation. The role of the pressure in the first pumping stage is systematically interrogated in this study for several model substances. Ion signal intensities and signal-to-noise ratios are significiantly enhanced if the pressure in the first pumping stage is increased and adjusted, and analyte fragmentation can be substantially reduced. Thus, besides thermal heating and the acceleration in the nozzle-skimmer region, which are usually optimized, the pressure in the first pumping stage is an additional important desolvation parameter.