Optimization of electrospray ionization conditions to enhance formation of doubly protonated peptide ions with and without addition of chromium(III).

Rationale: Production of multiply protonated ions by electrospray ionization (ESI) is important to the analysis of peptides by mass spectrometry. For small neutral and acidic peptides, addition of chromium(III) greatly increases the intensity of doubly protonated ions. The current study examines instrumental and solution parameters that maximize peptide ion charge by ESI.

Methods: The neutral and basic heptapeptides AAAAAAA (A7) and AAAKAAA (A3KA3) were used as test compounds and electrosprayed from a solution containing chromium(III) nitrate at a peptide to metal molar ratio of 1:10. Positive ion mode experiments were performed on a Bruker HCTultra PTM Discovery System quadrupole ion trap mass spectrometer. Source voltages and drying/nebulizer gases were systematically altered. The effects of rinsing, brand, and color of plastic microcentrifuge tubes (vials) employed were also investigated.

Results: Nebulizer gas pressure and drying gas flow rate are crucial parameters for production of [M + 2H] , while drying gas temperature alone has minimal effect. Optimization of the capillary exit and skimmer voltages are important both to maximize [M + 2H] and reduce unwanted ion dissociation. Protonation is enhanced and fewer impurity peaks are observed when solutions are prepared in colorless plastic vials that have been rinsed briefly with propan-2-ol (isopropanol).

Conclusions: Optimization of instrument and sample preparation factors for enhanced protonation with and without Cr(III) is necessary to allow maximum formation of [M + 2H] . Proteomics researchers should find these procedures to be of use for increasing multiply protonated signal intensity even in the absence of Cr(III). Copyright © 2017 John Wiley & Sons, Ltd.