Shifts in Peptide and Protein Charge State Distributions with Varying Spray Tip Orifice Diameter in Nanoelectrospray Fourier Transform Ion Cycltron Resonance Mass Spectrometry

Abstract

The influence of the diameter of the spray tip employed for nano-electrospray mass spectrometry (nano-ES-MS) upon mass spectral charge state distributions was investigated using angiotensin I (M_r = 1296), insulin (M_r = 5774), and ubiquitin (M_r = 8560) as test analytes. Under a variety of experimental conditions, the charge state distributions of the test peptides and protein consistently shifted toward higher values as the tip orifice diameter decreased. This finding indicates that the use of narrow diameter capillaries can promote the formation of higher charge state ions that are more reactive precursors in tandem mass spectrometry experiments. A detailed comparison of charge state distributions obtained for nanospray capillaries of varying diameters was undertaken while systematically varying experimental parameters such as sample flow rate, analyte concentration, solvent composition, and electrospray current. The general tendency to obtain higher charge states from narrow diameter capillaries was conserved throughout, but tips with smaller orifices were more sensitive to sample flow rate (the average charge state was lowered significantly as flow was raised), whereas tips with bigger orifices were more sensitive to analyte concentration and pH of the solution (as each was lowered, the average charge state increased).