Oligonucleotide charge states in negative ionization electrospray mass spectrometry are a function of solution ammonium ion concentration

The charge state distribution for oligonucleotides detected using negative ionization electrospray-mass spectrometry has been studied as a function of solution counterion concentration. In the absence of added buffer, an average charge state (Z) of - 7.2 is observed for a 10 mu M aqueous solution of a 14mer DNA at pH 7.0, with [M - 8H](8-) the most abundant ion. As the solution concentration of ammonium acetate increases from 0.1 to 33 mM, Z shifts to -3.8 with [M - 4H](4-) the most abundant charge state. The shift in most abundant charge state from [M - 7H](7-) to [M - 4H](4-) occurs abruptly between 1.0 and 10 mM NH4OAc. Above 100 mM NH4OAc, the value of Z plateaus at -3.1, with [M - 3H](3-) the most abundant charge state. The addition of 1-50 mM glycine to the analyte solution does not alter Z, suggesting that the changes in charge state observed by using ammonium acetate result from a solution equilibration of cations around the DNA strand, rather than nonspecific gas-phase proton transfers during the electrospray process. The fraction of neutralized phosphate groups reaches a maximum of 0.79 +/- 0.03 independent of length and sequence. (C) 1997 American Society for Mass Spectrometry