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 μM 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 cartons 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.
Griffey, R. H., Sasmor, H., & Greig, M. J. (1997). Oligonucleotide charge states in negative ionization electrospray-mass spectrometry are a function of solution ammonium ion concentration. Journal of the American Society for Mass Spectrometry, 8(2), 155–160. https://doi.org/10.1016/S1044-0305(96)00200-0