UTILITY OF ORGANIC-BASES FOR IMPROVED ELECTROSPRAY MASS-SPECTROMETRY OF OLIGONUCLEOTIDES

The sensitivity and accuracy of the mass spectrometric analysis of oli gonucleotides using electrospray ionization can be compromized when th e oligomer is adducted in the gas phase to cations such as sodium or p otassium. We have evaluated the addition of mM concentrations of a ser ies of organic bases with solution pK(b) values ranging from 11.5 to 5 .5 and gas-phase proton affinities ranging from 213 to 232 kcal/mol as a method for suppression of signals from alkali-adducted ions. Strong er bases such as triethylamine and piperidine reduce the signals from bound sodium most effectively, but also decrease the total ion current from oligonucleotide. Imidazole, with a solution pH of similar to 8.0 , provides modest suppression of sodium/potassium adduct ions, but up to a four-fold improvement in sensitivity. Go-addition of imidazole an d triethylamine or piperidine produces high ion abundance and good sup pression of cation-adducted species for samples of phosphodiester or p hosphorothioate oligomers which have not been desalted via preliminary precipitation or by high-performance liquid chromatography. Addition of high concentrations of imidazole generates a bimodal distribution o f charge states, which may reflect different gas-phase conformations f or single-stranded oligomers.