Negative ion electrospray mass spectrometry of nucleotides: ionization from water solution with SF6 discharge suppression

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Abstract
The total current and selected ion currents from the electrospray ionization (ES1) of 10−5 M solutions of cocaine hydrochloride and deoxycytidine monophosphate (dCMP) monosodium salt in methanol and water solvents were compared in positive and negative ion modes, respectively, without and with SF6, gas as a discharge suppressant. The ESI onset voltages (Von), were the same for the positive and negative ion modes. The Von, for methanol was much lower than that for water and in agreement with the equation of D. P. H. Smith, who attributes the difference to the higher surface tension of water. The onset of electric discharge (Vdis) without SF6, occurred at lower capillary voltages for the negative relative to the positive ion mode for methanol; but Vdis is much higher than Von for methanol, and discharges do not interfere with ESI operation. For water, Von ≈ Vdis in the absence of SF6, in the negative ion mode, and ESI operation is impossible without SF6, discharge suppression. The discharge problem in the positive ion mode is less severe, but SF6, is still very useful. A dynamic range of 10 −7–10−5 M was obtained by selected ion monitoring of [dCMP - H]− at 4.5 and 20 μL/min. flows. Subpicomole detection limits for the nucleotide salt were obtained under these conditions.