Ionization within a Cylindrical Capacitor:  Electrospray without an Externally Applied High Voltage

An interface was developed for mass spectrometry based on dc induction. Ions were produced as liquid flowed through a concentric cylindrical capacitor at room temperature. No externally applied high voltages affecting the spray tip were used, thus eliminating corona discharge problems in negative ion mode. The voltages applied to the capacitor were lower than those used for electrospray on the same instrument. Stable signals were observed for positive ions infused at flow rates from 50 nL/min to 200 μL/min, anions in the range 50−500 nL/min, without addition of sheath liquid or gas. The minimum concentration required to generate an interpretable product ion spectrum from the [M − 2H] 2 - precursor, for infused Salmonella typhimurium diphosphorylated lipid A, was improved by a factor of ∼500. Positive ion capillary LC/MS results for a horse apomyoglobin tryptic digest were similar to those observed for an optimized electrospray interface. Factor XIII b subunit, an 83-kDa protein, yielded an interpretable positive ion multiple charge envelope, under buffer conditions that did not allow collection of useful data with our low-flow electrospray source. Mass spectra were also generated for a single-stranded DNA 15-mer and 2‘,4‘-dideuteriotrifluoropropranolol, a small molecule that had been difficult to ionize by either electrospray or APCI.