Femtoamp and picoamp modes of electrospray and paper spray ionization

Abstract Sub nanoampere ionization was investigated using glass emitters and paper spray substrates. An electrometer was set up to measure ionization current as low as 20 fA, while the ion trap mass spectrometer was operated to analyze ion current as low as 1 ion per second. Femto amp (100 s fA) ionization currents were generated using glass emitters with 90 nm tip or 4 μm tips. Using the 4 μm emitter tips, both the fA mode and the regular nanoESI mode were generated. Mass spectra produced by the fA mode ESI were >3 order of magnitude less intense than those produced by nanoESI mode. Beyond generating ESI type of ions, fA mode ESI generates native cytochrome c charge states without using any buffering agent. Femto amp ionization was also observed when conducting paper spray ionization, as well as when ionizing dry paper substrate without solvent. Ultra-low current ionization allowed direct analysis of undiluted sample matrices without carryover contamination. Samples collected and stored on a dried paper substrate were analyzed with no sample preparation.