Californium-252 plasma desorption with Fourier transform mass spectroscopy

Plasma desorption (PD) such, as that induced by the 100-MeV fission products of 252Cf, is a particularly promising ionization method for large molecules, yielding molecular ion species even from trypsin, molecular weight 23,463. Further, with trypsin using nitrocellulose as the substrate, (M + 3H)3+ is the most abundant molecular ion species and (M + 6H)6+ is measurable, which greatly extends the mass values (m) observable for instruments with an upper m/z limit (z = number of charges). However, a 50-μCi 252Cf source only produces 55,000 fissions s-1, yielding ion currents that are generally much too low for scanning instruments. An instrument with unusual capabilities for these, as well as for simultaneous ion detection over a wide mass range, is the Fourier transform (FT) mass spectrometer. With FTMS, Hunt has measured (M + H)+ ions of cytochrome c, molecular weight 12,384, ionized in an exterior fast-atom-bombardment source. Here the authors describe techniques for obtaining PD/FT mass spectra for a variety of compounds with abundant molecular ion species of masses as high as 2016 (alamethicin)