Californium-252 plasma desorption with Fourier transform mass spectrometry.

Sir: Plasma desorption (PD), such as that induced by the 100-MeV fission products of z5zCf, is a particularly promising ionization method for large molecules (1-5), yielding molecular inn species even from trypsin, molecular weight 23463. Further, with trypsin using nitrocellulose as the substrate, (M + 3H)3+ is the most abundant molecular ion species and (M + 6H)fi+ is measurable ( 4 ) , which greatly extends the mass values (m) observable for instruments with an upper m/z limit (2 = number of charges). However, a 50-pCi 252Cf source only produces 55 000 fissions s-l, yielding ion currents that are generally much too low for scanning instruments. Until recently PD has only been used with time-of-flight (TOF) mass spectrometers of relatively poor resolution (1OOO) and poor capabilities for tandem mass spectrometry (2, 6). 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 (7-13). With FTMS, Hunt (14) has measured (M + H)+ ions of cytochrome e, molecular weight 1 2 384, ionized in an exterior fast-atombombardment source. A landmark PD/FTMS experiment on a single sample has been reported by Tabet and co-workers (15) showing abundant (M + K)+ ions a t m / z 594 from leuenkephalin. However, from more recent PD/FTMS experiments Hercules e t al. (16, 17) obtain spectra dominated by anomalous peaks, with the spectrum of the larger peptide gramicidin S (molecular weight 1141) showing only fragment ions such as m / t 994 (16). Here we 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).