Radial velocity distributions of molecular ions produced by matrix-assisted laser desorption/ionization

We have measured the radial velocity distributions of protonated intact dynorphin 1-13 ions, produced during matrix-assisted laser desorption/ionization (MALDI) (close to the threshold for ion production), and of the protonated dimer of the MALDI matrix alpha-cyano-4-hydroxycinnamic acid. For both ion species, the radial component of the velocity was found to be considerably smaller than the axial component, implying angular distributions for both matrix and polypeptide ions that are strongly forward peaked. This extreme forward peaking (cos(28)theta for the peptide ions and cos(12)theta for the matrix ions) lends support to the earlier proposals of a jet expansion model for MALDI. Our observation that the peptide ions exhibit a considerably higher degree of forward peaking than ions arising from the matrix awaits detailed explanation. The present results should prove useful for the design of highly efficient MALDI mass spectrometers. (C) 1997 Elsevier Science B.V.