Ghost peaks observed after atmospheric pressure matrix-assisted laser desorption/ionization experiments may disclose new ionization mechanism of matrix-assisted hypersonic velocity impact ionization

Abstract

RATIONALE Understanding the mechanisms of MALDI promises improvements in the sensitivity and specificity of many established applications in the field of mass spectrometry. This paper reports a serendipitous observation of a significant ion yield in a post-ionization experiment conducted after the sample has been removed from a standard atmospheric pressure (AP)-MALDI source. This post-ionization is interpreted in terms of collisions of microparticles moving with a hypersonic velocity into a solid surface. Calculations show that the thermal energy released during such collisions is close to that absorbed by the top matrix layer in traditional MALDI. The microparticles, containing both the matrix and analytes, could be detached from a film produced inside the inlet capillary during the sample ablation and accelerated by the flow rushing through the capillary. These observations contribute some new perspective to ion formation in both laser and laserless matrix-assisted ionization. METHODS An AP-MALDI ion source hyphenated with a three-stage high-pressure ion funnel system was utilized for peptide mass analysis. After the laser was turned off and MALDI sample was removed, ions were detected during a gradual reduction of the background pressure in the first funnel. The constant-rate pressure reduction led to the reproducible appearance of different singly- and doubly-charged peptide peaks in mass spectra taken a few seconds after the end of the MALDI analysis of a dried-droplet spot. RESULTS The ion yield as well as the mass range of ions observed with a significant delay after a completion of the primary MALDI analysis depended primarily on the background pressure inside the first funnel. The production of ions in this post-ionization step was exclusively observed during the pressure drop. A lower matrix background and significant increase in relative yield of double-protonated ions are reported. CONCLUSIONS The observations were partially consistent with a model of the supersonic jet from the inlet capillary accelerating detached particles to kinetic energies suitable for matrix-assisted hypersonic-velocity impact ionization.

Advances in Ionization for Mass Spectrometry

“…These authors demonstrated that the most efficient ionization was obtained from a surface with lower melting point, accessible K + ion, and rough texture. Moskovets proposed an explanation for the ionization of residual peptides in the inlet of the mass spectrometer after atmospheric pressure MALDI experiments . A linear ion trap with a three-stage high pressure ion funnel was used to detect the peptides without laser irradiation.…”

“…Moskovets proposed an explanation for the ionization of residual peptides in the inlet of the mass spectrometer after atmospheric pressure MALDI experiments. 147 A linear ion trap with a three-stage high pressure ion funnel was used to detect the peptides without laser irradiation. The ions appeared with decreased background pressure in the first funnel, which may indicate matrix-assisted hypersonic-velocity impact ionization of the peptides.…”