The Collision of Slowly Moving Heavy Biomolecules with Surfaces and Some Fundamental Studies Related to Matrix - Laser Desorption
Abstract
Heavy molecular ions with energies in the range of.1-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in two different tandem time-of-flight mass spectrometers. For 15 keV projectiles, the ratio of electron emission to secondary-ion emission decreases rapidly with increasing projectile mass. For projectiles from 300 u to 6000 u with energies from 300 eV to 30 keV, the electron -to-ion ratio is a function of velocity but largely independent of mass (for a given velocity). The ratio decreases rapidly at first with decreasing velocity but appears to approach a constant value of about 0.1 below a velocity {~}1.4times10^4 m/s (~1 eV/u). Ion emission is dominant for primary ions with energies {<}{ ~}2 eV/u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, with energy ~.08 eV/u (~3900 m/s). Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background; and the relative intensities of the background positive ions are largely independent of projectile. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range. Some properties of matrix-assisted laser desorption under field-free and high-field conditions were also studied. A two-grid acceleration region was used to test the velocity at which ions are emitted under zero extraction fields. In the same experiment, protein (and protein cluster) ions ranging from mass 5000 u to 94,000 u have a velocity of ~550 m/s, while the matrix ions (sinapinic acid) have a velocity of ~800 m/s, but for certain experiments, the matrix ions have a similar velocity to that of the peptide ions. The velocity of the peptide ions is almost independent of laser power from threshold to 16 times threshold. The velocity of the matrix ions increases rapidly with an increase in laser power. Under high-field conditions, both the matrix and peptide ions shift to longer time the spectrum with an increase in laser power, indicating an energy deficit or delay in ionization.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......151M
- Keywords:
-
- MALD;
- Physics: Molecular