Restricted Energy Transfer in Laser Desorption of High Molecular Weight Biomolecules

Producing ions from large molecules is of distinguished importance in mass spectrometry. In our present study we survey different laser desorption methods in view of their virtues and drawbacks in volatilization and ion generation. Laser induced thermal desorption and matrix assisted laser desorption are assessed with special emphasis to the recent breakthrough in the field (m/z \u3e 100,000 ions produced by matrix assisted laser desorption). Efforts to understand and describe laser desorption and ionization are also reported. We emphasize the role of restricted energy transfer pathways as a possible explanation to the volatilization of non-degraded large molecules

Ion Discrimination Effects in the Laser Microprobe Mass Analyzer

Different discrimination effects have been observed for atomic and polyatomic cluster ions when varying the ion lens potential of the laser microprobe mass analyzer. This is attributed to the chromatic aberration of the einzel lens, leading to different effects on ions with different energy distributions. Ion kinetic energy distributions were measured using the cut-off property of an ion reflector. The energies of elemental ions are higher and the energy distributions broader than those of polyatomic ions. Chemically different ion species have different energy distributions. An attempt is made to correlate the instrumental effects with results from ray tracing computer programs

The Role of Energy Deposition Processes in the Understanding of Laser Microprobe Analysis Mechanisms

After emphasizing the role of local energy deposition as a common feature of many microanalytical techniques we focus our attention to laser ionization processes in mass spectrometry of solids. Enhancement of ionization in the case of high power density laser pulses can be rationalized in terms of hydrodynamic equations. The mechanism of shock wave generation and plasma ignition as well as excess energy absorption is demonstrated. Model calculations show that a one component - one dimensional (1C-1D) description can account for such important features of the laser ionization process as energy distribution of the produced ions. The role of classical absorption in the determination of plasma formation threshold is unfolded. Present efforts to relate the results with the fine structure of mass spectra are outlined. Targets are most commonly strongly inhomogeneous in practical microprobing. The induced plasma ignition concept is introduced in order to describe poorly reproducible mass spectra in these situations

Laser microprobe mass spectrometry of quaternary phosphonium salts: direct versus matrix-assisted laser desorption

AbstractThe use of laser microprobe mass spectrometry (LMMS) for the structural characterization of thermolabile quaternary phosphonium salts has been evaluated. A comparison has been made between LM mass spectra obtained by direct analysis of “neat” organic salts and the corresponding “matrix-assisted” LM mass spectra. Main limitations of LMMS for the direct analysis of neat organic salts (i.e., no matrix) result from (1) formation of artifact ions that originate from thermal degradation and surface recombination reactions and (2) poor shot-to-shot reproducibility of the spectra. Dilution of the organic salts in a suitable, UV-absorbing matrix (e.g., nicotinic acid) significantly enhances the quality of the LM mass spectra. Improvements are: (1) an increase of the ion yield of preformed cations, 92) reduction or elimination of thermal decomposition and other deleterious surface reactions, and (3) a much better shot-to-shot spectral reproducibility. An interesting analytical feature is that these LM mass spectra, which contain only a few matrix peaks, can be obtained for subnanogram amounts of sample.The results also show that triphenylphosphonium salts with polycyclic aromatic substituents can be used as “molecular thermometers” to probe both the temperatures experienced by the sample molecules during the laser-induced sesorption ionization process and the internal energies of the desorbed ion species. In this way, quaternary phosphonium salts can be used for evaluating whether improvements have been achieved by applying different sample tretments. Comparison of four different matrices (i.e., nicotinic acid, ammonium chloride, glycerol, and 3-nitrobenzylalcohol) indicates that the effectiveness of a matrix to reduce thermal degradation and to decrese the internal energies of the ions depends on the UV-absorption characteristics and the volatilization/sublimation temperature of the matrix material