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

Atmospheric-pressure Molecular Imaging of Biological Tissues and Biofilms by LAESI Mass Spectrometry

Ambient ionization methods in mass spectrometry allow analytical investigations to be performed directly on a tissue or biofilm under native-like experimental conditions. Laser ablation electrospray ionization (LAESI) is one such development and is particularly well-suited for the investigation of water-containing specimens. LAESI utilizes a mid-infrared laser beam (2.94 μm wavelength) to excite the water molecules of the sample. When the ablation fluence threshold is exceeded, the sample material is expelled in the form of particulate matter and these projectiles travel to tens of millimeters above the sample surface. In LAESI, this ablation plume is intercepted by highly charged droplets to capture a fraction of the ejected sample material and convert its chemical constituents into gas-phase ions. A mass spectrometer equipped with an atmospheric-pressure ion source interface is employed to analyze and record the composition of the released ions originating from the probed area (pixel) of the sample. A systematic interrogation over an array of pixels opens a way for molecular imaging in the microprobe analysis mode. A unique aspect of LAESI mass spectrometric imaging is depth profiling that, in combination with lateral imaging, enables three-dimensional (3D) molecular imaging. With current lateral and depth resolutions of ~100 μm and ~40 μm, respectively, LAESI mass spectrometric imaging helps to explore the molecular structure of biological tissues. Herein, we review the major elements of a LAESI system and provide guidelines for a successful imaging experiment

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

Subcellular metabolite and lipid analysis of \u3ci\u3eXenopus laevis\u3c/i\u3e eggs by LAESI mass spectrometry

Xenopus laevis eggs are used as a biological model system for studying fertilization and early embryonic development in vertebrates. Most methods used for their molecular analysis require elaborate sample preparation including separate protocols for the water soluble and lipid components. In this study, laser ablation electrospray ionization (LAESI), an ambient ionization technique, was used for direct mass spectrometric analysis of X. laevis eggs and early stage embryos up to five cleavage cycles. Single unfertilized and fertilized eggs, their animal and vegetal poles, and embryos through the 32-cell stage were analyzed. Fifty two small metabolite ions, including glutathione, GABA and amino acids, as well as numerous lipids including 14 fatty acids, 13 lysophosphatidylcholines, 36 phosphatidylcholines and 29 triacylglycerols were putatively identified. Additionally, some proteins, for example thymosin β4 (Xen), were also detected. On the subcellular level, the lipid profiles were found to differ between the animal and vegetal poles of the eggs. Radial profiling revealed profound compositional differences between the jelly coat vitelline/plasma membrane and egg cytoplasm. Changes in the metabolic profile of the egg following fertilization, e.g., the decline of polyamine content with the development of the embryo were observed using LAESI-MS. This approach enables the exploration of metabolic and lipid changes during the early stages of embryogenesis

Direct Detection of Diverse Metabolic Changes in Virally Transformed and Tax-Expressing Cells by Mass Spectrometry

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