High resolution ion mobility-mass spectrometry for separation and identification of isomeric lipids.

Lipidomics is a particularly difficult analytical challenge due to the number and importance of isomeric species that are known or postulated in biological samples. Current separation and identification techniques are too often insufficiently powerful, slow or ambiguous. High resolution, low field ion mobility coupled to mass spectrometry is shown here to have sufficient performance to represent a new alternative for lipidomics. For the first time, drift-tube ion mobility separation of lipid isomers that differ only in position of the acyl chain, position of the double bond or double bond geometry is demonstrated. Differences in collision cross sections of less than 1% are sufficient for baseline separation. The same level of performance is maintained in complex biological mixtures. More than 130 high-precision reduced mobility and collision cross section values were also determined for a range of lipids. Such data can be the basis of a new lipidomics workflow, as the appropriate libraries are developed.

Ion yields of thin MALDI samples: dependence on matrix and metal substrate and implications for models.

Thin MALDI samples can perform differently than thicker samples, on metal substrates. Divergent results and models for the effect have been presented. Positive and negative yields are investigated here for three matrixes (2,5-dihydroxybenzoic acid (DHB), sinapinic acid, and alpha-cyano 4-hydroxycinnamic acid) on stainless steel and gold substrates. Samples were electrosprayed for uniformity and thickness control and imaged across a metal-metal boundary. Thin sample enhancement is found in both polarities for all three matrixes on a steel substrate. On gold, only alpha-cyano-4-hydroxycinnamic acid shows enhancement. These and earlier data are used to evaluate two models. The first is based on one-photon photoelectron emission from the metal; the second one, on two-photon matrix ionization at the metal interface. The surface-enhanced matrix photoionization model best fits the evidence, including the fluence dependence of electron emission from DHB on steel.

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization.

A quantitative model of primary ionization in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) is presented. It includes not only photochemical processes such as exciton pooling, but also the effects of the desorption event. The interplay of these two is found to be a crucial aspect of the MALDI process. The desorbing plume is modeled as an adiabatic expansion with entrained clusters. The parameters in the model are defined as much as possible via experiment or by analogy with known effects. The model was applied to the matrix 2,5-dihydroxybenzoic acid and found to reproduce the fluence dependence of the fluorescence yield and key features of the picosecond two-pulse ion generation efficiency curves. In addition, the model correctly predicts a fluence rather than irradiance threshold, the magnitude of the threshold, the magnitude of the ion yield, laser wavelength effects, plume temperatures, plume expansion velocities and the spot size effect.

Skin protection for photosensitized patients.

BACKGROUND AND OBJECTIVE: The goal of this study was to evaluate various creams for their capability to protect photosensitized skin from visible light. STUDY DESIGN/MATERIALS AND METHODS: Two cover creams and creams containing various combinations of Vaseline with TiO(2), ZnO, and Fe(2)O(3) were used to measure the reduced light transmission and the light absorption spectrum. In vitro and in vivo tests were performed to assess the protection from light by above mentioned compounds. RESULTS: The cover creams and the 50% TiO(2) cream showed similar efficacy in reducing light transmission, while the sunscreen was less efficient by a factor of 5. Cell protection by 25% TiO(2)+25% ZnO, TiO(2), or the cover creams was more efficient than protection by the sunscreen or other compounds. In vivo, the dark cover cream protected the skin by a factor of 3.4 better than the sunscreen. CONCLUSION: The dark cover cream has acceptable properties to protect photosensitized skin.

Two-photon ionization thresholds of matrix-assisted laser desorption/ionization matrix clusters.

Direct two-photon ionization of the matrix has been considered a likely primary ionization mechanism in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. This mechanism requires that the vertical ionization threshold of matrix materials be below twice the laser photon energy. Because dimers and larger aggregates may be numerous in the early stages of the MALDI plume expansion, their ionization thresholds are important as well. We have used two-color two-photon ionization to determine the ionization thresholds of jet cooled clusters of an important matrix, 2,5-dihydroxy benzoic acid (DHB), and mixed clusters with the thermal decomposition product of DHB, hydroquinone. The thresholds of the clusters were reduced by only a few tenths of an eV compared to the monomers, to an apparent limit of 7.82 eV for pure DHB clusters. None of the investigated clusters can be directly ionized by two nitrogen laser photons (7.36 eV), and the ionization efficiency at the thresholds is low.

A mass spectrometry and electron paramagnetic resonance study of photochemical and thermal aging of triterpenoid varnishes.

Photochemical and thermal aging of triterpenoid dammar and mastic resins used as varnishes on paintings were studied using graphite-assisted laser desorption/ionization mass spectrometry. This extends an earlier study on similar materials (Zumbühl et al., Anal. Chem. 1998, 70, 707-715) that focused on photoaging. Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as well as simultaneous loss of hydrogen. Oligomers up to tetramers are formed, while cleavage reactions lead to increased signal intensities in the mass ranges between the oligomers and below the monomers. No major differences were found between the mass spectra of samples aged in light or darkness, except that deterioration was faster in light. Electron paramagnetic resonance spectroscopy revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radical reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings lead to a unified explanation for yellowing of natural resin varnishes, one of the major degenerative changes in the appearance of paintings. It is also shown that the commercially available, nominally fresh resins are already in an advanced stage of oxidation and degradation. Energy-rich substances are formed upon irradiation with sunlight and are believed to restart the autoxidative chain reactions, regardless of storage conditions. As a result, varnishes are oxidized quite quickly (months) even when kept in darkness.

Secondary ion-molecule reactions in matrix-assisted laser desorption/ionization

Ion-molecule charge- and proton-transfer reactions in the desorption plume are considered for the case of matrix-assisted laser desorption/ionization (MALDI) with ultraviolet laser excitation, and it is proposed that they are major determinants of the observed mass spectrum. Specific MALDI phenomena which are discussed include the dominance of singly charged ions and analyte-matrix or analyte-analyte signal suppression. Should any be formed, highly charged products can be reduced by reaction with neutral matrix, yet singly charged ions cannot generally be neutralized in the same manner. Ion suppression effects can also be explained by similar reactions, which in some cases involve interconversion of dissimilar ion types. The plume is proposed often to be more under thermodynamic rather than kinetic control owing to these secondary reactions. UV/MALDI mass spectra should therefore be largely predictable, given sufficient thermodynamic information, and appropriate experimental conditions of sufficient analyte and plume density. Copyright 2000 John Wiley & Sons, Ltd.

Optimization of an ion-to-photon detector for large molecules in mass spectrometry.

Ion packets can be detected in time-of-flight mass spectrometry by collecting the photons that are produced during the impact of the packets with a scintillator. The photon yield is a function of the ion energy. It was found that post-acceleration of the particles in front of the scintillator was an efficient way of increasing signal intensities. For the same total ion energy, the intensities were larger with post-acceleration than when only increasing the initial ion kinetic energy. A venetian blind dynode, converting the primary ion beam into electrons/secondary ions, was also introduced. Positive or negative secondary particles produced on the dynode surface could be accelerated to the scintillator. Electrons were found to give the highest signals. Intensities similar to those measured with microchannel plates were found. The linearity and onset of saturation of the microchannel plates and the ion-to-photon detector were compared. At optimum operating conditions, the ion-to-photon detector gave around 10 times higher signals than the microchannel plates for heavy ions (150 kDa), with similar mass resolution. Copyright 1999 John Wiley & Sons, Ltd.

Graphite/Liquid mixed matrices for laser desorption/ionization mass spectrometry.

A combination of liquid matrix and graphite particulates (2 µm) has been proposed as a method suitable for the laser desorption/ionization mass spectrometry of peptides and proteins (Sunner, J.; et al. Anal. Chem. 1995, 67, 4335). Here we demonstrate the potential of this approach as a straightforward, and very general, method of achieving the ultraviolet laser desorption/ionization of a broad range of intermediate weight analytes. The desorption/ionization mechanism, the influence of preparative procedures, and the breadth of application of this methodology have been investigated. A simple and robust preparative procedure is presented for the analysis of proteins, oligosaccharides, and synthetic polymers. Detection sensitivities are in the femtomole region for lower molecular weight peptides and oligosaccharides. The graphite acts as an energy transfer medium by absorbing the UV radiation, leading to thermal desorption of the liquid matrix and analyte. The liquid matrix was observed to fulfill several important roles. In the case of peptides and proteins, which preferentially form protonated molecular ions, it acts as a protonating agent. It also enhances the signal intensities of cationized species (e.g., polysaccharides and polar polymers) by assisting their desorption. An excess of liquid matrix serves to cool the analyte during the desorption step and minimize decomposition. The presence of liquid matrices increases the sample lifetime at a particular desorption spot, minimizing the time-consuming search for "hot spots". The addition of cationizing salts has been shown to improve the quality of mass spectra obtained for polar polymers and extend the range of materials that can be investigated to include apolar synthetic polymers.