Ionic and neutral products of matrix-assisted laser desorption were simultaneously detected in a time-of-flight mass spectrometer. The neutrals were photoionized with coherent vacuum ultraviolet radiation. Mass spectra were obtained for primary and secondary alkyl amines in various absorbing matrices. Based upon relative signal intensities of directly formed analyte ions and photoionized analyte neutrals, the ratio of neutrals-to-ions ejected was found to be of the order of loo00 or greater with desorption irradiances near the threshold for ion formation. The ratio decreased with increasing irradiance of the desorption laser beam. This dependence is consistent with a collisional mechanism for analyte-ion formation during the desorption event that is initiated by photoionization of the matrix. Matrices which gave large signal intensities of directly formed analyte ions also gave large signal intensities of photoionized analyte neutrals. Conversely, matrices which gave small signal intensities of directly formed analyte ions also gave small signal intensities of photoionized analyte neutrals. These results suggest that the efficiency of analyte vaporization by matrix-assisted laser desorption is an intrinsic property of the matrix-analyte pair, independent of the ion-formation process.The mass range and sensitivity of laser desorption mass spectrometry can be increased by suspending the analyte in a strongly absorbing host matrix.'+ The matrix promotes conversion of the incident laser radiation into internal energy so that the analyte is efficiently ejected into the gas phase. Both ionic and neutral species are produced. In matrix-assisted laser desorption/ ionization (MALDI), directly formed ions in the desorption plume are extracted and analyzed. This method is widely used to obtain molecular-weight information on picomolar amounts of large biomolecules.5-8 Post-ionization of neutrals in the desorption plume using single-or multi-photon ionization (MALD/PI) is also possible, but generally suffers from low sensitivityg owin in part to the difficulty of tically, MALD/PI can provide greater flexibility in mechanistic studies by decoupling the desorption and ionization steps."Theoretical descriptions of the MALDI process treat the desorption and ionization steps separately. Several models have been proposed to account for rapid disintegration of the condensed phase with little or no decomposition of the analyte.'"16 For ultraviolet laser desorption, ionization is thought to occur by photoionization of the matrix followed by ion/molecule reactions with other matrix and analyte molecule^.'^ Experimental support for these mechanisms rests primarily on measurements of the kinetic-energy distribution ,lM9 laser irradiance dependence :* and types" of ions produced by MALDI. Further refinement of the models can be expected as the properties of neutral molecules become more fully characterized. A few mechanistic studies have been reported where neutrals produced by MALD were detected by postionization. Kinsel et al.'l used m...
Fatty acid methyl esters (FAMEs) were generated in situ, during pyrolysis, from whole-cell bacterial samples and analyzed by mass spectrometry (MS). The FAME profiles obtained by an in situ thermal hydrolysis methylation (THM) step were compared with gas chromatography (GC) and MS analyses of the chemically extracted and methylated fatty acids. This correlation was based on the ability of each technique to differentiate a representative group of 15 bacteria at the species level as predicted by principal component analysis. All three analyses, GC/FAME, pyrolysis-MS/FAME, and in situ THM-MS/FAME differentiated the studied bacterial sample set into three discrete clusters. The bacteria comprising each cluster were the same for all three analyses, showing that taxonomic information of the lipid profiles was preserved in the Py-MS/FAME and in situ THM-MS/FAME analyses of whole cells. Contributions from saturated, unsaturated, cyclopropyl, and branched bacterial fatty acids to the differentiation of microorganisms were identified for all three analyses. The in situ THM-MS/FAME approach is simple, requires small samples (approximately 2 x 10(6) cells/profile), and is rapid, with a total analysis time under 5 min/sample.
Metal‐organic frameworks (MOFs) with the “nbo” topology constitute a diverse suite of more than 100 nanoporous materials, but their use in applications such as chemical sensing and membranes is inhibited by a lack of methods for growing them as thin films. Here, layer‐by‐layer (LBL) and solvothermal growth of “nbo” films is demonstrated and it is established for the first time that interlinker steric hindrance is a critical factor in determining the effectiveness of the LBL method. Film growth is demonstrated for three “nbo” MOFs: NOTT‐100 and NOTT‐101, which have the R‐3m space group and are deposited by the LBL method, and PCN‐14, with the R‐3c space group, which is deposited by a solvothermal approach. Continuous and dense films of NOTT‐100 and NOTT‐101 are obtained and LBL growth is verified by observing deposition with a quartz crystal microbalance technique, which also yields the temperature dependence. Oxygen plasma treatment is found to be a useful tool for promoting the MOF film growth under solvothermal conditions. Effective mechanical coupling of these films to the substrate is demonstrated by growing them on surface acoustic wave sensors, which respond reversibly to vapors of water, acetone, and n‐hexane.
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