Desorption techniques, mass spectrometry

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

The most discriminating technique for proving the identity and purity of analyte peak of a chromatogram, especially for analyzing biological samples and natural products, is by using online LC-UV/MS or GC-MS/FTIR methods [15]. Alternatively, one could use a combination of TLC and MS, where direct determination on the TLC plates is made by matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) [16]. 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

Other MS based analytical approaches have occasionally been applied to ancient resin samples, in particular for paint varnishes. Such techniques include FABS (fast atom bombardment mass spectrometry) [35], MALDI (matrix assisted laser desorption-ionization mass spectrometry) and GALDI (graphite assisted laser desorption-ionization mass spectrometry) [36 38]. 

Two recently developed mass spectrometric techniques have had a major impact on the analysis of large biomolecules matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS). MALDI-MS was first introduced by Karas and Hillenkamp66 and Tanaka et al.61 in 1988 and has experienced an exponential development. It has become a widespread soft ionization technique for bioorganic samples, especially large biomolecules. Fenn and co-workers68 first published the successful soft ionization technique for... 

Separation techniques in hyphenation with elucidation of the structure of organometallic compounds [electrospray ionization-mass spectrometry (ESI-MS) and matrix assisted laser desorption ionization-mass spectrometry (MALDI-MS)]... 

Kussmann M, Nordhoff E, Rahbek-Nielsen H, Haebel S, Rossel-Larsen M, Jakobsen L, Gobom J, Mirgorodskaya E, Kroll-Kristensen A, Palm L, Roepstorff P (1997) Matrix-assisted laser desorption/ionization mass spectrometry sample preparation techniques designed for various peptides and protein analytes. J Mass Spectrom 32 593-601... 

Burrell M, Earnshaw C, Clench M (2007) Imaging matrix assisted laser desorption ionization mass spectrometry a technique to map plant metabolites within tissue at high spatial resolution. J Exp Bot 58(4) 757-763. doi 10.1093/jxb/erll39... 

FAB is most often compared to the soft ionization method known as field desorption (FD) mass spectrometry, a technique in which the sample, deposited on an emitter wire coated with microcrystalline carbon needles, is desorbed under the influence of a high electric field gradient. As usual, bioorganic systems are best represented by both techniques (21, 33). Though FAB is the easier of the two, they are complementary, FAB being particularly suited for the case of extreme thermal lability and FD for the case of chemical lability or matrix interference. Cerny et al. (33) compare the two techniques for the study of coordination complexes and conclude FD is better for molecular-ion determination, while FAB provides better fragmentation information, which is useful in elucidating structures. 

Larhrib, H. Wells, M.H. Rubinstein, M.H., et al. Characterization of PEGs using matrix-assisted laser desorption/ ionisation mass spectrometry and other related techniques. Int. J. Pharm. 1997, 14, 187-198. 

Other techniques, such as capillary electrophoresis and [matric-assisted laser desorption ionization] mass spectrometry, provide qualitative analyses, often with greater speed and sensitivity. Nevertheless, AAA by high-performance liquid chromatography (HPLC) complements other structural analysis techniques, such as peptide sequencing, and remains indispensable for quantifying proteinaceous materials. 

Selva and coworkers - reported on their experiences to apply various mass spectrometric techniques to the analysis of -carotene and carotenoids and their adducts formed in aqueous solution. El mass spectrometry and field desorption (FD) mass spectrometry were applied to aqueous mixtures of -carotene and /J-cyclodextrin, and the polyene was found to be detectable . Tandem mass spectrometry can be applied to identify fi-carotenone as a minor component in complex carotenoid mixtures. EI/MIKE spectrometry of the molecular ion m/z 600) was used in this case . A previous study was focused on the characterization of. vecw-carotcnoids using EI/MIKE and CID spectrometiy . The more recent ionization methods, viz. MALDI and its variant working without a matrix, laser desorption/ionization (EDI), as well as electrospray ionization (ESI) mass spectrometry, have also been applied to this topic. MALDI and LDI mass spectrometry were used to analyse mixtures of -carotene and y-cyclodextrin in aqueous solution. Adduct ions were not observed using these mctbods. ... 

S. Hillenkamp, F De Vries, M.S. Comparative Mass Spectrometric Analyses of Photofrin Oligomers by Fast Atom Bombardment Mass Spectrometry, UV and IR Matrix-Assisted Laser Desorption/lonization Mass Spectrometry, Electrospray Ionization Mass Spectrometry and Laser Desorption/Jet-Cooling Photoionization Mass Spectrometry, J. Mass Spectrom. 34, 661-669 (1999). Powell, K.D. Fitzgerald, M.C. Accuracy and Precision of a New H/D Exchange- and Mass Spectrometry-Based Technique for Measuring the Thermodynamic Properties of Protein-Peptide Complexes, Biochemistry 42,4962-4970 (2003). 

Elucidation of degradation pathways and identification of transformation products (TPs) is of crucial importance in understanding their fate in the environment and requires the employment of advanced instrumental techniques. Analytical methods that can be used for this purpose include Uquid chromatography with diode array or fluorescence detector (LC-DAD/FL), nuclear magnetic resonance (NMR), infrared spectroscopy (IR), matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS), gas... 

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