Applications of Molecular MS

The applications for molecular MS have grown enormously in recent years. MS (molecular and atomic) is used by clinical chemists, pharmaceutical chemists, synthetic organic chemists, petroleum chemists, geologists, climatologists, molecular biologists, marine biologists, environmental 

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The corresponding liquid-phase chemistry can be used to promote ion formation by appropriate choice of solvent and pH, salt addition to form M.Na+ or M.NH4+, and postcolumn addition of reagents. The primary applications of ESI-MS are in the biopolymer field. The phenomenon of routine multiple charging is exclusive to electrospray, which makes it a very valuable technique in the fine chemical and biochemical field, because mass spectrometers can analyse high-molecular-mass samples without any need to extend their mass range, and without any loss of sensitivity. However, with ESI, molecules are not always produced with a distribution of charge states [137], Nevertheless, this phenomenon somehow complicates the determination of the true mass of the unknown. With conventional low-resolution mass spectrometers, the true mass of the macromolecule is determined by an indirect and iterative computational method. 

Application of HPLC-MS to the analysis of a black tea liquor was studied in a paper by Bailey 39 a great deal of useful information could be obtained without sample pretreatment. A tea liquor was applied to a wide-pore HPLC column connected to a mass spectrometer by a VG Plasmaspray interface. Pseudo-molecular ions were obtained from the flavanols, flavanol gallates, chlorogenic acids, 4-coumarylquinic acids, and caffeine, but the flavanol glycosides were extensively fragmented by the interface. Fragments were obtained from unresolved polymer that supported its previous designation as a flavanol polymer. 

The gas phase limitation imposed by the very nature of the GC-MS experiment limits its applicability to chemical space. The Venn diagram in Fig. 19.4 illustrates an approximate mapping of applicability of the combined chromatography-mass spectrometry techniques discussed here. Using only relative molecular mass and polarity as two dimensions that can map chemical space, the figure indicates that the applicability of GC-MS is limited to those relatively small, thermally stable, nonpolar compounds that can be readily volatized (or made to volatilize by derivatization [26]) to pass through a gas chromatographic column. 

Several additional useful publications demonstrating practical applications of CE/MS methods for neurotransmitter analysis and neuropharmaceutical studies are those of Larsson and Lutz (2000) (neuropeptides including substance P) Hettiarachchi et al. (2001) (synthetic opioid peptides) Varesio et al. (2002) (amyloid-beta peptide) Zamfir and Peter-Katalinic (2004) (gangliosides) Peterson et al. (2002) (catecholamines and metanephrines) Cherkaoui and Veuthey (2002) (fluoxetine) and Smyth and Brooks (2004) (various lower molecular weight molecules including benzodiazepines, steroids, and cannabinols). 

Some newer applications of FAC-MS have been described, which suggest that the technology has a strong role to play in the drug discovery process for an extremely broad range of molecular interactions. 

During the last decade, research efforts in the field of LC-MS have changed considerably. Technological problems in interfacing appear to be solved, and a number of interfaces have been found suitable for the analysis of flavonoids. These include TSP, continuous-flow fast-atom bombardment (CF-FAB), ESI, and APCI. LC-MS is frequently used to determine the occurrence of previously identified compounds or to target the isolation of new compounds (Table 2.11). LC MS is rarely used for complete structural characterization, but it provides the molecular mass of the different constituents in a sample. Then, further structural characterization can be performed by LC-MS-MS and MS-MS analysis. In recent years, the combination of HPLC coupled simultaneously to a diode-array (UV-Vis) detector and to a mass spectrometer equipped with an ESI or APCI source has been the method of choice for the determination of flavonoid masses. Applications of LC-MS (and LC-MS-MS) in flavonoid... 

Kerns and co-workers demonstrated the application of LC/MS and LC/MS/MS standard method approaches in preclinical development for the metabolite identification of buspirone, a widely used anxiolytic drug (Kerns et al., 1997). The success of this method relies on the performance of the LC/MS interface and the ability to generate abundant ions that correspond to the molecular weight of the drug and drug metabolites. The production of abundant molecular ions is an ideal situation for molecular weight confirmation because virtually all the ion current is consolidated into an adduct of the molecular ion (i.e., [M+H]+, [M+NH3]+). 

The following discussion will be concerned primarily with applications of the ms/ms technique in the synfuel area. Attempts will be made to illustrate the unique capabilities of the ms/ms analysis with examples taken from our work on coal liquefaction products. Figure 5 shows the positive ion chemical ionization (PCI) mass spectrum of the coal liquid in question (SRC II mid heavy distillate, total bottoms). This spectrum is actually the normalized sum of approximately 500 individual mass spectra taken while the SRC II was thermally vaporized from a solids probe into the source of a mass spectrometer, and represents the molecular weight profile of this distillate fraction. Since isobutane Cl gives to a first approximation only protonated molecular ions (and no fragment ions), the peaks represent the individual components in the SRC II arranged incrementally by molecular weight. 

NM R experiments (ID and 2D, homonuclear and heteronuclear) are the preeminent techniques for the determination of molecular structures. However, careful application and analysis of mass spectral data can provide sufficient information to postulate tentative structures. In this respect, the application of tandem MS experiments, sometimes in conjunction with selective derivatization of the unknown compound, can be very informative about the stmcture. The high-resolution mass spectral data are critical to the support of NMR-deduced stmctures by providing molecular formulae for unknowns. 

As was first described in 1997 [4], the application of MALDI MS to the direct analysis of proteins in tissue sections is illustrated in Figure 12.1. The general procedure is to obtain a thin section of a sample of interest, apply matrix (which is necessary for the MALDI process), and then acquire mass spectra at discrete x-, y-coordinates over the entire sample section. Each individual mass spectrum contains signals corresponding to proteins (and other endogenous compounds, including peptides, lipids, and metabolites) present in the sample at a unique set of coordinates. The intensities of any given molecular species may then be plotted as a function of position on the sample surface. 

Zhao, I.Y. et al., Application of MALDI-MS/MS molecular imaging software for small molecule profiling in tissue samples, Proceedings of the 51st ASMS Conference, Montreal, Quebec, Canada, 2003. 

The application of FD MS to the sodium salts of 147a-c has shown [M + Na] + ions and intense doubly charged ions, which enabled identification of their molecular weights109. 

Other Techniques. A growing technique related to lc/ms and regarded as complementary to it is that of capillary zone electrophoresis/mass spectrometry (cze/ms) (22). Using cze/ms, high resolution separation of water-soluble compounds is accompHshed by the principles of electrophoresis (qv). The sample is then coupled to the mass spectrometer by electrospray ionization (23) or a fast atom bombardment interface (fab) to produce molecular ions (24). Biotechnology applications of cze/ms have great potential (25). 

The application of GC-MS to PAH-analysis will allow considerable simplification of the work-up procedure. As far as solubility and dynamic range considerations will permit, the major PAH in airborne particulate matter samples can be accurately measured in the electron impact single or multiple ion monitoring mode, using the molecular ions of the respective PAH as specific ions, in the presence of much larger amounts of aliphatic hydrocarbons and carboxylic acids. Polar and ionic material is first removed from the combined benzene-methanol extract by liquid-liquid partition in water-diethylether. After drying, the addition of diazomethane results in derivatisation of the acidic components and the sample can be injected onto the column (Van Vaeck and Van Cauwenberghe (30)). 

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