Ionization methods, mass identification

Metastable atom bombardment (MAB) is a novel ionization method for mass spectrometry invented by Michel Bertrand s group at the University of Montreal, Quebec, Canada, and described by Faubert et al.38 For the identification of bacteria by MS, MAB has a number of significant advantages relative to more familiar ionization techniques. Electron ionization (El) imparts so much excess energy that labile biomolecules break into very small fragments, from which the diagnostic information content is limited since all... 

Recently, a quantitative electrospray ionization/mass spectrometry method (ESI/MS) has been developed to analyze the molecular profile, or hpidome of different lipid classes in very small samples. In this method, total lipid extracts from tissues or cultured cells can be directly analyzed. By manipulating the ionization method, the mass spectrographs of polar or even non-polar lipids can be obtained [8]. This method and the use of lipid arrays allow precise and quantitative identification of the lipid profile of a given tissue, and map functional changes that occur. 

Chromatographic methods are used to separate the components in a mixture, but in a complex mixture, a single chromatographic method or step many not separate all components. In these cases, using simple retention time to identify the components will not suffice and the identification of components in the mixture will be incorrect. Thus, the addition of a method of identification such as mass spectrometry (MS) or Fourier transform infrared (FTIR) is essential. In some cases, it may even be necessary to confirm either an FTIR or MS identification by the same method applied in a different way. For example, FTIR may be followed by MS, or electron ionization (El) MS followed by chemical ionization (Cl) MS or by an entirely different method. 

To summarize, mass spectrometry has successfully been used for the identification of compounds containing a Zn—C bond, which have a large diversity of structures and complexity. These complexes have been subjected to different ionization methods (such as El, Cl, EAB and ESI) and in many cases they generated numerous Zn-containing fragment ions. Under soft (Cl or FAB) experimental conditions, some of these compounds produced protonated molecules [M-t-H]" " or even protonated dimerlike species [M2H — R]+. Electron ionization was successful for the characterization of many volatile Zn-containing compounds. Peaks of molecular ions M+ were frequently observed, but the majority of the mass spectra were dominated by Zn—C bond dissociation products. 

With increasing energy of the incident photons the photoionization process is accompanied by a rupture of valence bonds, leading to various ionized fragments, the identification of which requires complementary methods of analysis. During the last decade much progress has been achieved in the mass spectrometry of the photoionization products in various diatomic or polyatomic gases under vacuum u.v. irradiation. 

Without authentic standards or a spectroscopic detection scheme, compound identification is difficult. Our choice of a desorption/ionization method is potassium ionization of desorbed species (K+IDS) with mass spectrometric detection (n. 12) which provides a rapid qualitative tool for compound identification. Using K+IDS, molecular weight data is available and fragmentation is minimal. Ions appear as M[K], the mass of the analyte plus 39 Da, the mass of potassium. Hence, structure identification is possible based on a knowledge of starting materials and the molecular weight data afforded by K+IDS. 

A common method for identification of organic compounds is mass spectrometry (MS) in combination with GC. After separation of the component by GC the mass spectrometer transform the analyte into gaseous ions in vacuum in the ion source. For electron impact ionization this results in different mass fragmentation patterns with different mass-to-charge ratios (m/z). From this fragmentation pattern it will be possible to identify the compound by comparison with commercial mass spectral libraries. Identification of unknown compounds can be facilitated by... 

They generally lack sensitivity and a direct relation to molecular structure. GC-MS is fast, direct, and very sensitive, and the spectrum provides a result which puts identification beyond dispute. Computer-assisted systems are now available which embody extensive drug reference libraries and can be automatically searched to identify unknown spectra. The further development of chemical ionization and mass fragmentography methods using stable isotopes now permits very accurate quantitative work. 

A sensitive liquid chromatography/ electrospray ionization tandem mass spec-trometrical (LC/ESI-MS/MS) method was developed for the identification and quantification of two polyisoprenylated benzophe-nones, xanthochymol and isoxanthochymol, in the extracts of the fruit rinds, stem bark, seed pericarps and leaves of G. indica and in... 

The widely used strategy for protein identification is depicted in Figure 8.15. This strategy thus is to cleave the protein either by trypsin, V8 protease, Lys-C endoprotease or by a reactant such as BrCN. The mixture then is analysed by mass spectrometry to obtain the molecular masses of the largest possible number of peptides. The two ionization methods, MALDI and ESI, are used. MALDI is best to use if one wants to avoid chromatographic separation, because it yields very simple spectra, has a better sensitivity and is not so sensitive to the presence of contaminants. However, ESI can be coupled directly with HPLC or capillary electrophoresis (CE) if a separation is wanted. Furthermore, ESI is often used with mass spectrometers that allow MS/MS data to be easily obtained. 

Tandem mass spectrometry (MS/MS) is a new analytical technique applied to problems in food and flavor analyses. Rapidity of analysis, a high discrimination against chemical noise, and the ability to analyze mixtures for functional groups are attributes of MS/MS that make it attractive for such problems. Sanple collection and pretreatment differ frcm methods used in GC/MS. Correct choice of an ionization method is paramount. Daughter ion MS/MS spectra are used for conpound identification via comparison with those of authentic compounds, and parent and neutral loss spectra are useful in functional group analysis. Applications to direct analysis of volatiles emitted from fruits and to spice analyses are considered. 

Mass spectrometers are often used in combination with other instruments. Since a mass spectrometer is an identification instrument, it is often paired with a separation instrument like a chromatograph. Sometimes two mass spectrometers are paired, so that a mild ionization method can be followed by a more vigorous ionization of the individual fragments. 

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