Mass spectrometry spectral libraries

Maximum benefit from Gas Chromatography and Mass Spectrometry will be obtained if the user is aware of the information contained in the book. That is, Part I should be read to gain a practical understanding of GC/MS technology. In Part II, the reader will discover the nature of the material contained in each chapter. GC conditions for separating specific compounds are found under the appropriate chapter headings. The compounds for each GC separation are listed in order of elution, but more important, conditions that are likely to separate similar compound types are shown. Part II also contains information on derivatization, as well as on mass spectral interpretation for derivatized and underivatized compounds. Part III, combined with information from a library search, provides a list of ion masses and neutral losses for interpreting unknown compounds. The appendices in Part IV contain a wealth of information of value to the practice of GC and MS. 

METHOD REPRODUCIBILITY AND SPECTRAL LIBRARY ASSEMBLY FOR RAPID BACTERIAL CHARACTERIZATION BY METASTABLE ATOM BOMBARDMENT PYROLYSIS MASS SPECTROMETRY... 

To record a mass spectrum it is necessary to introduce a sample into the ion source of a mass spectrometer, to ionize sample molecules (to obtain positive or negative ions), to separate these ions according to their mass-to-charge ratio (m/z) and to record the quantity of ions of each m/z. A computer controls all the operations and helps to process the data. It makes it possible to get any format of a spectrum, to achieve subtraction or averaging of spectra, and to carry out a library search using spectral libraries. A principal scheme of a mass spectrometer is represented in Fig. 5.2. To resolve more complex tasks (e.g., direct analysis of a mixture) tandem mass spectrometry (see below and Chapter 3) may be applied. 

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... 

Modem mass spectral databases allow the automated searching of very extensive mass spectral libraries.6 This has made the identification of compounds by mass spectrometry a far more straightforward task. One must understand, however, that such databases are no substitute for the careful analysis of each mass spectrum and that the results of database matchup are merely suggestions. 

To many, mass spectrometry is synonymous with El mass spectrometry. This view is understandable for two reasons. First, historically, El was universally available before other ionization methods were developed. Much of the early work was El mass spectrometry. Second, the major libraries and databases of mass spectral data, which are relied upon so heavily and cited so often, are of El mass spectra. Some of the readily accesible databases contain El mass spectra of over 390,000 compounds and they are easily searched by efficient computer algorithms. The uniqueness of the El mass spectrum for a given organic compound, even for stereoisomers, is an almost certainty. This uniqueness, coupled with the great sensitivity of the method, is... 

MS/MS is an empirical method of analysis. As is evident from the exanples presented, the interpretation of a daughter ion MS/MS spectrum is often based on the same correlation principles derived frcm electron and chemical ionization mass spectrometry. More often, the comparison of the spectrum obtained to that of the authentic capound is used for identification. This is a fundamentally unsatisfying procedure. While electron and chemical ionization spectra can be compared to a spectral library which has been compiled over the past thirty years, no comparable library of MS/MS spectra exists. Data systems may be used within individual... 

Although individual laboratories find it useful to compile their own reference library files, access to very large collections of mass spectra and to published data [55] is essential. A compilation of many thousands of spectra by the Aldermaston Mass Spectrometry Data Centre and the Division of Computer Research and Technology at the National Institutes of Health [56-58] has been made available commercially. The file can be searched in a number of ways using an interactive conversational mass spectral search system via a teletype and acoustic link over telephone lines. 

Photoionization is not a common technique in mass spectrometry, but it was reported to be utilized for both Py-GC/MS [61, 62] and for Py-MS [61]. Compared to El spectra, for which extended mass spectral libraries are available, the photoionization (PI) spectra are not standard and therefore not library searchable. However, the ionization being softer, the molecular ion is commonly more intense providing additional information in certain cases when the El mass spectrum is not diagnostic. 

The common procedure to generate silylated pyrolysates is to perform pyrolysis in a filament system followed by off-line derivatization with BSTFA. The chromatographic separation was done on a DB-5 column (60 m long, 0.32 mm i.d., 0.25 pm film thickness) using a temperature gradient between 50° C and 300° C with detection by mass spectrometry. The compounds identified by mass spectral library search in the pyrograms from Figures 12.3.3 and 12.3.4 are listed in Table 12.3.2. 

In the modern analytical laboratory, gas chromatography-mass spectrometry (GC-MS) is a vital tool in the characterization and identification of unknowns. The advantages of GC-MS are accuracy in quantitation, low detection limits, tentative identification of unknowns by spectral library search, and a high degree of reliability and versatility ( 1). Of all the chemicals known, however, only a fraction (=20 ) are amenable to analysis by GO. The remaining compounds, because of their high molecular weight, thermal instability, or ionic and/or polar character, are not suited to direct GC determination. 

W. Ion suppression effects in liquid chromatography-electrospray-ionization transport-region collision induced dissociation mass spectrometry with different serum extraction methods for systematic toxicological analysis with mass spectral libraries. J Chromatogr B 2002 773 47-52. 

Sanders, M. Josephs, J., Schwartz, J. Tymiak, A. DiDonato, G. Rapid Identification of Natural Products Using a Modified Ion Trap Mass Spectrometer and MS/MS Spectral Library Searching, in Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, Dallas, Texas, June 13-17, 1999. 

It is often difficult to determine the degree to which the chemistry proceeded on the entire library population and whether peaks in a mass spectrum are due to the product, side reactions, reagents, solvents, or impurities. Diversity Sciences developed mass-spectral methods to distinguish all components that are cleaved from a solid support and implemented the method into the analytical construct. While early studies demonstrated promising results for fragmentation methods with tandem mass spectrometry (MS/MS), stable isotopes were routinely implemented as signature peaks for the identification of compounds that are produced from solid-phase reactions [27]. 

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