Mass Spectral Library

S. E. Stein, D. R. Scott,/. Am. Soc. Mass Spectrom. 1994, 5, 856-866. Optimization and testing of mass spectral library seardi algorithms for compound identification. 

The National Institutes of Health-EPA mass spectral library is used to identify analyzed components of a sample by comparing their mass spectra with those of authentic specimens held in the library. 

We thank the DuPont Company for providing the resources for the preparation of this book. VG Fisons and NIST also have graciously permitted the use of spectra from their mass spectral libraries. Roger Patterson,... 

Comparison with mass-spectral libraries is the easiest way of interpreting mass spectra (and often the only way for non-mass spectroscopists). The NIST/EPA/NIH (NIST 02) electron-ionisation mass spectral library... 

The spray paint can was inverted and a small amount of product was dispensed into a 20 mL glass headspace vial. The vial was immediately sealed and was incubated at 80°C for approximately 30 min. After this isothermal hold, a 0.5-mL portion of the headspace was injected into the GC/MS system. The GC-MS total ion chromatogram of the paint solvent mixture headspace is shown in Figure 15. Numerous solvent peaks were detected and identified via mass spectral library searching. The retention times, approximate percentages, and tentative identifications are shown in Table 8 for the solvent peaks. These peak identifications are considered tentative, as they are based solely on the library search. The mass spectral library search is often unable to differentiate with a high degree of confidence between positional isomers of branched aliphatic hydrocarbons or cycloaliphatic hydrocarbons. Therefore, the peak identifications in Table 8 may not be correct in all cases as to the exact isomer present (e.g., 1,2,3-cyclohexane versus 1,2,4-cyclohexane). However, the class of compound (cyclic versus branched versus linear aliphatic) and the total number of carbon atoms in the molecule should be correct for the majority of peaks. 

Mass Spectral Library[35] and the Wiley Registry of Mass Spectral Data[36] are now available to help mass spectrometrists in the identification of unknowns. 

Standard Reference Data Program, NIST/EPA/NIH Mass Spectral Library, Version 2.0f, National Institute of Standards and Technology, Gaithersburg, MD. 

Whatever the analyser and the analytical conditions chosen, the spectrum obtained corresponds to the sum of the spectra of all the individual compounds present in the sample investigated. Spectra have thus to be cautiously interpreted using a set of reference data from single commercial or synthesised compounds, reference raw and aged natural substances and mass spectral libraries. 

Of course one may employ automated library searches ( library percent reports ) to check for compound identities, but algorithms for library matching are not infallible, and mass spectral libraries are not exhaustive, thus some compounds of interest will likely not be identified. Additional dilemmas are presented by mere reliance on retention times and library percent reports to ascertain the presence of common or unique peaks from among multiple mass spectral data files. As illustrated in Table 2.1, the TICs from the GC-MS of urine from four elephants evidence a peak at essentially the same retention time, but the library search results are inconclusive as to their common identity or lack thereof. As will be seen below, our novel macros can assist in making such decisions for a large number of peaks. 

Provided El spectra have been measured under some sort of standard conditions (70 eV, ion source at 150-250 °C, pressure in the order of 10 " Pa), they exhibit very good reproducibility. This is not only the case for repeated measurements on the same instrument, but also between mass spectrometers having different types of mass analyzers, and/or coming from different manufacturers. This property soon led to the collection of large El mass spectral libraries, either printed [76-78] or computerized. [79] The best established El mass spectral databases are the NIST/EPA/NIH Mass Spectral Database and the Wiley/NBS Mass Spectral Database, each of them giving access to about 120,000 evaluated spectra. [80-83]... 

Mass Spectral Library Searches. J. Am. Soc. Mass Spectrom. 1994, 5,316-323. 

GC/MS was the primary tool for identifying the first DBFs, and it remains an important tool for measuring and identifying new DBFs. Large mass spectral libraries (NIST and Wiley databases, which contain >200,000 spectra) enable rapid identifications. When DBFs are not present in these databases, high-resolution... 

A thorough search of the reconstructed ion chromatogram (RIC) was made to determine the mass spectra of all detectable components. The spectra were compared with those of reference standards when available or with the spectra from the mass spectral library. Molecular weights of more abundant constituents are shown in Figure b, many additional minor components were also characterized. A complete list of compounds characterized in this sample by GC-MS is given in Table I. 

When identifying an eluted compound by comparing its mass spectrum with a mass spectral library, false matches are frequent. If retention index is used as a second characteristic, false matches are reduced. 

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... 

LC-MS uses different types of soft chemical ionization that produces molecular ions and no fragmentation pattern. In MS/MS instruments the molecular ions can be fragmented by collision with a gas for example, He. This fragmentation can be used for identification of a compound. No mass spectral libraries exist for LC-MS hence identification of unknown compounds is more time-consuming than for GC-MS. For known compounds LC-MS is a very sensitive and specific method, using LC-MS/MS systems the analytical performance can be increased even more. LC-MS analysis is especially suitable for non-volatile POMs such as non-ionic surfactants in house dust samples (Clausen et al., 2003). 

Generally, the mass spectra corresponding to peaks in the gas chromatograms obtained in the El mode at 70 eV are assigned by comparisons with mass spectral libraries. The fragments were assumed to be characteristic of the original structure (Galletti and Bocchini, 1995 Schulten, 1996). However, one often-described disad-... 

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. 

NIST Standard Reference Database 1A, NIST/EPA/NIH Mass Spectral Library with Search Program (Data Version NIST 02, Software Version 2.0). 

Marquet, P., N. Venisse, E. Lacassie, et al. 2000. In-source CID mass spectral libraries for the general unknown screening of drugs and toxicants. Analusis 28 925-934. 

The identification of the lipids is a very challenging task. The lack of comprehensive mass spectral libraries often limits the identification of compounds in LC-MS and shotgun methods. Some spectral libraries are available, such as the Human Metabolome Database (http /www.hmdb.ca), the METLIN Metabolite Database (http /metlin.scripps.edu) (24), and the MassBank (http /www. massbank.jp) (25). However, construction of universal spectral databases for API-MS is challenging due to the poor reproducibility and high interinstru-ment variability of fragmentation patterns. 

There are a variety of techniques available to aid in the identification of sample components matching retention times, standard addition, internal standard, isotopic labeling, enzyme peak shift, and UV and mass spectral libraries. 

Hopley C, Bristow T, Lubben A, Simpson A, Bull E, Klagkou K, Hemiman J, Langley J (2008) Towards a universal product ion mass spectral library - reproducibility of product ion spectra across eleven different mass spectrometers. Rapid Commun Mass Spectrom 22 1779-1786... 

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