Universal Mass Spectral Libraries

The NIST/EPA/NIH Mass Spectral Library is probably the most popular and most widely distributed library for GC-MS instruments. The 2011 edition has been largely expanded by the number of El (electron-ionization) mass spectra with the addition of Kovats RIs. MS/MS mass spectra are increasingly included. Extensive spectra evaluation and quality control has been involved in the new edition of the NIST database. Each spectrum was critically examined by experienced mass spectrometrists, and each chemical structure has been examined for correctness and consistency, using both human and computer methods (Ausloos etal., 1999). Spectra of stereoisomers have been intercompared, chemical names have been examined by experts and lUPAC names provided. CAS registry numbers have been verified. 

The NIST library is available with a new version of the NIST Mass Spectral (MS) Program (v.2.0g) and the enhanced versions of MS Interpreter and AMDIS, the mass spectral interpretation tools with thermodynamics-based interpretation of fragmentation and chemical substructure analysis. The binary format has not changed from the 2002 version, although several new files have been added that associate equivalent compounds and link individual compounds to the RI library. Raw data files are provided in both an SDFile format (structure and data together) as well in earlier formats. The SDFile format holds the chemical structure as a MOLFile and the data in a simple ASCII format. The NIST MS Search Program is also part of many commercial instrumental GC-MS software suites. 

The 2014 edition of the NIST database now contains 276,248 spectra of 242,466 compounds (mainlib), an increase of 13% from the 2011 version. The library includes also 33,782 replicate spectra (replib) of differing spectral quality. 

Other major enhancements have been made to the prior version including many replacements with higher quality spectra, a thorough review of chemical names and merging of the previous salts library into the main library. 

The contents of the NIST 14 Mass Spectral and RI Libraries are specified as follows  

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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. 

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... 

The application of MS/MS or in-source-CID for identification of pollutants brought problems, since no product ion libraries were available. Product ion spectra either had to be interpreted for compound identification or standard comparison had to be performed provided that standards were available. To overcome this disadvantage, analysts prepared their own libraries suitable for the instrument generated on or for instruments of the same type. Attempts at generating mass spectral libraries for polar compounds determined by API methods were reported and the results obtained with their application in real environmental samples were discussed [291, 292]. The generation of IT-MS product ion spectra and their use for identification provided the most promising results so far. The preparation of a universally applicable product ion library for the identification of polar compounds, i.e. of a library which could work with various equipments for identification, still remains wishful thinking. 

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... 

Cleij, P. Reproducibility of Mass Spectral Peak Intensities as the Basis of an Automated Library Search Method for Identification of Organic Compounds. D.Sc. Dissertation, State University of Utrecht, 1984. 

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