The NIST Mass Spectral Database: Extending the Evaluation

S.E. Stein, A. Mikaya, Jane Klassen, Zhu Damo (Guest Researcher), D. Tchekhovskoi (Contractor), C.L. Clifton, and W.G. Mallard

Objective: To provide a fully evaluated mass spectral database with tested and documented search algorithms that will enable the positive identification of unknown organic compounds using gas chromatography/mass spectrometry (GC/MS).

Problem: Modern organic analytical chemistry is critically dependent on instrumental analysis. For qualitative analysis, there is no better tool than the mass spectrometer. Like many other analytical techniques, it is best used with a library of reference spectra. Even with good reference spectra, the data must be processed correctly. Robust algorithms that have been thoroughly tested to eliminate flaws are needed.

Approach: The ongoing work after the release of the NIST mass spectral data base, NIST 98, will add new fully evaluated mass spectra to the database of mass spectra with 129,136 evaluated spectra for 107,886 compounds. New data from laboratory work at NIST, spectra purchased from commercial firms, and contributed spectra are being added. As in prior years, emphasis is being placed on the evaluation of the most important spectra and on improvements of the search software.

Results and Future Plans: The addition of the over 17,000 spectra from the National Institute of Materials and Chemical Research of Japan is an important milestone. These spectra are of the highest quality and are primarily focussed on common compounds. The evaluation has been done using the same techniques established for NIST 98. The data were examined for reasonable neutral losses, for air peaks, for impurities, and for errors in transcription. In every case where a change had to be made, agreement between at least two evaluators was required. The analysis was always conservative; if a given spectrum or spectral feature was not clearly in error, it was not changed. Evaluation is continuing on the spectra obtained from a number of chemical suppliers.

Development of algorithms to aid in the evaluation, as well as to provide users of the NIST software with tools for analyzing spectra from compounds that are not in the database, is ongoing. The software needed for the basic searching has been steadily improved. The new release adds a number of features that make it easier to use the proven NIST algorithms. In addition, the ability to add user-drawn structures and synonyms to the user data has been implemented.

Work is continuing on the addition of retention indices to the database. The retention index is a measure of the time it takes a compound to elute from the gas chromatography column. For many compounds, which might otherwise be confused just using the mass spectrum, the retention index makes it possible for a more definitive positive identification to be made. An example of an important class of compounds for which the retention time data is especially useful is the hydrocarbons, many of which have very similar mass spectra. The first stage of compiling and entering the data has begun. Tools are being developed for both evaluation of the data and for prediction of retention times for compounds for which data are not available.

Publications:

Ausloos, P., Clifton, C., Lias, S.G., Mikaya, A., Sparkman, O.D., Stein, S.E., Tchekhovskoi, D., Zaikin, V., and Zhu, D., "The Critical Evaluation of a Comprehensive Mass Spectral Library," J. Amer. Mass. Spect. 10, 287 (1999).