We described mass spectral libraries and their uses in this forum in 1994 (1), and we can assume that at least a few things
have changed in the intervening 15 years. Certainly the standard mass spectral libraries contain more archived spectra, the
computers used to search the libraries are much faster, and the software is more sophisticated. In the larger perspective,
mass spectrometry (MS) is now applied to a much wider variety of analytical problems than before, and more types of mass spectra
(using different ionization methods) are recorded. Major advances in instrumentation means that more accurate mass measurements
are recorded, leading to information that more accurately predicts the ion empirical formula. Overall, the basic use of a
mass spectral library remains the same, and that is to assist in the identification of molecular structure, and the identification
of a particular compound, via a match between a measured mass spectrum and the mass spectrum stored within the library for
the known compound. It has often been suggested (informally at least) that the breadth of modern spectral libraries and the
speed with which they can be searched will result in a decay of both basic and advanced spectral interpretation skills on
the part of the analyst. Phrased within a simple question: "Why interpret when you can just search?" There is an apparent
push toward search rather than interpretation. This push is not so much driven by the increased speed of the library search
with modern data systems but is instead catalyzed and amplified by the fact that mass spectra are acquired much faster than
before, and hundreds of samples (if not more) can be analyzed in a single day. Although it can seem that there is little time
for thoughtful spectral interpretation, interpretation short courses such as those offered annually at ASMS remain full of
those wanting to learn the skill. There are certain applications, such as forensic MS, in which the truly unknown nature of
some of the samples can reduce the usefulness of a library and magnifies the interpretive skill of the analyst. Interpretive
acumen is a valuable skill that will never be replaced.
Where do libraries of mass spectral data get their content? In the early days, mass spectra were shared freely among the members
of the user community. The informal collection process soon evolved into a standard collection process and format, which included
an evaluation of the spectral quality (and therefore the propriety for inclusion in the library). Users were encouraged to
submit mass spectra recorded for new compounds, and many service laboratories in academic settings did so over a period of
many years. However, as the libraries grew, the newly contributed mass spectra were more and more likely to be unique to a
particular synthesis or research project and perhaps less likely to be of interest to members of the broader community, who
were unlikely to encounter that particular compound themselves. The incremental cost for including the additional mass spectra
was very low, however, and each new spectral–structure correlation could, in theory, be used to confirm and extend, or offer
an exemption to, the standard "rules" of spectral interpretation. Concurrent with the growing importance of MS in support
to the research efforts of the pharmaceutical industry, it also became clear that valuable competitive information could be
gleaned from the mass spectra submitted by one's colleagues (and competitors). The uninhibited flow of mass spectral contributions
into libraries slowed, and the dissemination of mass spectral information from the base of industrial users became much more
tightly controlled. Within a very short time, the contribution of new data became almost the exclusive venue of the academic
laboratories, and even there, the implications for potential proprietary information made an impact. Finally, the electron
ionization (EI) mass spectra that formed the core of the original mass spectral libraries were supplanted by mass spectra
recorded with other ionization methods including chemical ionization and later electrospray ionization mass spectra. Large,
complex sets of data from gas chromatography (GC)–MS, liquid chromatography (LC)–MS, and MS-MS also needed to be accommodated
within a modernized format and structure. Annotating and transferring such large data sets was cumbersome and awkward, and
to this day, we still have not fully managed the glut of mass spectral data produced by our analytical instruments.
