David Sparkman and Steve Stein showed me some new exact mass search capabilities introduced with the NIST 11 Library at ASMS conference in Denver. They thought that these new capabilities might be useful in the identification of “known unknowns.”
An unknown to an investigator, in many cases, is often known in the chemical literature. We refer to these types of compounds as “known unknowns.” The term originated from a quote by Donald Rumsfeld:
Recent changes in the software allow one to search by accurate mass values with an associated error window. The results allow one to display and sort the number of synonyms or associated databases. Often the most likely component would be the one with the highest numbers for these fields. Steve Stein refers the importance of this type of information in the identification of unknowns as “prior probability.”
I evaluated the software using a group of compounds that I had employed previously to evaluate ChemSpider and CAS. The results were very encouraging. I have attached my initial results and some instructions for searching Warfarin’s exact mass. The approach was very successful for drugs, pesticides, natural products. However, many polymer antioxidants and UV stabilizers were not found. Thus, the choice of which database to search, NIST/ChemSpider/or CAS, will be dependent on the user’s needs.
The molecular weight distribution is somewhat lower for the NIST database than either the CAS Registry or Chemspider. Thus the latter two databases will probably be more useful for higher molecular weight compounds found in LC-MS analyses.
We heavily use NIST for searching both our EI and MS/MS and searching commercial EI and MS/MS databases. The current version of NIST includes 64,511 MS/MS spectra that can be very useful for identifying “known unknowns.” Furthermore, we routinely run the same sample by LC-MS and GC-MS and utilize EI spectra in addition to our in-source MS/MS spectra.
Here is a file showing how to use the new NIST exact mass search for identifying a “known unknown” by accurate mass. Of course, one can also search by molecular formula.
Here are the results from my evaluation with 90 compounds:
Links for our two papers describing our work with CAS and ChemSpider are shown below: