In many cases, an unknown to an investigator is actually known in the chemical literature,
a reference database, or an internet resource. We refer to these types of compounds
as "known unknowns." ChemSpider is a very valuable internet database of known compounds
useful in the identification of these types of compounds in commercial, environmental,
forensic, and natural product samples. The database contains over 26 million entries
from hundreds of data sources and is provided as a free resource to the community.
Accurate mass mass spectrometry data is used to query the database by either elemental
composition or a monoisotopic mass. Searching by elemental composition is the preferred
approach. However, it is often difficult to determine a unique elemental composition
for compounds with molecular weights greater than 600 Da. In these cases, searching
by the monoisotopic mass is advantageous. In either case, the search results are refined
by sorting the number of references associated with each compound in descending order.
This raises the most useful candidates to the top of the list for further evaluation.
These approaches were shown to be successful in identifying "known unknowns" noted
in our laboratory and for compounds of interest to others.
© American Society for Mass Spectrometry, 2011