An unknown to an investigator, in many cases, is often known in the chemical literature. We refer to these types of non-targeted species as "known unknowns." The term originated in a much different context in a quote by Donald Rumsfeld.
Researchers have note that the US EPA's CompTox Chemistry Dashboard could be more useful than ChemSpider for the identification of "known unknown" using accurate mass data. Using the approach discussed below is particulary important when reference EI or MS/MS spectra are not present in commercial mass spectral libraries.
"The performance of the Dashboard for identifying known unknowns was evaluated against that of the online ChemSpider database, one of the primary resources used by mass spectrometrists, using multiple previously studied datasets reported in the peer-reviewed literature totaling 162 chemicals. These chemicals were examined using both applications via molecular formula and monoisotopic mass searches followed by rank-ordering of candidate compounds by associated references or data sources. A greater percentage of chemicals ranked in the top position when using the Dashboard, indicating an advantage of this application over ChemSpider for identifying known unknowns using data source ranking."
The link to the Dashboard is shown below. Be sure for accurate mass data to use the pull down menu for "Search/Advanced Search."
The number of structures in ChemSpider is about 120 M and SciFinder/CAS Registry is 275 million. Much more than the 1.3 million chemicals in the US EPA's CompTox Chemistry Dashboard.
Thus, links to these approaches on my website should still be useful.