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One of the greatest misconceptions in analytical chemistry is that every unknown compound is completely new. In reality, many unknowns encountered in environmental, pharmaceutical, forensic, industrial, and natural product analyses have already been reported somewhere in the chemical literature or exist within one of the world's chemical reference databases. These compounds are unknown to the investigator but known to science. We refer to these compounds as "known unknowns."
The challenge is not discovering a new compound—it is finding the correct one among hundreds of millions of possibilities. Modern high-resolution mass spectrometry, combined with trusted chemical databases and systematic interpretation, provides an efficient path to that answer. The approaches listed below are particulary important when reference EI or MS/MS spectra are NOT present in commercial mass spectral libraries.
A Systematic, Evidence-Based Approach
Successful identification rarely depends on a single measurement. Instead, it requires combining multiple independent pieces of evidence to progressively narrow the list of possible candidates.
Typical evidence includes:
- Accurate molecular mass
- Elemental composition
- Isotopic pattern and isotope ratios
- Substructural information
- MS/MS or EI fragmentation
- Sample history and chemistry
- Associated Databases and literature references
- Orthogonal analytical information
Rather than relying on one "magic" search, each piece of information increases confidence until only the most probable identification remains.
Spectral and "Spectraless" Databases
Many scientists immediately think of spectral libraries such as NIST when identifying unknowns. These libraries remain invaluable whenever experimental spectra are available for comparison.
Equally important, however, are very large "spectra-less" chemical databases such as CAS Registry, SciFinder, and ChemSpider. Although these resources generally do not contain searchable EI or LC-MS/MS spectra, they contain enormous collections of known chemical structures, literature references, patents, synonyms, and associated chemical information. They can often be searched using molecular formula, monoisotopic mass, average molecular weight, elemental composition, or structural constraints. Candidate lists can then be refined using reference counts, keywords, substructure searching, fragmentation, isotopic evidence, and sample context. This combination of spectral and non-spectral resources is frequently the key to identifying difficult "known unknowns."
Why Accurate Mass Matters
High-resolution accurate mass measurements dramatically reduce the number of possible elemental compositions and candidate structures. When combined with isotope ratio measurements and molecular ion confirmation, they provide a powerful starting point for database searching.
Accurate mass alone is seldom sufficient for a definitive identification. Confidence comes from integrating accurate mass with fragmentation behavior, isotopic fidelity, chromatography, chemical knowledge, and information retrieved from trusted databases.
The Goal
The objective is not simply to obtain a database hit—it is to develop an identification supported by multiple, independent lines of evidence. By integrating high-quality accurate mass measurements, isotope ratios, fragmentation, chromatography, trusted databases, literature information, and chemical reasoning, investigators can confidently identify many compounds that initially appear to be unknown.
Resources on This Website
-Known Unknowns Identification CAS Registry
-Known Unknowns Identification ChemSpider
-Known Unknowns Identification EPA Dashboard
-Illicit Drugs Identifications Overview
-LCGC Article