How Enhanced Molecular Ions in Cold EI Improve Compound Identification by the NIST Library

Tal Alon, Chemistry, Tel Aviv University, Tel Aviv, Israel
Aviv Amirav, Chemistry, Tel Aviv University, Tel Aviv, Israel

Library based compound identification with electron ionization mass spectrometry is a well-established identification method which provides sample compounds names and structures up to the isomer level. The library (such as NIST) search algorithm compares different EI mass spectra in the library's database with the measured EI mass spectrum, assigning each of them a similarity score called 'Match' and an overall identification probability. Cold EI is electron ionization of vibrationally cold molecules in supersonic molecular beams. Cold EI provides mass spectra with all the standard EI fragment ions combined with enhanced Molecular Ions and high-mass fragments. As a result, Cold EI mass spectra differ from those provided by standard EI and tend to yield lower matching scores. However, in most cases library identification actually improves with Cold EI, as library identification probabilities for the correct library mass spectra increase, despite the lower matching factors.

We examined the way enhanced molecular ion abundances affect library identification probability and the way Cold-EI mass spectra, which include enhanced molecular ions and high-mass fragments, typically improve library identification results. Several computer simulations were conducted, which incrementally modified the relative abundance of the various ions and analyzed the resulting mass spectra.

The simulation results support previous measurements, showing that while enhanced molecular ion and high-mass fragments lower the matching factor of the correct library compound, the matching factors of the incorrect library candidates are lowered even more, resulting in a rise of the identification probability for the correct compound.

This experimentally demonstrated behavior can be explained by the fact that high-mass ions, and especially the molecular ion, characterize a compound more than low mass ions and therefore carry more weight in library search identification algorithms. These ions are uniquely abundant in Cold EI which therefore enable enhanced compound-characterization along with improved NIST library based identification.