A Comparison of Electron Ionization Mass Spectra Obtained at 70 eV, Low Electron Energies and with Cold EI and Their NIST Library Identification Probabilities

AbstractElectron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ions abundances were compared as obtained with 70 eV EI, with low eV EI (such as 14 eV) and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their NIST library identification probabilities. We found that: Low eV EI is not a soft ionization method and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions. Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra result in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real ‐world applications. Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds, yet it retains the low mass fragment ions hence Cold EI mass spectra can be effectively identified by the NIST library. Different standard EI ion sources provide diffe rent 70 eV EI mass spectra. Among the Agilent technologies ion sources the "Extractor" exhibits relatively abundant molecular ions compared with the "Inert" ion source while the "High efficiency source" (HES) provides mass spectra with depleted molecular ions compared with the "Inert" ion source or NIST library mass spectra.These concl...
Source: Journal of Mass Spectrometry - Category: Chemistry Authors: Tags: RESEARCH ARTICLE Source Type: research