Electron affinities of polynuclear aromatic hydrocarbons and negative-ion chemical-ionization sensitivities

Negative-ion chemical-ionization mass spectrometry (NICI MS) has the potential to be a very useful technique in identifying various polycyclic aromatic hydrocarbons (PAHs) in soil and sediment samples. Some PAHs give much stronger signals under NICI MS conditions than others. On the other hand, positive-ion signals are largely comparable under the same source conditions. An extensive set of newly re-evaluated experimental electron affinities (EAs), or free energies of electron attachment, are now available, as well as reliable predicted electron affinities from quantum theoretical calculations or from solution reduction potentials and theoretically predicted solvation energies. In order to show a high negative-ion sensitivity, a PAH must have an EA that exceeds a threshold of approximately of 0.5 eV. Comparisons between the negative-ion to positive-ion sensitivities (N/P ratios) and these new electron affinities show a rough correlation between the two, but naphthacene and perylene are exceptions to this relationship with much lower sensitivities than expected from their high EA values. By calculating the EA for a PAH, one can predict whether a sensitivity enhancement under NICI MS conditions is to be expected. Since aliphatic hydrocarbons and many other substances have negative or very low EAs, NICI MS is expected to be a good technique for detecting PAHs in samples contaminated with other hydrocarbons or compounds with low EAs.