Origin of adduct ions in the electron‐capture mass spectrum of tetracyanoethylene

Abstract

The high-pressure electron-capture (HPEC) mass spectrum of tetracyanoethylene (TCNE) is dominated by unexpected hydrogen atom and hydrocarbon radical adduct ions when methane is used as the buffer gas. The origin of these unexpected ions was investigated by three separate mass spectrometric experiments: the electron-capture (EC) rate constant of TCNE was determined and integrated into a previously developed kinetic model of HPEC ion source events; electron impact mass spectra of TCNE were obtained following exposure of the ion source surfaces to irradiated methane and irradiated carbon dioxide; and TCNE was determined by gas chromatographic introduction into the HPEC ion source with multiple ion monitoring. All of these experiments suggest that the reactions leading to the major adduct ions observed in the HPEC mass spectrum of TCNE are initiated by alteration of TCNE on the walls of the ion source rather than in the gas phase.

High‐pressure electron capture mass spectrometry

“…The product of this reaction undergoes EC to form an unexpected ion, MR-. Although the literature of HPECMS contains numerous articles that invoke this mechanism in attempting to explain unexpected ions in HPEC spectra (10,(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77), this process is expected to be of importance only for those compounds that do not normally attach thermal electrons rapidly (26,78), or for those that do attach electrons rapidly only to form molecular anions that undergo fast thermal electron detachment (as described in Section 1II.D). Process D is not expected to be of significance in affecting HPECMS responses to low concentrations of strongly responding EC-active compounds (26,78).…”

“…Although the literature of HPECMS contains numerous articles that invoke this mechanism in attempting to explain unexpected ions in HPEC spectra (10,(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77), this process is expected to be of importance only for those compounds that do not normally attach thermal electrons rapidly (26,78), or for those that do attach electrons rapidly only to form molecular anions that undergo fast thermal electron detachment (as described in Section 1II.D). Process D is not expected to be of significance in affecting HPECMS responses to low concentrations of strongly responding EC-active compounds (26,78). This result is primarily because the rate constants for moleculeradical reactions will typically be several orders of magnitude lower than the rate constants for fast EC reactions (26,78), and the concentrations of gas phase radicals will be less than two orders of magnitude greater than the electron concentration (see Section 11).…”

“…Process D is not expected to be of significance in affecting HPECMS responses to low concentrations of strongly responding EC-active compounds (26,78). This result is primarily because the rate constants for moleculeradical reactions will typically be several orders of magnitude lower than the rate constants for fast EC reactions (26,78), and the concentrations of gas phase radicals will be less than two orders of magnitude greater than the electron concentration (see Section 11). Although the inclusion of radical species in unexpected ions is often noted in the EC spectra of strongly responding compounds, it has been shown under standard HPECMS conditions, where the concentration of the EC-active compound is very low (see Section II), that the neutral precursors of these unexpected ions are formed by reactions occurring on the surfaces of the ion source rather than in the gas phase (26,78).…”

“…Wall-assisted reactions of the analyte, M, have been shown to be responsible for the appearance of unexpected ions in the HPEC mass spectra of a large rn KNIGHTON, SEARS, A N D GRIMSRUD number of EC-active compounds (26,74,(78)(79)(80). Usually, these compounds have double bonds or aromatic rings to which wall-absorbed free radicals can be added.…”

“…6(A), where the HPEC mass spectrum of tetracyanoethylene in methane buffer gas is shown (78). In addition to the M-ion, numerous other ions of equal or greater relative intensities are observed, each of which indicates the inclusion of additional hydrogen atoms or simple organic radicals, R. It has been shown (78) that these ions are formed by a sequence of reactions, including the addition of either two H atoms or an H atom and a methyl or ethyl radical, R, to tetracyanoethylene upon its contact with the wall.…”