Formation of protonated phosphonates in the ion‐trap mass spectrometer under electron impact conditions

Abstract

The gas-phase behaviour of organophosphorus compounds such as diethylmethylphosphonate towards electron impact (EI) ionization occurring in a quadrupole ion-trap instrument is investigated. If all ions observed in conventional EI mass spectra are detected, ion/molecule reactions are seen to occur leading to formation of protonated molecules and adduct ions such as 2Mf' and [2M + HI+. The keto-to-enol-like isomerization of the molecular M" ion seems to favour proton transfer to neutral M species and a "self chemical ionization" ion is observed with corresponding fragment ions.Concomitant formation of MH' and odd-electron M+' ions has been observed under electron impact conditions for various classes of compounds, organophosphorus esters for example. In particular, under G U M S conditions using a conventional electron impact (EI) ion source, the relative abundance of [ M + H ] + ions increases as the sample pressure rises.' The presence of heteroatoms such as oxygen or nitrogen in aliphatic molecules can favour this process called 'self ionization'. By using mass spectrometers that allow ion storage, such as ion-trap instruments and ion cyclotron resonance mass spectrometers, 'self chemical ionization' is more easily observed since longer residence times result in ion/molecule collisions even at low sample pressures.2,"4,5Phosphonate compounds are characterized by an [M + l]+/(M+') ratio larger than that predicted for the natural isotopic ratio.6 For instance, diethylmethylphosphonate was characterized by a low abundance M+' ion (mlz 152) accompanied by a signal at m/z 153 (30% of M+*) (see Fig. 2). However, if all expected fragment ions observed in conventional EI mass spectra (and reported in library data) are detected, several experiments performed in ion-trap mass spectrometers show that the MH+ ion intensity depends on conditions of sample ion preparation and ion residence times in the instrument. This behaviour suggests that MH+ is produced by ion/molecule reactions between M+'(and/or fragment ions) and neutral molecules, M. Furthermore, adduct ions such as the dimeric [2M + HI+ species can be observed when concentrations or residence times are increased. In this preliminary study, the effect of the concentration and the storage time on the yield of MH+ ions is investigated and a mechanism of formation is discussed.

Use of a hand-portable gas chromatograph–toroidal ion trap mass spectrometer for self-chemical ionization identification of degradation products related to O-ethyl S-(2-diisopropylaminoethyl) methyl phosphonothiolate (VX)

“…In a quadrupole ion trap mass spectrometer the non-ionized molecules present may be available to serve as targets for the self-CI process, especially where internal ionization is used. Both protonation [9,10] and formation of protonated dimers ([2M+H] + ) [10,11] have been shown to result from self-CI in ion trap instruments. Where protonation occurs by self-CI, evidence for [M+H] + ions may be observed in mass spectra at levels greater than expected for 13 C isotope effects, including instances where M +•· would not be observed under typical EI analysis conditions using a transmission quadrupole instrument.…”

Gas‐phase reactivity of the OP(OCH3)2+ phosphonium ion with aliphatic esters in a quadrupole ion trap. Spontaneous elimination of ketenes

“…Zeller et al 2 reported that protonated TMPA protonates neutral TMPI in experiments performed with a Fourier transform ICR mass spectrometer. Formation of protonated phosphonates has been observed in the quadrupole ion trap (QIT) spectrometer under electron ionization (EI) 3. The study of the gas‐phase reactivity of long‐lived radical cation of trimethylphosphine oxide has demonstrated that this ion is a Brønsted acid and likely to have a distonic structure with a carbon radical center 4.…”

Gas-phase ion-molecule reactions in organophosphorus esters

“…6,8 The artifacts in the mass spectra have been ascribed to ion-molecule reactions. This paper reports results on ion-molecule reactions of trimethyl phosphite, triethyl phosphite, dimethyl phosphonate, trimethyl phosphate and 2,2-dichlorovinyl dimethyl phosphate (dichlorvos) studied by ion trap and/or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS).…”

“…The mass spectra of alkyl and aryl phosphates and phosphites have been extensively studied, 1 -5 and ion trap mass spectrometry has recently been used in the investigation of the gas-phase behavior of diethyl methyl phosphonate. 6 Moreover, ion-molecule reactions of some phosphates and phosphites have been investigated by conventional (field scanning) ion cyclotron resonance mass spectrometry 7 and ion trap mass spectrometry. 8 In this context, the aim of the present work was to carry out a detailed study of the gas-phase ion chemistry of some organophosphorus esters through the investigation of the species obtained by electron ionization and of the factors affecting their behavior in self-condensation processes.…”