Methane chemical ionization of pesticides by ion‐trap technology: Spectral characteristics and data precision

Abstract

Examination of the methane chemical ionization (CI) spectra produced in an ion trap under automatic reaction control has revealed a strong concentration dependence phenomenon even at trace levels (ppm). The spectra produced during the elution profile of various pesticides via capillary gas-chromatographic introduction have indicated a large percentage of an electron ionization (EI) spectral component superimposed on the CI spectrum. This duplicity of spectral character can be an important asset for confirmation of presence and it does not present a problem of obtaining acceptable precision and accuracy on quantification of pesticide residues. Identification of residues using the standard spectral library databases derived from other sources might require careful scrutiny before acceptance.The choice of chemical ionization (CI) for the analysis of trace levels of pesticides by gas chromatography/ mass spectrometry (GUMS) has been well documented in the literature.' In particular, the selection of methane as reagent gas has been d e m~n s t r a t e d~,~ to reduce potential interferences from background ions or matrix components while providing higher specificity through production of ions corresponding to protonated molecules for the pesticides of interest. Therefore, it was anticipated that the same analytical approach on an ion trap would serve as a more costeffective mechanism to regulatory pesticide analysis by providing full-scan data for detection, confirmation and quantification.However, during the development of such a multiresidue pesticide analytical protocol for pesticides in various fruits and vegetables, it was noticed that the full-scan methane CI spectra produced by the ion trap under automatic reaction control (ARC) were hybrid in nature. Closer examination of the spectra during typical elution profiles revealed a concentration dependence between electron ionization (EI) and C1-derived ions for the compound under investigation. This experimental observation was particularly disturbing for a number of reasons. Firstly, the advantage of reducing ion contributions from background and matrix components through the application of CI had been effectively removed. Secondly, the hybrid CI spectra of chlorinated compounds could not easily be assigned a chlorine number since the molecular-ion cluster was often a mixture of molecular (via EI) and protonated molecule ions (via CI). Thirdly, the comparisons of full-scan data could not be directly related to the standard pesticide library developed from previously recorded spectra from quadrupole and magnetic scanning instruments.While concentration dependence during G C elution has been observed previously for various drugs, the cause and effect has been attributed to thermal l a b i l i t~.~ In the case of the ion trap, however, such an explanation cannot be advanced since these pesticides have already been shown to be thermally stable under

Application of nonresonance excitation to ion trap tandem mass spectrometry and selected ejection chemical ionization

“…Cairns et al [32] have shown that ion trap chemical ionization rcn spectra may be complicated by the presence of ions that originate from the electron ionization (En of the analyte, which results in a combined EI and CI spectrum. Insertion of a time period between the ionization and reaction periods, in which the sample fragment ions from EI are selectively ejected, produces a chemical ionization spectrum without the contamination of ions from electron ionization.…”

“…During the reaction period, the rf trapping field is raised and the resulting sample CI ions effectively are stored in the trap for analysis. Cairns et al [32] found that under certain conditions, a combined EI and CI spectrum was still observed in ion traps. Cairns et al [33] further demonstrated improved performance by modifying the rf scan function.…”

Comparison of an Ion-Trap and a Quadrupole Mass Spectrometer using Diazepam as a Model Compound

“…The PC[ spectra of diazepam is shown in Figure 2. Under CI conditions, older ITMS produced spectra containing a mixture of El and CI that was concentration dependent (14). This was caused by reactive radical ions and was corrected by additional waveforms applied to the endcaps to eject unwanted reagent gas ions during the reaction phase (15).…”

Determination of dioxins/furans and PCBs by quadrupole ion-trap gas chromatography-mass spectrometry

“…Chemical ionization (CI) has been used in the selected positive-ion monitoring mode for the analysis of various hazardous compounds found in the environment (Facchetti, 1989;Cairns, Chiu, & Siegmund, 1992) as well as for PCB mixtures in environmental sample extracts (Safe, 1990(Safe, , 1993. Compared to HRGC-ECD, this technique suffers from decreased molecular sensitivity with increasing molecular weight because the ionization ef®ciency is not dependent on the molar chlorine content.…”