Previous studies using the ion trap as a potential replacement for an array of element-sensitive detectors indicated that only semi-quantitative results at the 50-100 ppb range could be obtained for 245 pesticides because of interfering endogenous sample matrix compounds. The development of a new clean-up procedure for a wide range of fruits and vegetables, specifically for use in an ion trap, has been undertaken to improve recoveries at the low ppm level to within analytically acceptable ranges (80-120%). This clean-up involved the use of three solid-phase extractions. The use of a C-18 SepPak column removed the majority of hydrocarbon-like molecules while the anion-exchange column (QMA) removed all colored compounds and flavors, and finally the use of an aminopropyl column removed all sugars. Preliminary results from this study using 24 selected pesticides in four different sample matrices have indicated an acceptable improvement over previous results.
The ion trap has been demonstrated to be able to detect and quantify 245 target pesticides extracted via the Luke method while providing concurrent confirmation of presence via full scan data at the sub-ppm level. The precision and accuracy of the analytical approach was determined to be no greater than 15% relative standard deviation. A comparison study of over 100 incurred residues analyzed by the ion trap and gas chromatography with an array of element-selective detectors has indicated that sample clean-up will probably be necessary before quantification is acceptable for all target compounds. The data obtained using a combination of gas chromatography and mass spectrometry and presented for 250 target pesticides constitutes the basic information required to duplicate and extend the methodology.
The thermospray ionization mass spectra of selected carbamate pesticides were obtained using both single-stage and triple-stage quadrupole mass spectrometry in conjunction with high-performance liquid chromatography. With the use of the single-quadrupole mass spectrometer and ammonium acetate as the mobile phase, enhanced sensitivity for the analytes of interest was obtained by monitoring the relatively intact [M + 1]+ or [M + 18]+ adduct ions. Further structural characterization of the carbamates was not readily obtainable owing to the lack of diagnostic bond cleavages. The single-stage quadrupole analyses were therefore complemented by triple-stage quadrupole analyses. Here, collisionally activated daughter ion spectra exhibited structure-specific fragmentations. In addition, the enhanced selectivity and specificity provided by tandem mass spectrometry allowed use of the technique as a rapid screening tool for carbamates without the need for the chromatographic separation step.
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
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