( th 25 mL of acetonitrile. The final sol...)Fast Analysis of Phenoxyacetic and Phenoxypropionic Herbicides in Vegetable Matrix by Negative Ion Electrospray LC/MS/MS

Results and Discussion

The MS/MS conditions shown above are determined with the Automated MS/MS Breakdown software (also see Varian LC/MS Application Note 11 and 12).

The LC method uses a fast gradient program for a complete analysis in less than ten minutes. Due to the structural similarity of the acidic herbicides, incomplete resolution is observed under faster separation conditions.

The relative selectivity of LC/MS (SIM mode) and LC/MS/MS (MRM mode) are directly compared using spiked matrix samples. For pure standards, the sensitivity in the SIM mode is excellent, but even at the highest spiked concentration of 5 ppb, matrix interferences are strong. In Figure 2, SIM plots for all four herbicides shows extra peaks and baseline instability. For mecoprop, an interfering peak clearly overlaps with the analyte. As with a LC/UV detector, reliable quantitation is not possible with SIM, especially at lower concentrations.

Using the same spiked matrix extract, the increased selectivity of MS/MS is readily apparent. The MRM chromatograms in Figure 3 are interference free. The baseline is clear from any false positive peaks and no cross talk effect is observed when similar product ions are monitored in the same segment time (Table 1 and Figure 3). With the elimination of these matrix interferences, quantitation with high confidence is possible.

The MRM method has excellent linearity for all the herbicides from 0.1 to 5 ppb as shown for dichlorprop in Figure 4. The developed MRM method is also very sensitive with a LOD of 0.1 ppb for all the pesticides under the described instrument conditions.

Conclusion

This 1200L LC/MS/MS method is simple, fast, and hig
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