39
Figure 5: Matrix matched calibration curves and chromatograms for standards at 0.001 mg/kg for peaks from: A. GC analysis of leptophos in celery and lemon, and B. LC analysis of carbofuran in corn and kale.
Figure 6: The percentage of pesticides detected in the 0.01 mg/kg standard for each matrix using both GC and LC.
The dwell time calculated for this compound using the autodwell function was 0.006 s. The resulting chromatogram of three replicate injections of 0.010 mg/kg of flutolanil in a celery matrix can be seen in Figure 3. Even with the fast scanning speed, 19 points were collected across the peak and the RSD of three consecutive injections in matrix was 5.2%. The same is true for the LC method used for this analysis.
Pesticides in Matrix
Matrix matched standards were prepared in celery, lemon, corn, and kale over a range of 0.001 to 0.050 mg/kg, and replicate injections made using the LC and GC methods. A TIC overlay for a selection of pesticides is shown in Figure 4, with 0.010 mg/kg in celery extract from both the A. APGC, and B. UHPLC analyses. The data were fitted with the best fit calibration: for the UHPLC data, the response was shown to be linear, whereas the APGC response over the range investigated was non-linear and so it was fitted with a quadratic calibration. A majority of the compounds in both analysis methods had correlation coefficient (R2
)
values of 0.995 or greater. Figure 5 shows the matrix matched calibration curves and the peak response at 0.001 mg/kg of a representative pesticide from each analysis method in the four matrices. Residuals from triplicate injections at each calibration point were within ±20%. Ion ratios were also shown to be within 30% tolerance of the reference values.
Figure 7: Percentage of compounds detected at 0.01 mg/kg in each matrix and associated RSDs.
For convenience, all sample extracts were spiked at the default MRL of 0.01 mg/kg. Figure 6 demonstrates the number of pesticides in each method detected in the spiked matrices at 0.01 mg/kg. However many pesticides could also be detected at 0.001 mg/kg as demonstrated in Figure 5 which shows leptophos (APGC compound) and carbofuran (UHPLC
compound) in the different matrices. The precision of the measurements was excellent with more than 90% of the detected pesticides exhibiting RSDs of peak area of <10% (n=3). The exception was the APGC analysis of the kale matrix, which had more than 80% of pesticides exhibiting RSDs of <10% (Figure 7).
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