Evaluation of a modified QuEChERS method for the extraction of insecticide and fungicide residues in processed peaches
mg kg-1 for dichloran, azoxystrobin and fenthion and levels of 2.0 and 6.0 mg kg-1 for malathion. The recoveries varied from 75 to 118% with RSD below 20% for all compounds. In case of inter-day precision, the recoveries varied from 78 to 104% while the RSD ranged from 1.5 to 13.1%. These results, which comply with SANCO [45] and ANVISA, are shown in Table III. Overall, the results for intra-day and inter-day precision, linearity and accuracy suggest that this method is acceptable for the analysis of the selected pesticides in processed peach samples. Typical chromatograms of the spiked peach sample, extracted using the proposed QuEChERS method (a) and of the blank peach sample extract after the QuEChERS method (b) indicate the absence of interfering peaks at the same retention times as the analytes as well as good separation of the measured compounds, thus, indicating good selectivity (Fig. 1).
Figure 1. LC-DAD chromatograms and the identification spectra of the detected compounds of: (a) spiked peach sample; (b) blank peach sample extract after the QuEChERS method. The concentration of the pesticides dichloran (tR 7.519 min), azoxystrobin (tR 8.093 min) and fenthion (tR 14.249 min) was 2.5 mg kg-1, and 8.0 mg kg-1 for malathion (tR 10.238 min), obtained at a wavelength of 230 nm
3.1.3 Matrix effect Several factors affect the matrix effect, such as the chemical structure, concentration of analytes, time of interaction, the kind and concentration of the matrix [34]. In food samples, the recovery rate is not always satisfactory due to the fact that the matrix effects of the chromatographic analysis can lead to erroneous estimates of the levels of analytes [50,51]. The quantification of compounds can be affected by the presence of co-extracted matrix compounds. To compensate the matrix effect, the use of an analytical curve prepared in matrix extracts free of pesticides is recommended [52]. The existence of the matrix effect was evaluated by comparing the areas of pesticides in the blank matrix extract spiked after extraction (set 2) and the standard prepared in the solvent (set 1), according to Equation 2 [13]. (2)
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The ME calculated in this manner may be referred to as an “absolute� matrix effect. This aspect of the matrix effect assessment is highly relevant for the development of selective chromatographic methods [13]. ME values of 100% indicate that there is no matrix effect; values above 100% indicate signal enrichment and must be determined carefully. When values are below 100%, there is signal suppression [53]. Despite the complexity of the matrix, a significant matrix effect was not observed, due to the chromatographic separation, free from interferences, at the same retention times as the analytes under investigation. The values for matrix effect observed were between 87-105% (Fig. 2)
Figure 2. Relations between ME (%), R (%) and PE (%) for the analytes dichloran, azoxystrobin and fenthion at concentration levels of 0.5 mg kg-1 and for malathion at 4.0 mg kg-1. Error bars indicate RSD values (%)
3.1.4 Process Efficiency The process efficiency (PE) characterizes the overall performance of the proposed method and is defined by the comparison of the peak areas obtained in sets 1 and 3. It shows a matrix effect combined with the recovery of the analytes. The process efficiency was calculated according to Equation 3 [13]. (3) PE (%) values around 100% generally indicate that both matrix effect and recovery are around 100%. Generally, signal enrichment caused by matrix effect causes an increase in process efficiency and signal suppression can cause low efficiency in the process, even with high recoveries [54]. In Fig. 2, the relations between ME (%), R (%) and PE (%) for the analytes dichloran, azoxystrobin and fenthion at concentration levels 0.5 mg kg-1 and for malathion, at 4.0 mg kg-1, are shown. As can be seen, the process efficiency was higher than 70% for all compounds, indicating that the process was efficient with good recoveries and without any matrix effect.
Br J Anal Chem