Comparative evaluation of QuEChERS and the ‘dilute and shoot’ QuPPe extraction methods coupled with LC-MS/MS for the analysis of mepiquat residue in sweet potatoes: addressing residual soil impact on recovery efficiency

Mepiquat (MPQ) is a quaternary ammonium salt widely used as a growth regulator in agriculture. A recent report from the European Union Reference Laboratory for Single Residue Methods (EURL SRM) reported MPQ residues in Egyptian sweet potatoes at levels that raised some concern. Hence, this study aimed to develop and validate a fast, effective extraction method coupled with LC-MS/MS for MPQ analysis in Egypt. Initially, the QuEChERS citrate buffered method was tested, but it showed significant fluctuation in spike recovery (30%-72%). A systematic investigation revealed that recovery differed depending on the amount of residual soil in the sweet potato samples. Two key observations were made: (1) Clay loam negatively affected recovery more than sandy loam, and (2) the "dilute and shoot" Quick Polar Pesticides (QuPPe) method was less impacted by residual soil. By applying exponential decay equation modelling, it was proved that residual soil can remain on the sweet potato even after 10 min washing. A study on the effect of washing for 10 min was done using real contaminated samples and it showed that about 30% of the incurred pesticide was lost. Hence, dry scrubbing was used for the removal of the soil on the sweet potato, in addition to the QuPPe method for sample extraction. This provides a predictive framework for understanding how residual soil influences recovery, which could be extended to other polar pesticides and root crops. The QuPPe method was validated for specificity, selectivity, accuracy, and precision, ensuring its suitability for quantitative MPQ analysis in sweet potatoes. A survey of 30 samples showed that only four tested positives for MPQ, with one exceeding the maximum residue limits.