On-line preconcentration and determination of six sulfonylurea herbicides in cereals by MEKC with large-volume sample stacking and polarity switching

A new MEKC method with large‐volume sample stacking and polarity switching was developed for on‐line preconcentration and detection of sulfonylurea herbicide (SUH) residues in cereals, including nicosulfuron (NS), thifensulfuon (methyl) (TFM), tribenuron‐methly (TBM), sulfometuron‐methyl (SMM), pyrazosulfuron‐ethyl (PSE), and chlorimuron‐ethyl (CME). In order to achieve a high resolution and enrichment factor, several parameters were optimized, such as the pH of the running buffer, the concentration of the BGE and the SDS, the separate voltage, the sample size, the pH, and the electrolyte concentration of the sample. The optimal running buffer was composed of 30 mM borate and 80 mM SDS at pH 7.0. The borate concentration in the sample was 30 mM and the pH value of the sample was the same as that of the running buffer. The concentrating voltage and the separating voltage were –15 kV and 15 kV, respectively. The sample size was 1.455 kPa × 780 s (33.11 cm). Under the optimum conditions, for NS, TFM, TBM, SMM, PSE, and CME, the enrichment factors were 613, 642, 835, 570, 709, and 599; the LODs were 0.29–0.50 ng/g, 0.22–0.36 ng/g, 0.60–0.89 ng/g, 0.39–0.72 ng/g, 0.28–0.56 ng/g, and 0.31–0.57 ng/g; the LOQs of six SUHs were all 5 ng/g; the average recoveries of the spiked sample were 86.68–92.99%, 80.73–93.65%, 81.49–94.40%, 82.97–95.1%, 82.96–98.84%, and 80.41–92.94%, respectively.