Guan-Hua Chen

Determination of eight triazine herbicide residues in cereal and vegetable by micellar electrokinetic capillary chromatography with on-line sweeping

A new method was developed for the determination of eight triazine herbicide residues in cereal and vegetable samples by on-line sweeping technique in micellar electrokinetic capillary chromatography (MEKC). Some factors affecting analyte enrichment and separation efficiency were examined. The optimum buffer was composed of 25 mM borate, 15 mM phosphate, 40 mM sodium dodecylsulfate (SDS) and 3% (v/v) of 1-propanol at pH 6.5. The separation voltage was 20 kV and the sample was injected at 0.5 psi for 240 s. The detection wavelength was set at 220 nm with the capillary temperature being at 25 °C. Under the optimized conditions, the enrichment factors were achieved from 479 to 610. The limits of detection (LODs, S/N = 3) ranged from 0.02 to 0.04 ng/g and the limits of quantification (LOQs) of eight triazine herbicides were all 0.1 ng/g. The average recoveries of spiked samples were 82.8-96.8%. This method has been successfully applied to the determination of the triazine herbicide residues in cereal and vegetable samples.

Determination of nicotinyl pesticide residues in vegetables by micellar electrokinetic capillary chromatography with quantum dot indirect laser‐induced fluorescence

A new assay was developed by use of micellar electrokinetic capillary chromatography with indirect LIF fluorescence for the determination of thiamethoxam, acetamiprid, and imidacloprid residues in vegetables, in which the cadmium telluride quantum dots (QDs) synthesized in aqueous phase were used as fluorescent background substance and their excitation and emission wavelengths matched with LIF detector by engineering their size. The factors that affected the peak height and the resolution were optimized. The running buffer was composed of 4.4 μM cadmium telluride QDs as fluorescent background substance, 40 mM borate and 60 mM SDS, and its pH was adjusted to 8.0. The separation voltage was 25 kV. Under the optimum conditions, the detection limits were 0.05, 0.01, and 0.009 mg/kg; the linear dynamic ranges were 0.5–30, 0.1–30, and 0.1–30 mg/L; and the average recoveries of spiked samples were 72.0–101.2, 74.0–106.7, and 77.8–105.1% for thiamethoxam, acetamiprid, and imidacloprid, respectively. The assay can meet the requirement of maximum residue limits to these three pesticides in the regulations of European Union and Japan, and has been applied for determining their residues in vegetables.

Molecularly imprinted solid-phase extraction in the analysis of agrochemicals.

The molecular imprinting technique is a highly predeterminative recognition technology. Molecularly imprinted polymers (MIPs) can be applied to the cleanup and preconcentration of analytes as the selective adsorbent of solid-phase extraction (SPE). In recent years, a new type of SPE has formed, molecularly imprinted polymer solid-phase extraction (MISPE), and has been widely applied to the extraction of agrochemicals. In this review, the mechanism of the molecular imprinting technique and the methodology of MIP preparations are explained. The extraction modes of MISPE, including offline and online, are discussed, and the applications of MISPE in the analysis of agrochemicals such as herbicides, fungicides and insecticides are summarized. It is concluded that MISPE is a powerful tool to selectively isolate agrochemicals from real samples with higher extraction and cleanup efficiency than commercial SPE and that it has great potential for broad applications.

ON-LINE PRECONCENTRATION IN CAPILLARY ELECTROPHORESIS FOR ANALYSIS OF AGROCHEMICAL RESIDUES

Capillary electrophoresis (CE) has been recognized as a highly attractive separation technique for determining agrochemical residues in agricultural produces and environmental matrices due to its extremely high column efficiency, rapid analysis, and less reagent consumption. However, a small sample size and a short optical path length make the low concentration samples detected with an ultraviolet (UV) detector difficult or even impossible without sample preconcentration. The on-line concentrating techniques promise to extremely increase sample size without the changes of CE instrument and make the detection of agrochemical residues possible by CE. This review describes these on-line preconcentration techniques and comments their applications in the determination of agrochemical residues.

Determination of Strobilurin Fungicide Residues in Fruits and Vegetables by Micellar Electrokinetic Capillary Chromatography with Sweeping

A new assay of micellar electrokinetic capillary chromatography with sweeping was developed to determine azoxystrobin, kresoxim-methyl and pyraclostrobin in fruits and vegetables. The key factors affecting resolution and peak height were studied and the optimum conditions were obtained for separation and enrichment. The running buffer consisted of 40 mM borate, 25 mM sodium dodecyl sulfate and 15% acetonitrile, and its pH was adjusted to 8.4. The sample was injected for 677 nL and the separation voltage was 25 kV. Under the optimum conditions, the enrichment factors of azoxystrobin, kresoxim-methyl and pyraclostrobin were 861, 550 and 403; the linear dynamic ranges were all 0.01-5.0 mg/L; the limits of detection were 0.002, 0.001 and 0.002 mg/kg; the recoveries of spiked samples were 85.1-98.5%, 87.5-97.0% and 89.1-99.1%, respectively. The assay can meet the requirement of maximum residue limits for these three strobilurin fungicides, and has been applied for determining their residues in fruits and vegetables.

Determination of Dissociation Constants of Resverastrol and Polydatin by Capillary Zone Electrophoresis

Abstract A method to determine the dissociation constants of resverastrol and polydatin by capillary zone electrophoresis was developed. First, resverastrol and polydatin have to be baseline separated to measure their effective electrophoretic mobilities accurately. Second, the first order dissociation constants of resverastrol and polydatin, which are 9.49 and 9.40, can be obtained from the non‐linear regression between their effective electrophoretic mobilities and the H+ activities of the running buffer. The correlation coefficients of the non‐linear regression are 0.9956 and 0.9982, respectively. The reliability of the method is validated by the result of the hydroquinone of which the first order dissociation constant is known.

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.

Determination of Six Functional Compounds in Crataegus pinatifida BGE by Capillary Zone Electrophoresis

Abstract The method of capillary zone electrophoresis was developed to separate four pharmacologically active flavones, one flavonoid, and cholorgenic acid in Crataegus. Experiments showed that the baseline separation of six compounds were finished within 15 min, with an optimized buffer system containing 40 mmol/L borax and 10% (V/V) acetonitrile, and being adjusted to pH 8.20 by 90% (V/V) H3PO4. The relative standard deviation of the concentration of analytes in samples varied in the range of 2.73–6.52% and the recovery of spiked sample within 94.25–98.23% (five replicates), and detection limits were below 0.55 mg/L for each analyte. The distribution of the six active compounds was found to differ quantitatively in different parts of Crataegus. Vitexin‐2″‐O‐rhamnose was the most flavone in leaves, but in fruit epicatechin was very high and hyperside was not detected in this species of Crataegus. There were very little active compounds in flowers.

Quantum dots applied to methodology on detection of pesticide and veterinary drug residues

The pesticide and veterinary drug residues brought by large-scale agricultural production have become one of the issues in the fields of food safety and environmental ecological security. It is necessary to develop the rapid, sensitive, qualitative and quantitative methodology for the detection of pesticide and veterinary drug residues. As one of the achievements of nanoscience, quantum dots (QDs) have been widely used in the detection of pesticide and veterinary drug residues. In these methodology studies, the used QD-signal styles include fluorescence, chemiluminescence, electrochemical luminescence, photoelectrochemistry, etc. QDs can also be assembled into sensors with different materials, such as QD-enzyme, QD-antibody, QD-aptamer, and QD-molecularly imprinted polymer sensors, etc. Plenty of study achievements in the field of detection of pesticide and veterinary drug residues have been obtained from the different combinations among these signals and sensors. They are summarized in this paper to provide a reference for the QD application in the detection of pesticide and veterinary drug residues.

Analysis of Six β-Lactam Residues in Milk and Egg by Micellar Electrokinetic Chromatography with Large-Volume Sample Stacking and Polarity Switching

A new micellar electrokinetic chromatography method with large-volume sample stacking and polarity switching was developed to analyze amoxicllin, cephalexin, oxacillin, penicillin G, cefazolin, and cefoperazone in milk and egg. The important parameters influencing separation and enrichment factors were optimized. The optimized running buffer consisted of 10 mM phosphate and 22 mM SDS at pH 6.7. The sample size was 1.47 kPa × 690 s, the reverse voltage was 20 kV, and the electric current recovery was 95%. Under these optimum conditions, the enrichment factors of six β-lactams were 193-601. Their LODs were <0.26 ng/g, and LOQs were all 2 ng/g, which was only 1/50-1/2 of the maximum residual limits demanded by U.S. and Japanese regulations. The intraday and interday RSDs of method were lower than 3.70 and 3.91%, respectively. The method can be applied to determine these six antibiotic residues in egg and milk.