Development of soybean certified reference material for pesticide residue analysis

Assessment of the long-term stability of pesticide residues under freezing conditions in brown rice and soybean certified reference materials

The objective of this study was to assess the long-term stability of pesticide residues in brown rice and soybean. The long-term stability of pesticide residues in brown rice and soybean was assessed for 5415 days (over 14 years) and 1801 days (about 5 years), respectively. The samples-certified reference materials (CRMs) 7504-a (brown rice) and 7509-a (soybean) -were prepared by freeze-pulverization. Two target pesticides (etofenprox and fenitrothion) were selected for brown rice and four (chlorpyrifos, diazinon, fenitrothion, and permethrin) for soybean. Our analytical results for long-term stability based on highly reliable isotope dilution mass spectrometry were in the range of expanded uncertainty (k=2) for the certified values of each CRM. The concentration showed a decreasing trend in none of the target pesticides when the samples were stored at temperatures between -20 °C and -30 °C, which indicated that the target pesticides were stable for the tested long terms.

Improvement of the characterization uncertainty of a matrix-certified reference material for accurate measurement of H2SiF6 mass fraction in industrial fluorosilicic acid by using a direct measurement procedure

The role of metrology in the industrial and manufacturing sectors is of paramount importance to ensure informed decision-making whether for product quality control, process monitoring and R&D activities. However, to guarantee the quality and reliability of analytical measurements, the development and use of appropriate reference materials (CRMs) is essential. In particular, certified reference materials (CRMs) are extensively used to validate analytical methods in a multitude of applications, measure uncertainty, improve the accuracy of measurement data, as well as to establish the meteorological traceability of analytical results. In this paper, we report the improvement of the characterization uncertainty of an in-house matrix reference material by direct determination of the concentration of fluorosilicic acid recovered from the fertilizers production industry. The certified reference material was characterized by the potentiometric method as a novel and direct approach for the determination of H2SiF6 concentration and the results were compared against a reference measurement procedure based on molecular absorption spectrophotometry (UV-VIS). The approach adopted in the work resulted in the improvement of the uncertainty of the CRM by decreasing the characterization uncertainty, which constitutes the major contribution to the overall uncertainty. The newly obtained characterization combined standard uncertainty was 2.0 g.kg-1, which gives an expanded uncertainty (k = 2 with a confidence interval of 95%) of the CRM of 6.3 g.kg-1 instead of 11.7 g.kg-1 reported in previous works. This improved CRM can be used to improve the uncertainty of the analytical methods used for the determination of H2SiF6 mass fraction and, therefore, to improve the accuracy of measurement data.

Development of Avocado Reference Material for Pesticide Residue Analysis

Background

Pesticide quantitation in foods relies on the availability of matrix reference materials, which, however, are scarce because of the general instability of pesticides. In particular, no avocado reference material has been developed.

Objective

This research aimed to develop a reference material to support the determination of pesticide residues in avocado.

Method

Avocado was spiked with 11 selected pesticides at levels of 0.01–0.35 mg/kg. The production process included the assessment of between-unit heterogeneity, stability during dispatch, and best storage conditions according to ISO 17034. Reference values were estimated through an interlaboratory comparison study involving laboratories of demonstrated competence and adhering to ISO/IEC 17025. The corresponding expanded uncertainties were calculated as 4–19% in compliance with the Guide to the Expression of Uncertainty in Measurement.

Results

The reference material was sufficiently homogeneous and stable at 4°C during the entire study period (365 days) for most of the pesticides, and at 40°C during 10 days, but it was unstable at 50 and 60°C during 10 days. Four pesticides showed downward trends; however, this behavior was considered in the uncertainty budget. As this material complied with all requirements of proficiency testing, it was used in an interlaboratory proficiency test designed to investigate analytical performance and assist laboratories in improving the quality of measurement results.

Conclusions

The presented material can be used for the development of novel analytical methods or in-house reference materials and adds to the scarce supply of reference materials for the determination of pesticides in vegetable matrixes with high oil contents and intermediate water contents.

Highlights

A novel reference material has been developed for the reliable and accurate quantitation of multiple pesticides in avocado, allowing researchers to avoid the preparation of in-house reference materials for this purpose.

Internationally harmonized certified reference materials and proficiency testings for pesticide residue analysis

Matrix certified reference materials (CRMs) and proficiency testings (PTs) are effective for evaluation of the quality of analytical results. The National Metrology Institute of Japan has developed five kinds of CRMs and has provided eight PTs so far for the quantification of pesticide residues in foods. Target pesticides were sprayed on growing crops, and the harvests were used for the preparation of CRMs and PT samples. In most cases, multiple analytical methods based on isotope dilution mass spectrometry were used to ensure the reliability of certified values (for CRMs) and reference values (for PT samples). These activities were carried out with corresponding to the international standards such as ISO 17034 and ISO/IEC 17043. An overview of the development of CRMs and the implementation of PTs is described, with some examples.

Multiresidue Analytical Method for Pesticides in Soybean Extract

Developed and validated a fast, simple and effective method based on the use of DLLME technique and determination by GC-MS of 26 pesticides in SE. To carry out the extraction of the pesticides of the matrix, 70 μL of mix of pesticides (1.5 μg/mL) was added to 5.0 mL of SE, containing 1.0 g of sodium chloride and 3.0 mL of acetonitrile. The results of validation were suitable. The calibration curve was linear in the range of 0.500-5.0 μg/mL. The method showed a limit of detection and quantification of 0.17 μg/mL and 0.50 μg/mL, respectively. The recovery recovering between 47% and 115%, with relative standard deviation (RSD) of <20% for fortification levels (range of 1.0-3.0 μg/mL). The method validated can be applied for routine analysis in soy-based drinks, considering it is fast, easy to perform and has satisfactory validation results.

Proficiency testing by the National Metrology Institute of Japan for quantification of pesticide residues in grain samples from 2012 to 2018

From 2012 to 2018, proficiency testings (PTs) were carried out to support the analytical skill development of participants. Grains were sprayed with various target pesticides and used as material samples for PTs. It was found that 27-56% and 10-43% of participants used the Japanese official multiresidue method and a solid-phase extraction technique with the Quick, Easy, Cheap, Effective, Rugged, and Safe (modified QuEChERS, known as "STQ" in Japan) method, respectively. Reported results were evaluated using two types of z-scores: one was based on consensus values calculated from the analytical results reported by the participants, and the other was based on values obtained by the National Metrology Institute of Japan (NMIJ). Acceptable z-scores based on the consensus values and NMIJ reference values were achieved by 78-95% and 67-94% of the participants, respectively. Many laboratories improved their z-scores by continuing participation in our PT.

Gas chromatography-isotope dilution mass spectrometry method validation for target pesticides in soybeans

The production of certified reference materials requires the application of highly accurate methods for characterisation. A gas chromatography-isotope dilution mass spectrometry method, setting ambitious performance criteria, was developed for eight selected pesticides in soybeans. Pressurised liquid extraction was followed by automated gel-permeation chromatography and solid-phase extraction clean-up. Pesticides identification respected a Commission Decision and guidelines of the Directorate General for Health and Food Safety (DG SANTE). Reliable quantification involved stable isotopically labelled analogues as internal standards. Validation, according to ISO/IEC 17,025 and DG SANTE guidelines, assessed linearity, LOD/LOQ, trueness, selectivity, precision, stability and robustness. Mean recoveries ranges (83-109%, relative standard deviations < 3%), repeatability (2.2-4.8%), day-to-day variation (0.6-4.2%) and combined uncertainty (1.2-4.2%) were fit for purpose. The method is highly accurate and suitable for certification of the selected pesticides in soybean matrix reference material. Chemical compounds studied in this article: Diazinon (PubChem CID: 3017); malathion (PubChem CID: 4004); chlorpyrifos (PubChem CID: 2730); captan (PubChem CID: 8606); endosulfan (PubChem CID: 3224); tebuconazole (PubChem CID: 86,102); iprodione (PubChem CID: 37,517); cypermethrin (PubChem CID: 2912).

Difference between Consensus Value of Participants' Results and Isotope-Dilution Mass Spectrometric Results in Proficiency Testing for Pesticide Residues in Husked Wheat

Proficiency testing was organized by the National Metrology Institute of Japan (NMIJ) as a measure of analytical competency in the quantification of pesticide residues in husked wheat powder. Seventy-one participants submitted analytical concentrations of the target pesticides (diazinon, fenitrothion, malathion, and etofenprox) along with details of the analytical method employed. Two types of assigned values were obtained for each target pesticide, i.e., the participants' analytical results and the results obtained by NMIJ based on isotope-dilution mass spectrometry (IDMS). The former values were lower than the latter due to the incomplete recovery yield of the target pesticides during the pretreatment process. The discrepancy between the two assigned values was particularly pronounced for malathion because of the longer duration of water-soaking used for the test samples. Two corresponding types of z-scores were then calculated to evaluate the analytical performance of the participants, where the z1-score indicates the performance of a participant relative to all participants, and the z2-score indicates the relative deviation of the analytical results of the participant from the IDMS value.

Evaluation of the impact of matrix effect on quantification of pesticides in foods by gas chromatography-mass spectrometry using isotope-labeled internal standards

The impact of the matrix effect in GC-MS quantification of pesticides in food using the corresponding isotope-labeled internal standards was evaluated. A spike-and-recovery study of nine target pesticides was first conducted using paste samples of corn, green soybean, carrot, and pumpkin. The observed analytical values using isotope-labeled internal standards were more accurate for most target pesticides than that obtained using the external calibration method, but were still biased from the spiked concentrations when a matrix-free calibration solution was used for calibration. The respective calibration curves for each target pesticide were also prepared using matrix-free calibration solutions and matrix-matched calibration solutions with blank soybean extract. The intensity ratio of the peaks of most target pesticides to that of the corresponding isotope-labeled internal standards was influenced by the presence of the matrix in the calibration solution; therefore, the observed slope varied. The ratio was also influenced by the type of injection method (splitless or on-column). These results indicated that matrix-matching of the calibration solution is required for very accurate quantification, even if isotope-labeled internal standards were used for calibration.

Development of a new cucumber reference material for pesticide residue analysis: feasibility study for material processing, homogeneity and stability assessment

The feasibility of the production of a reference material for pesticide residue analysis in a cucumber matrix was investigated. Cucumber was spiked at 0.075 mg/kg with each of the 15 selected pesticides (acetamiprid, azoxystrobin, carbendazim, chlorpyrifos, cypermethrin, diazinon, (α + β)-endosulfan, fenitrothion, imazalil, imidacloprid, iprodione, malathion, methomyl, tebuconazole and thiabendazole) respectively. Three different strategies were considered for processing the material, based on the physicochemical properties of the vegetable and the target pesticides. As a result, a frozen spiked slurry of fresh cucumber, a spiked freeze-dried cucumber powder and a freeze-dried cucumber powder spiked by spraying the powder were studied. The effects of processing and aspects related to the reconstitution of the material were evaluated by monitoring the pesticide levels in the three materials. Two separate analytical methods based on LC-MS/MS and GC-MS/MS were developed and validated in-house. The spiked freeze-dried cucumber powder was selected as the most feasible material and more exhaustive studies on homogeneity and stability of the pesticide residues in the matrix were carried out. The results suggested that the between-unit homogeneity was satisfactory with a sample intake of dried material as low as 0.1 g. A 9-week isochronous stability study was undertaken at -20 °C, 4 °C and 18 °C, with -70 °C designated as the reference temperature. The pesticides tested exhibited adequate stability at -20 °C during the 9-week period as well as at -70 °C for a period of 18 months. These results constitute a good basis for the development of a new candidate reference material for selected pesticides in a cucumber matrix.

Proficiency testing for determination of pesticide residues in soybean: comparison of assigned values from participants' results and isotope-dilution mass spectrometric determination

Proficiency testing (PT) for the determination of pesticide residues in soybean samples was organized by the National Metrology Institute of Japan (NMIJ). The candidate certified reference material, NMIJ CRM 7509-a, that was prepared from the raw soybeans containing target pesticides (diazinon, fenitrothion, chlorpyrifos, and permethrin) was used as the test sample. Forty participants submitted two sets of analytical results along with the details of the analytical method and conditions they applied. Two types of assigned values were established for each target pesticide: one was derived from the analytical results of the participants, and the other was provided from the analytical results by isotope-dilution mass spectrometry (IDMS). The latter values were 7.4-16% higher than the former values, plausibly because the analytical values from the IDMS measurements were not affected by the recovery ratio of the target pesticides during the analytical process. Thus, two kinds of z-scores were calculated for individual participants using the corresponding assigned values: one (z1-score) showed the relative performance score for the present PT and the other (z2-score) could be used for evaluation of the trueness of their analytical methods.