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

Quantitative Determination of Organophosphorus, Pyrethroid, and Dithiolane Pesticide Residues in Brown Rice Using Supercritical Fluid Extraction and Liquid Chromatography-Tandem Mass Spectrometry

BACKGROUND

Supercritical fluid extraction (SFE) is a fast, versatile, and solvent-efficient automatic extraction method. Despite its advantages, the results of our proficiency tests imply that the applicability assessments of SFE for pesticide residues were insufficient.

OBJECTIVE

In this study, as analytical method using SFE was optimized and validated by testing the incurred and fortified brown rice samples with organophosphorus (OP), pyrethroid (PYR), and dithiolane (DIT) pesticides.

METHOD

A validation study using the incurred sample with etofenprox, fenitrothion, and isoprothiolane was performed by comparing the analytical results obtained using the SFE and solid-liquid extraction with homogenization (SLE), which is a well-validated official multi-residue extraction method. The tests on the fortified samples were also performed for seven pesticide residues, chlorpyrifos, diazinon, O-ethyl O-4-nitrophenyl phenylphosphonothioate (EPN), etofenprox, fenitrothion, isoxathion, and isoprothiolane, at three fortification levels, 0.001, 0.01, and 0.1 mg/kg.

RESULTS

In the test on the incurred samples, optimized SFE-to-SLE analytical values (CSFE/CSLE) were 99.2-100.1%, with RSD lower than 3%. In contrast, the analytical-to-spiked concentrations in the tests on the fortified samples were 96.4-105.0%, with RSD lower than 8.8%.

CONCLUSIONS

These results indicate that the proposed SFE method, which is well validated with the incurred brown rice sample, is useful for determining OP, PYR, and DIT pesticide residues in brown rice.

HIGHLIGHTS

The proposed SFE method satisfies EU and Japanese maximum residue limits (MRLs). The consumption of solvent can be reduced to one-fourth of that of SLE using the proposed SFE method.

Advances of stable isotope technology in food safety analysis and nutrient metabolism research

Food quality, safety, and the regulatory metabolism of food nutrients in cells are primary factors in determining human health. However, residues of undesirable or hazardous compounds in food products and dysregulation in the nutrient metabolism inevitably occur occasionally. For years, chromatography-mass spectrometry technology has been recognized as an essential research tool in food analysis and nutrient metabolism research, and it is more accurate and robust when coupled with stable isotopes. In this study, we summarize the applications of stable isotope technology in the quantification of contaminant residues (pesticides, veterinary drugs, mycotoxins, polycyclic aromatic hydrocarbons, and other hazardous compounds) in foods and in the nutrients (glucose, lipids, amino acids and proteins) metabolism research. The aim of this review was to serve as a reference for providing effective analysis techniques for protecting food quality and human health, and to pave the way for the broader application of stable isotope technology.

APMP-APLAC joint proficiency testing programs for elemental analysis in food with metrological reference values: Assessment of participants' performance considering measurement uncertainties

The Asia Pacific Metrology Program (APMP) and the Asia Pacific Laboratory Accreditation Cooperation (APLAC) joint Proficiency Testing (PT) programs for toxic elements such as cadmium (Cd) and lead (Pb) or nutritional elements such as iron (Fe) and zinc (Zn) in food were organized by the Korea Research Institute of Standards and Science (KRISS) with the aim of enhancing the quality of measurement and metrological traceability in various economies of the Asia Pacific region by evaluating the performance with rigorous evaluation. Three APMP-APLAC joint PT programs for elemental analyses were carried out by KRISS sequentially, where candidate certified reference materials (CRMs) were used as the PT materials and metrologically traceable certified reference values (RVs) were used as the PT assigned values for the evaluation of participants' results, which allows reliable evaluation of participant performance. This article describes the operation of the PTs and the overall performance of the participating laboratories. The effectiveness of these joint PT programs and trends in PT performance assessment are also discussed. These PT programs confirm the significant importance of using the metrologically traceable RVs instead of the consensus values from participants as the PT assigned value for reliable assessment. The lack of understanding of the concept of coverage factor, degree of freedom, standard uncertainty, and expanded uncertainty was revealed by some participants in these PT programs. Interpreting the zeta-scores or E_n scores, which are derived by using measurement uncertainties, in conjunction with the z-scores is highly meaningful for assessing participants' ability in measurement capabilities and measurement uncertainty evaluation. Assessment of participants' performance considering measurement uncertainties helps the participants to check how reasonable their measurement uncertainty estimation was. The results of PTs also demonstrated that these PT programs are useful for improving the measurement capability of the laboratories, whereas more capability-building in uncertainty evaluation is required for further improvement.

Pesticide residues in fruits and vegetables in Indonesia: findings of five-year proficiency testing

The first proficiency testing of pesticides in fruits and vegetables in Indonesia is reported. This report covers the findings of five-year proficiency testings. Every year, from 2016 to 2020, 18-25 laboratories join the proficiency testings and analyze 5-11 pesticides in tomato, orange, lettuce, brown rice, strawberry respectively. The number of laboratories participating in the proficiency testings tends to increase, although only 38 % of the laboratories are able to report all pesticides. More than 72 % of participants use QuEChERS or its modifications for sample preparation, all participants use gas chromatography or liquid chromatography for separation, at least 20 % of participants still rely on detectors other than mass spectrophotometer for detection, and 20 %-60 % of participants use matrix-matched calibration for quantification. The performance of laboratories is evaluated as z-score with an average of 90.8 % achieves satisfactory results while 3.3 % and 5.9 % achieve questionable and unsatisfactory results correspondingly. Overall, the performance of laboratory participants during proficiency testings is good. However, improvement is still needed, especially for the number of target pesticides for multi-residue pesticide analysis. Moreover, unsatisfactory z-scores are likely to be resulted from laboratories which use conventional solvent extraction, use detectors other than mass spectrometers, and are not accredited.

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.

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.

Evaluation of the impact of matrix effects in LC/MS measurement on the accurate quantification of neonicotinoid pesticides in food by isotope-dilution mass spectrometry

The use of isotope-labeled internal standards is the most widely accepted approach to overcome the matrix effects on quantification of pesticides in food by LC/MS. We evaluated the impact of the matrix effects on quantification of six neonicotinoid pesticides, acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, and thiamethoxam, in food by using deuterated internal standards. The calibration curves for each pesticide were obtained by using matrix-free and matrix-matched calibration solutions with blank brown rice, carrot, and green onion extracts. For brown rice and carrot, the matrix effects were not observed. In contrast, the slopes of calibration curves for each pesticide were influenced by presence of green onion extracts in calibration solutions (variability of the slopes was 4-9%), because the ratios of peak area for native pesticide to those for internal standards were influenced by matrix. The spike-and-recovery test with green onion was also performed. The analytical values obtained by using matrix-free calibration solution were biased from the spiked concentration, whereas those obtained by using matrix-matched calibration solution were comparable to the spiked concentration. These results indicate that matrix-matched calibration solution should be used for accurate quantification of neonicotinoid pesticides in food by LC/MS using deuterated internal standards.

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.