Keisuke Nakamura

Organic luminescent radicals have emerged as promising open-shell emitters for advanced optoelectronic and sensing applications. Among them, the carbazole-substituted tris(2,4,6-trichlorophenyl)methyl radical (Cz-TTM) exhibits remarkable photostability and emission efficiency compared with the parent TTM radical. Nevertheless, Cz-TTM can be visibly degraded by indoor light irradiation and its photodegradation mechanism is poorly understood. Here, we systematically investigate the photochemical behavior of Cz-TTM in solutions, together with the influence of impurities on the photophysical properties of the compound. We found that emissive and non-emissive impurities are introduced depending on the purification methods and the light irradiation conditions. Impurities cause underestimation of photoluminescence quantum yield through competitive absorption. Photodegradation under continuous irradiation undergoes a multi-step reaction via metastable intermediates, which react with dissolved oxygen upon local excitation of the TTM radical moiety. Quantum-chemical calculations indicate a ring-closure followed by dechlorination within the TTM moiety as the key degradation pathway. The superior photostability of Cz-TTM to TTM originates from its stabilized first excited doublet energy level, which increases the activation energy for ring-closure reaction in Cz-TTM. These combined experimental and theoretical insights reveal the fundamental factors governing the photostability of TTM-type luminescent radicals and offer design principles for next-generation organic luminescent materials.

Background: The Quick, Easy, Cheap, Effective, Rugged, and Safe (QuEChERS) method has been widely applied to various compounds and matrices. However, the cleanup process of QuEChERS with dispersive solid-phase extraction (d-SPE) is considered to be not always sufficient. Therefore, a modified QuEChERS method using small-scale SPE (abbreviated as “modified QuEChERS”) for the cleanup process has seen an increasing number of users in Japan; it is therefore necessary to evaluate its performance. Objective: To evaluate the performance of modified QuEChERS for the analysis of nine organophosphorus pesticides (OPPs) in spinach, and to compare the various parameters among modified QuEChERS, QuEChERS, and the Japanese official multiresidue method (JOMM). Methods: Modified QuEChERS was evaluated by a recovery test using blank spinach samples and quantification using a pesticide-containing QC spinach sample. A matrix effect was also evaluated in GC–MS using a stomach-shaped spiral, large-volume injection (LVI). Various parameters including accuracy, time, cost, and cleanup effect were compared in modified QuEChERS, QuEChERS, and JOMM. Results: The mean recoveries, the intra-day repeatability, and the inter-day reproducibility for modified QuEChERS were acceptable according to the validation guideline. The concentration of fenitrothion in the pesticide-containing QC spinach sample obtained by modified QuEChERS was comparable to that obtained by QuEChERS and JOMM. It was found that the cleanup of the blank spinach sample by modified QuEChERS using small-scale SPE can be more effective than QuEChERS using d-SPE. A matrix effect was newly evaluated in our analysis by GC–MS using a stomach-shaped spiral LVI. Conclusions: The modified QuEChERS is an accurate and simple method of analyzing OPPs in spinach. Highlight: The modified QuEChERS is not only accurate but also a fast, cost-effective, and environment-friendly method for analyzing OPPs in spinach.

A polystyrene (PS) certified reference material (CRM) for the analysis of decabromodiphenyl ether (BDE 209) was issued. PS disk was prepared by injection molding of the mixture of versine PS and BDE 209. The certification of the PS CRM was conducted by two analytical methods with different sample preparation methods using isotope dilution mass spectrometry (IDMS). The certified value, w_CRM, was 978 mg/kg, and this value coincided with the regulation value of BDE 209 in the Restriction of Hazardous Substances directive (1000 mg/kg). The uncertainties related to certification, u_wmean, inhomogeneity, u_hom, and long- and short-term instability, u_sts and u_lts, respectively, were evaluated based on the mass fraction of BDE 209. The u_wmean, u_hom, u_sts, and u_lts were 0.0265, 0.0046, 0.0061, and 0.0099 (relative), respectively, and the expanded uncertainty for this CRM was determined as 57 mg/kg (coverage factor is 2). Additionally, the quantitative capability of the thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) method was evaluated. In TD-GC/MS, the analytical values of the developed CRM obtained by the external and internal standard methods with matrix-free calibrants were out of the range of the w_CRM (almost 10% larger or smaller), whereas those with matrix-matched calibrants agreed with the w_CRM. In contrast to these results, the analytical values obtained by TD-GC/MS using IDMS were consistent with the w_CRM no matter if matrix-free or matrix-matched calibrants were used. These results indicated that, for quantification of BDE 209 in PS, the trueness and precision of TD-GC/MS can be enhanced by applying IDMS without matrix-matched calibrants.

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. In this study, as analytical method using SFE was optimized and validated by testing the incurred and fortified brown rice samples with organophosphorus, pyrethroid, and dithiolane pesticides. 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. In the test on the incurred samples, optimized SFE-to-SLE analytical values (C_SFE/C_SLE) were in 99.2–100.1%, with RSD lower than 3%. In contrast, the analytical-to-spiked concentrations in the tests on the fortified samples were in 96.4–105.0%, with RSD lower than 8.8%. These results indicate that the proposed SFE method, which is well validated with the incurred brown rice sample, is useful for determining organophosphorus, pyrethroid, and dithiolane pesticide residues in brown rice. The proposed SFE method satisfies EU and Japanese maximum residue limits. The consumption of solvent can be reduced to one-fourth of that of SLE using the proposed SFE method.

A supercritical fluid extraction (SFE) method was presented for the determination of neonicotinoid pesticides in green onion. We optimized the SFE parameters, namely the pressure, temperature, type and concentration of the modifier solvent, and extraction time, by testing on the incurred green onion applied with four commercial neonicotinoid formulations at the harvesting stage. The analytical values of the four neonicotinoids, obtained by 5 min dynamic SFE with a 25% (v/v) methanol/supercritical carbon dioxide mixture at 80 °C and 25 MPa, were in good agreement with those obtained by solid-liquid extraction using a homogenizer. This latter homogenization method is employed as a Japanese official method for the analysis of pesticide residues in food. The SFE-to-homogenization analytical value ratios were in the range 98.8–100.0%. Spike-and-recovery testing with spiked green onion was also performed by optimized SFE. The ratios of the analytical-to-spiked concentrations for six neonicotinoids obtained by the SFE method were 96.1–102.1% for 0.4 mg/kg and 93.9–104.1% for 4.0 mg/kg spiking levels. These values satisfied the Japanese validation guidelines for the testing method of pesticides in food. These results indicate that SFE is applicable for the analysis of neonicotinoids in green onion and enables automatic extraction with a small amount of solvent.

A pressurized liquid extraction (PLE) method was presented for the determination of six neonicotinoid pesticides, acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, and thiamethoxam in green onion. The critical parameters of PLE, e.g. extraction solvent, temperature, pressure, number of cycles, and static extraction time, were optimized by test on the spiked green onion with six neonicotinoids and the incurred green onion applied with four commercial neonicotinoid insecticide formulations (acetamiprid, dinotefuran, imidacloprid, and thiamethoxam). As a result, the recoveries of six neonicotinoids obtained by one cycle PLE with acetonitrile at 140 °C and 50 bar for 10 min were 94.7–99.5%. These results were acceptable according to the validation guideline for testing method of agricultural chemicals in food by Ministry of Health, Labour, and Welfare in Japan. PLE was also validated by the test on the incurred green onion. The analytical values of four neonicotinoids obtained by PLE were good agreement with those obtained by solid–liquid extraction with homogenizer, which is employed for Japanese official method for the analysis of pesticide residues in food (the ratios of analytical values obtained by PLE to those obtained by solid–liquid extraction were 99.7–101.2%). These results indicate that PLE is applicable for the determination of neonicotinoids in green onion.

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.

The local structure of Rb ⁺ in a cation‐exchange resin soaked in superheated water up to 160 °C is studied via X‐ray absorption fine structure (XAFS) analysis. The first hydration shell structure of Rb ⁺ is not changed when Rb ⁺ is transferred from bulk superheated water into the resin soaked therein. Our previous chromatographic study with utilizing superheated water as an eluent indicated that separation selectivity between alkali cations decreases and the relative affinity of small ions toward ion‐exchange resins is enhanced. This strongly suggests that countercations in superheated water undergo different interactions compared to those in ambient conditions. However, XAFS spectral analysis for a Rb ⁺ ‐form resin show that Rb ⁺ remains hydrated similar to in bulk superheated water. In contrast, XAFS of a minor amount of Rb ⁺ in a Cs ⁺ ‐form resin suggests that even at ambient temperatures, Rb ⁺ tends to form direct ion pairs with sulfonate cation‐exchange groups. The invasion of sulfonate groups into the first hydration shell of Rb ⁺ becomes more obvious as temperature increases. The hydration structure of Rb ⁺ cannot be maintained when it is surrounded by poorly hydrated Cs ⁺ because of the limited availability of water molecules for forming complete hydration shells around Rb ⁺ . The sulfonate groups invade the first coordination shells of Rb ⁺ to compensate for the shortage of water molecules.