Probing Matrix Effects on the Heterogeneous Photochemistry of Neonicotinoid Pesticides, Dinotefuran and Nitenpyram

Abstract

The environmental fate of neonicotinoids (NNs) is a key regulatory issue due to their widespread distribution, mobility, and role in honeybee colony collapse disorder. Here, we explore the potential matrix effects of the commercial formulations on the photochemistry of two NNs, dinotefuran (DNF) and nitenpyram (NPM). The commercial formulations and the pure reagents, both embedded in KBr pellets, were irradiated at wavelengths from 254 to 350 nm. Loss of the NN and product formation were followed using Fourier transform infrared spectroscopy and direct analysis in real time mass spectrometry. Quantum yields for loss of NN in commercial formulations were within experimental error of the pure NN. However, quantum yields for loss in KBr were consistently smaller than those previously measured for thin films of pure compounds, suggesting increased quenching of excited intermediates in the dense solid KBr matrix. At 350 nm where NPM absorbs, the quantum yield for loss in the formulation was about half that of the pure compound, which was attributed to a reduction in the NPM absorption cross section due to the presence of water taken up by additional formulation ingredients. Major products of DNF and NPM photolysis were the same in the formulation and in the pure compound. However, in contrast to previous thin film measurements, unexpected infrared peaks at 2222 and 2136 cm −1 during photolysis of DNF in KBr were identified as N 2 O trapped in the KBr matrix and NO + , respectively. Overall, the presence of other compounds in the formulations did not significantly alter the photochemistry compared to the pure compounds.

Oxidation of solid thin films of neonicotinoid pesticides by gas phase hydroxyl radicals

“…The major solid phase photolysis products are the desnitro and urea derivatives, which form from cleavage of the N–N bond in the CN–NO 2 chromophore, resulting in a CNH (desnitro) or CO (urea) group respectively. 42,45…”

“…An abundance of work is available on the reactions and fate of NNs in aqueous systems. 2–4,26,30–41 In addition, previous studies from this lab have reported the heterogeneous photolysis of solid NNs 42–45 as would be the case for example for NNs on leaf, soil, and dust surfaces, and the oxidation of the NN nitenpyram by gas phase ozone. 46 However, little is known about the heterogeneous reactions of solid NNs with the hydroxyl (OH) radical, which is a major atmospheric oxidant that is ubiquitous in air.…”

“…The major solid phase photolysis products are the desnitro and urea derivatives, which form from cleavage of the N-N bond in the C]N-NO 2 chromophore, resulting in a C]NH (desnitro) or C]O (urea) group respectively. 42,45 Table 2 summarizes the measured accurate masses of the [M + H] + ions from the IMD-OH reaction, the corresponding molecular formulae, the errors between the measured accurate masses and the exact masses, and the molecular structures of the products proposed based on the MS/MS fragmentation patterns summarized in the table and reasonable reaction mechanisms. In all chromatograms, the [M + H] + ion of the parent IMD at m/z 256 is observed.…”