LC-IRMS Persulfate Oxidation: Case Study on Neonicotinoid-Related Structures.

RATIONALE: Liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) is used to analyze stable carbon isotope ratios of polar nonvolatile compounds. However, challenges with the persulfate-based oxidation interface have been reported, particularly for molecules with recalcitrant structures like those found in neonicotinoids. This study systematically investigates the oxidation efficiency of neonicotinoid-related structures in a commercial LC-IRMS. METHODS: Neonicotinoid proxies of varying molecular complexity were evaluated for carbon recovery and stable carbon isotope ratio accuracy. LC-IRMS parameters such as oxidant concentration, reaction time, temperature, acid concentration, and the presence of AgNO(3) catalyst were varied. Carbon recoveries and δ(13)C biases were determined by injecting an oxidation-independent inorganic carbon standard under identical conditions. Elemental analyzer isotope ratio mass spectrometry (EA-IRMS) was used to normalize δ(13)C values. RESULTS: Several neonicotinoid derivatives exhibited low carbon recovery and significant δ(13)C bias. Increasing oxidant concentration, reactor temperature, and reaction time improved recoveries but did not fully mitigate isotopic biases. The addition of AgNO(3) improved carbon recoveries for most derivatives but introduced variability in δ(13)C values, likely due to shifts in reaction mechanisms. A workflow to identify oxidation problems during method development was proposed. CONCLUSIONS: Optimization of LC-IRMS oxidation parameters is critical for urea, guanidine, and nitroguanidine derivatives and similar compounds. A systematic evaluation of oxidation efficiencies under different conditions is needed for optimal mineralization and thus more accurate δ(13)C ratios.