Rationale: Liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) is used to analyze stable carbon isotope ratios of polar non-volatile 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 ¹³C 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 ¹³C values. Results: Several neonicotinoid derivatives exhibited low carbon recovery and significant δ ¹³C 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 ¹³C 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 ¹³C ratios.