Niemann Felix, Gruhlke Annika, Jochmann Maik A, Schmidt Torsten C
Faculty of Chemistry, Instrumental Analytical Chemistry, University of Duisburg-Essen, Essen, Germany.
Centre for Water and Environmental Research (ZWU), Essen, Germany.
Rapid Commun Mass Spectrom. 2025 Aug 30;39(16):e10067. doi: 10.1002/rcm.10067.
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.
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 catalyst were varied. Carbon recoveries and δC 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 δC values.
Several neonicotinoid derivatives exhibited low carbon recovery and significant δC bias. Increasing oxidant concentration, reactor temperature, and reaction time improved recoveries but did not fully mitigate isotopic biases. The addition of AgNO improved carbon recoveries for most derivatives but introduced variability in δC values, likely due to shifts in reaction mechanisms. A workflow to identify oxidation problems during method development was proposed.
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 δC ratios.
液相色谱 - 同位素比率质谱法（LC - IRMS）用于分析极性非挥发性化合物的稳定碳同位素比率。然而，已报道过基于过硫酸盐的氧化界面存在挑战，特别是对于具有顽固结构的分子，如新烟碱类化合物中的那些分子。本研究系统地研究了商业LC - IRMS中与新烟碱类相关结构的氧化效率。
评估了不同分子复杂性的新烟碱类替代物的碳回收率和稳定碳同位素比率准确性。改变了LC - IRMS参数，如氧化剂浓度、反应时间、温度、酸浓度以及硝酸银催化剂的存在情况。在相同条件下注入与氧化无关的无机碳标准物来测定碳回收率和δC偏差。使用元素分析仪同位素比率质谱法（EA - IRMS）对δC值进行归一化。
几种新烟碱类衍生物表现出低碳回收率和显著的δC偏差。增加氧化剂浓度、反应器温度和反应时间可提高回收率，但并未完全消除同位素偏差。添加硝酸银提高了大多数衍生物的碳回收率，但引入了δC值的变异性，这可能是由于反应机制的变化。提出了一种在方法开发过程中识别氧化问题的工作流程。
对于尿素、胍和硝基胍衍生物以及类似化合物，优化LC - IRMS氧化参数至关重要。需要在不同条件下对氧化效率进行系统评估，以实现最佳矿化，从而获得更准确的δC比率。
