During the analysis of neonicotinoid pesticide standards (thiamethoxam, clothianidin, imidacloprid, acetamiprid, and thiacloprid) by mass spectrometry, the degradation of these pesticides (M-C=N-R is degraded into M-C=O, M is the skeleton moiety, and R is NO₂ or CN) was observed in the atmospheric pressure ionization interfaces (ESI and APCI). In APCI, the degradation of all the five neonicotinoid pesticides studied took place, and the primary mechanism was in-source ion/molecule reaction, in which a molecule of water (confirmed by use of H₂ ¹⁸O) attacked the carbon of the imine group accompanying with loss of NH₂R (R=NO₂, CN). For the nitroguanidine neonicotinoid pesticides (R=NO₂, including thiamethoxam, clothianidin, and imidacloprid), higher auxiliary gas heater temperature also contributed to their degradation in APCI due to in-source pyrolysis. The degradation of the five neonicotinoid pesticides studied in ESI was not significant. In ESI, only the nitroguanidine neonicotinoid pesticides could generate the degradation products through in-source fragmentation mechanism. The degradation of cyanoamidine neonicotinoid pesticides (R=CN, including acetamiprid and thiacloprid) in ESI was not observed. The degradation of neonicotinoid pesticides in the ion source of mass spectrometer renders some adverse consequences, such as difficulty interpreting the full-scan mass spectrum, reducing the sensitivity and accuracy of quantitative analysis, and misleading whether these pesticides have degraded in the real samples. Therefore, a clear understanding of these unusual degradation reactions should facilitate the analysis of neonicotinoid pesticides by atmospheric pressure ionization mass spectrometry.

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在通过质谱分析新烟碱类农药标准品（噻虫嗪，布比尼丁，吡虫啉，对乙酰氨基酚和噻虫啉）时，这些农药的降解（MC = NR降解为MC = O，M为骨架部分，R为NO ₂或在大气压电离界面（ESI和APCI）中观察到CN。在APCI中，研究的所有五种新烟碱类农药均发生了降解，其主要机理是源内离子/分子反应，其中一个水分子（通过使用H ₂ ¹⁸ O确认）侵蚀了亚胺的碳。基团伴随NH ₂ R的损失（R ＝ NO ₂，CN）。对于硝基胍类新烟碱类农药（R = NO ₂（包括噻虫嗪，可比丁和吡虫啉），较高的辅助气体加热器温度也由于其在源内的热解作用而导致其在APCI中的降解。在ESI中研究的五种新烟碱类农药的降解并不显着。在ESI中，只有硝基胍新烟碱类农药才能通过源内裂解机制生成降解产物。在ESI中未观察到氰基idine新烟碱类农药（R = CN，包括对乙酰氨基酚和噻虫啉）的降解。质谱仪离子源中新烟碱类农药的降解会带来一些不利后果，例如难以解释全扫描质谱图，降低定量分析的灵敏度和准确性，以及误导这些农药在实际样品中是否已降解。所以，

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