Analysis of nicotinamide adenine dinucleotide (NAD⁺/NADH)-modified electrodes is important for in vitro monitoring of key biological processes. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze NAD⁺/NADH-modified gold electrodes. Interestingly, no obvious characteristic peaks of nicotinamide fragment could be observed in the mass spectra of NAD⁺/NADH in their neutral sodium pyrophosphate form. However, after acidification, the characteristic peaks for both NAD⁺ and NADH were detected. This was due to the suppression effect of inner pyrophosphoric salts in both neutral molecules. Besides, it was proved that the suppression by inner salt was intramolecular. No obvious suppression was found between neighboring molecules. These results demonstrated the suppression effect of inner salts in ToF-SIMS analysis, providing useful evidence for the study of ToF-SIMS ionization mechanism of organic molecule-modified electrodes.

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烟酰胺腺嘌呤二核苷酸（NAD ⁺ / NADH）修饰电极的分析对于关键生物学过程的体外监测非常重要。在这项工作中，飞行时间二次离子质谱法（ToF-SIMS）用于分析NAD ⁺ / NADH修饰的金电极。有趣的是，在中性焦磷酸钠形式的NAD ⁺ / NADH的质谱图中未观察到明显的烟酰胺特征峰。但是，酸化后，NAD ⁺的特征峰和NADH被检测到。这是由于两个中性分子中内部焦磷酸盐的抑制作用所致。此外，已证明内盐的抑制是分子内的。在相邻分子之间未发现明显的抑制作用。这些结果证明了ToF-SIMS分析中内盐的抑制作用，为研究有机分子修饰电极的ToF-SIMS电离机理提供了有用的证据。

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