Laboratory of Chemical Physics, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, United States.
Anal Chem. 2022 Apr 19;94(15):5729-5733. doi: 10.1021/acs.analchem.2c00264. Epub 2022 Apr 8.
Due to similar reactivity of organic hydroperoxides (OHPs), an HPLC separation step is typically required for their indirect (chemical) quantification in mixtures. The high sensitivity of chemical shifts to chemical structure makes NMR an ideal tool for the simultaneous quantification of OHPs in mixtures, but the concentration of these analytes in the samples of interest is usually well below the sensitivity of standard NMR experiments. This sensitivity problem can be mitigated by taking advantage of the fact that the magnetization of the HO resonance recovers at the rate of hydrogen exchange with water, which is significantly faster than longitudinal relaxation, thus enabling very fast scanning for signal-to-noise enhancement. An adaptation of the pulse is described that suppresses the water signal by more than 4 orders of magnitude, yielding uniform excitation of peroxide signals without interference of the ca. 10-fold stronger HO resonance. We demonstrate the method for a mixture of OHPs and report the chemical shifts for multiple OHPs that are of interest in atmospheric chemistry. As shown for hydroxymethyl hydroperoxide, the chemical decay of OHPs can be tracked directly by NMR spectroscopy.
由于有机过氧化物 (OHPs) 的反应性相似,因此通常需要通过 HPLC 分离步骤才能对混合物中的 OHPs 进行间接(化学)定量。化学位移对化学结构的高灵敏度使 NMR 成为混合物中 OHPs 同时定量的理想工具,但感兴趣样品中这些分析物的浓度通常远低于标准 NMR 实验的灵敏度。通过利用 HO 共振的磁化强度以与水的氢交换速率恢复的事实,可以缓解此灵敏度问题,这比纵向弛豫快得多,从而能够进行非常快速的扫描以增强信号与噪声的比值。描述了一种 脉冲的适应方法,该方法将水信号抑制超过 4 个数量级,从而在没有约 10 倍强的 HO 共振干扰的情况下均匀激发过氧化物信号。我们展示了该方法在 OHPs 混合物中的应用,并报告了大气化学中多个 OHPs 的化学位移。如羟甲基过氧化物所示,OHPs 的化学衰减可以直接通过 NMR 光谱进行跟踪。
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