Sandia National Laboratories, Department of Organic Materials Science, Albuquerque, NM 87185, USA.
Int J Mol Sci. 2020 May 30;21(11):3938. doi: 10.3390/ijms21113938.
Materials often contain minor heterogeneous phases that are difficult to characterize yet nonetheless significantly influence important properties. Here we describe a solid-state NMR strategy for quantifying minor heterogenous sample regions containing dilute, essentially uncoupled nuclei in materials where the remaining nuclei experience heteronuclear dipolar couplings. NMR signals from the coupled nuclei are dephased while NMR signals from the uncoupled nuclei can be amplified by one or two orders of magnitude using Carr-Meiboom-Purcell-Gill (CPMG) acquisition. The signal amplification by CPMG can be estimated allowing the concentration of the uncoupled spin regions to be determined even when direct observation of the uncoupled spin NMR signal in a single pulse experiment would require an impractically long duration of signal averaging. We use this method to quantify residual graphitic carbon using C CPMG NMR in poly(carbon monofluoride) samples synthesized by direct fluorination of carbon from various sources. Our detection limit for graphitic carbon in these materials is better than 0.05 mol%. The accuracy of the method is discussed and comparisons to other methods are drawn.
材料中通常含有少量难以表征的异相，但这些异相却显著影响着重要的性质。在这里，我们描述了一种固态 NMR 策略，用于定量分析材料中含有稀散、本质上非偶合核的少量异相区域，这些区域中的剩余核体会经历异核偶极耦合。偶合核的 NMR 信号会退相，而使用 Carr-Meiboom-Purcell-Gill (CPMG) 采集技术，可以将非偶合核的 NMR 信号放大一到两个数量级。通过 CPMG 进行信号放大的程度可以进行估计，即使在单脉冲实验中直接观察非偶合核的 NMR 信号需要不切实际的长时间信号平均，也可以确定非偶合自旋区域的浓度。我们使用这种方法通过 C CPMG NMR 来定量聚（一氟碳）样品中的残余石墨碳，这些样品是通过直接从各种来源的碳进行氟化反应合成的。我们在这些材料中对石墨碳的检测限优于 0.05mol%。讨论了该方法的准确性，并与其他方法进行了比较。
