Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA.
J Magn Reson. 2013 Sep;234:141-6. doi: 10.1016/j.jmr.2013.06.018. Epub 2013 Jun 29.
Proton linewidths decrease with increasing magic-angle spinning (MAS) rates. However, without spin dilution by deuteration, even with the fastest MAS rates available today, the narrowest proton linewidths are obtained by using the combined rotation and multiple pulse spectroscopy (CRAMPS) method. Direct observation under windowed CRAMPS typically introduces several tens of times more noise, partly because wideband analog filters (e.g. 5 MHz) must be used or sometimes even bypassed. Here we report that it is possible to keep using narrowband analog filters (about 50 kHz cutoff frequency) in CRAMPS by taking advantage of the time delay caused by the filters, which is inversely proportional to the cutoff frequency. This delay coincides with typical CRAMPS cycle times, enabling acquisition of the data point in the next detection window. The noise of such CRAMPS spectra is only about 5 times larger than MAS-only spectra. This new method allows CRAMPS to be performed on systems that lack wideline hardware (wideband filters and fast ADCs), for example, older spectrometers originally intended for solution NMR.
质子线宽随魔角旋转（MAS）速率的增加而减小。然而，如果不通过氘化进行自旋稀释，即使使用当今最快的 MAS 速率，也只能通过组合旋转和多次脉冲光谱（CRAMPS）方法获得最窄的质子线宽。在窗口化的 CRAMPS 下直接观察通常会引入数十倍的噪声，部分原因是必须使用或有时甚至绕过宽带模拟滤波器（例如 5 MHz）。在这里，我们报告说，通过利用滤波器引起的时间延迟（与截止频率成反比），可以在 CRAMPS 中继续使用窄带模拟滤波器（截止频率约为 50 kHz）。这种延迟与典型的 CRAMPS 周期时间相吻合，从而可以在下一个检测窗口中采集数据点。这种 CRAMPS 光谱的噪声仅比仅 MAS 光谱大约 5 倍。这种新方法允许在缺乏 wideline 硬件（宽带滤波器和高速 ADC）的系统上执行 CRAMPS，例如最初用于溶液 NMR 的旧光谱仪。
