NMR probes cryogenic

Applications of Small-Volume High-Sensitivity and Cryogenic NMR Probes 31... 

The most important factors in determining the s/n ratio or sensitivity of an NMR experiment in a cryogenic NMR probe, insofar as the hardware itself is concerned, are the temperature of the coil, 7/ the resistance of the coil, Rc the temperature of the sample, Ts and the resistance added to the coil by the presence of the sample or the sample resistance, Rs. The temperature of the rf coil in a cryogenic NMR probe is typically in the range of 15-30K commercial examples of the Varian Cold-probe operate at 25 K. The preamplifier noise temperature is generally in the range of 10-15 K and the coil resistance is small compared to a conventional room temperature NMR probe. The first two terms in the expression below provide the basis for the vast improvement in the performance of a cryogenic relative to a conventional NMR probe. The sample temperature, Ts, and sample resistance, Rs,... 

Another expression that illustrates how Rs impacts on cryogenic NMR probe performance is ... 

In the example of a 3 mm tube being run coaxially in a 5 mm cryogenic NMR probe, the filling factor, r, is defined by the following equation, where ... 

One additional aspect of sample preparation that impinges on the performance of a cryogenic NMR probe remains to be discussed. Again, the Rs term that plays an important role in Eqs. (3) and (4) has an important role in this issue of the effects of solvent on cryogenic NMR probe performance. The role of Rs on the effect of... 

Fig. 13. Results obtained with 4 mm samples in a 500 MHz gradient inverse triple resonance cryogenic NMR probe, (a) Non-spinning resolution of the -methanol multiplet for a 30 mm solvent column in a 4 mm tube, (b) Non-spinning resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube. As expected from Fig. 12, the resolution is lower with a solvent column of this short (the optimal solvent column for a 4 mm tube is 30 mm) is shown in Panel A. (c) Resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube with the sample spinning at 20 Hz. For very scarce samples when it is necessary to resort to the shortest possible solvent column height to facilitate the acquisition of high-quality 2D-NMR data, it may be beneficial to spin the sample during the acquisition of the proton reference spectra.

NMR solvents and examining the effects of solvent on s/n ratio losses in cryogenic NMR probes leads to some rather interesting results that are manifest in part, with 5 vs. 3 mm NMR tubes. These data are shown in Fig. 14. As shown, better results are uniformly obtained with 3 rather than 5 mm NMR tubes, especially for polar solvents such as methanol and with buffers such as 100 mM Tris. 

To illustrate the performance advantage of a cryogenic NMR probe, a pair of overnight HMBC spectra were acquired for a sealed 3 mm NMR tube containing... 

Fig. 14. Plot of solvent vs. s/n ratio losses in 3 and 5mm NMR tubes in a cryogenic NMR probe. Better probe performance is obtained in all cases with a 3 mm tube rather than a 5 mm tube, even for polar pro tic solvents such as methanol and 100 mM Tris buffer. (Data generously provided by D. Avizonas and T. de Swiet, Varian NMR Instruments, Palo Alto, CA. Reproduced with permission.)...

APPLICATIONS OF SMALL-VOLUME HIGH-SENSITIVITY AND CRYOGENIC NMR PROBES... 

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