Convection in liquid-state NMR : expect the unexpected

Temperature gradients in liquid-state NMR samples are unavoidable, but undesirable: they lead to sample convection, and consequently to signal attenuation in experiments that use field gradients. This paper illustrates how widely the dependence of sample convection velocity on the temperature at which the sample is maintained can differ between different probes and different spectrometers, including the first such results for cryoprobe systems, and highlights the importance of understanding this dependence if the effects of sample convection are to be kept to an acceptable minimum. It is sometimes thought that efficient sample temperature control should suffice to avoid convection: alas, this is not true, and rapid sample convection can occur even with the best hardware. Previous experiments have shown that the effects of convection can sometimes be avoided by setting the sample temperature regulation to one particular temperature; here it is shown that no such temperature exists in some probes. The issue of convection is all too often swept under the carpet; these results confirm that it is a more general problem than is commonly realized.


Table of contents:
 Figure S1: Representative examples of fitted experimental data over a wide range of temperatures and convection rates. Table S1: Values of decremented and incremented delays, d5 and d3, used in the convection experiments. Table S2: Nominal values of gradient (G/cm) for the five spectrometers used in the convection velocity measurements. Electronic Supplementary Material (ESI) for RSC Advances.This journal is © The Royal Society of Chemistry 2016 In the main text we demonstrate the convection velocity behavior in a range of spectrometers, probes and NMR tube geometries.The Figure S1 shows representative examples (400 MHz, BBI probe using a 5 mm NMR tube) of fitted experimental data over a wide range of temperatures (5, 9, 13, 17, 21, 25, 29, 33 and 37 °C) and convection rates.The total delay imbalance ΔΔ is twice the difference between d3 and d5: The pulse sequence IS_test_convexp_list was used to perform the convection experiments in combination with the (attached) macro (AU programme) entitled convection.

Figure S2 :
Flow chart of the main steps in determining convection velocities. Pulse program code (IS_test_convexp_list) for convection velocity measurements in Bruker spectometers. TopSpin AU program (convection) used in combination with the above pulse sequence. TopSpin AU program (PHC0) for automatic phase correction. TopSpin AU program (multi_pp2) for automatic peak picking.

Figure S2 :
Figure S2: Flow chart of the main steps in determining convection velocities.

Table S1 :
Values of decremented and incremented delays d5 and d3 in the attached pulse sequence used in the convection experiments, and the corresponding diffusion delay

Table S2 :
Nominal values of gradient (G/cm) for the five spectrometers used.