Performance of cryogenic probes as a function of ionic strength and sample tube geometry

Abstract

The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions.

Temperature-Induced Transitions in Disordered Proteins Probed by NMR Spectroscopy

“…Additionally, due to several pK a values, this buffer is also applicable to the study of acid unfolded proteins. In general, buffer and salt concentration should be kept low as the ionic strength decreases the sensitivity of especially cryogenic NMR probes (24). This is particularly important for polyionic buffers such as phosphate (25).…”

NMR-Metabolic Methodology in the Study of GM Foods

“…For example, a 400 mM phosphate buffer was sufficient for buffering the aqueous solution of metabolites from 25 mg of lyophilized tomato powder [5,9]. On a different note, a relevant loss of sensitivity was observed when a high buffer concentration was used, particularly with cryogenically cooled probes [19]. The phosphate buffer is a suitable one for NMR applications because it has only exchangeable protons that do not produce additional 1 ) present in plant tissue extracts.…”

Nuclear Magnetic Resonance Analysis of Natural Organic Matter

“…At present, commercial probes are restricted to solution -state NMR, although cryogenically cooled HR -MAS and solid -state NMR probes are in development. In an organic solvent system, cryogenically cooled probes provide a three -to fourfold enhancement in sensitivity (Kim et al, 1995 ;Odoj et al, 1998 ;Voehler et al, 2006 ). This advantage decreases as the salinity of the sample increases.…”

“…This advantage decreases as the salinity of the sample increases. It has been recently shown that with samples that have > 4 M NaCl, the use of a smaller diameter temperature probe (1 -4 mm) can be advantageous over a cryo -probe, especially for " mass limited " samples (Voehler et al, 2006 ). However, in NOM research, the sample is often not limited (i.e., at least 50 mg of sample is available).…”