title: 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.
authors.affiliation: Department of Chemistry, Center in Molecular Toxicology, Vanderbilt University, Nashville, TN 37235, USA.
authors.forename: Markus W
authors.initials: MW
authors.lastname: Voehler
authors.forename: Galen
authors.initials: G
authors.lastname: Collier
authors.forename: John K
authors.initials: JK
authors.lastname: Young
authors.forename: Michael P
authors.initials: MP
authors.lastname: Stone
authors.forename: Markus W
authors.initials: MW
authors.lastname: Germann
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D005615:Freezing; D007477:Ions; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation* / Q000379:methods*; D015336:Molecular Probe Techniques; D011506:Proteins / Q000032:analysis* / Q000737:chemistry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers
chemical_list: D007477:Ions; D011506:Proteins