Isotope concentric tubes

If concentric tubes are used, with the heavy isotope species in one compartment and the light one in the other, meticulous efforts must be taken to ensure equal concentration in both compartments to avoid effects due to concentration differences. As this is difficult, concentric tube experiments are not recommended for the study of isotope effects in equilibrium systems. 

The employment of NMR-active isotopes permits to access experimental parameters which are intrinsically difficult to measure, unless a significant concentration of the sugar is present in the NMR tube. For instance, aqueous solutions of N-acetyIncuraminic acid, labeled with 13C at Cl, C2, and/or C3, were analyzed to detect and quantify the various chemical species present in equilibrium at different pHs. In fact, in addition to the expected a and (3 pyranose forms, acyclic keto, keto hydrate and enol forms were identified on the basis of 13C NMR spectroscopic data. Besides, DFT methods were employed to predict the effect of enol and hydrate structure on the coupling constant values Jc,u and /c c involving C2 and C3, finding that 2/c2,h3 can be safely used to differentiate the cis and tram isomers of the enol forms.9... 

The reaction is monitored by 1H NMR with sample preparation as follows A 0.3-mL aliquot of the reaction mixture is removed and concentrated under reduced pressure for 10 min. The resulting residue is dissolved in 0.5 mL of methyl sulfoxide-d (DMSO-d6> (Cambridge Isotope Labs) and filtered through a pipette with a glass wool plug directly into an NMR tube. The sample is checked on a Bruker ARX-500 MHz instrument. The checker used a Bruker 300 MHz instrument, which sufficed. 

Examples of separations of isotopes are in Table 19.10(b). The concentration of U-235 listed there was accomplished in a cascade of 2100 columns, each with an effective height of 14.6 m, inner tube 5 cm dia, gap 0.25 mm, hot surface 87-143°C, and cold surface 63°C, just above the condensation temperature at the operating pressure of 6.7 MPa. Although the process was a technical success. 

The thermal diffusion method of isotope separation has broad application to liquid-phase as well as gaseous-phase separations. The apparatus widely used for this purpose consists of a vertical tube provided with an electrically heated central wire. The gaseous or liquid mixture containing the isotopes to be separated is placed in the tube, and heated by means of the wire. In such an apparatus two effects act to separate the isotopes. Thermal diffusion tends to concentrate the heavier isotopes in the cooler outer portions of the system, while the portions near the hot wire are enriched in die lighter isotopes. At the same time, thermal convection causes the hotter fluid near the hot wire to rise, while the cooler fluid in the outer portions of the system tends to fell. The overall result of these two effects causes die heavier isotopes to collect at the bottom of the tube and the lighter at the lop, whereby both fractions may be withdrawn... 

Two sets of experiments were conducted using six radioactive tracers simultaneously. Starting concentrations are given in Table III. Radionuclides were eluted from the cells with ground-water solutions using a peristaltic pump. Effluent was collected at 90-min intervals by means of a fraction collector connected to the cell with small-bore Teflon tubing. Concentrations of the various isotopes in the effluent were measured by gamma spectrometry. 

Commonly, the sample produced for heteionuclear triple-resonance 3-D NMR experiments must be ImM or greater in macromolecule concentration in approximately 400-600uL of solution in a SmM NMR tube. However, it should be noted that there are efforts underway in several laboratories aimed at developing probes which can accommodate larger sample volumes. The solution used is routinely 90% H2O with 10% D2O added for field locking. The sample should be free of impurities, especially other proteins which may copurify and may also be labeled with and N isotopes. 

Substrate titration experiments were carried out in a 5 mm NMR tube. GK was dissolved in a D2O buffer containing 20 mM perdueterated Tris and 100 mM KCl, pH 7.5 (pH meter reading without correction for deuterium isotope effects). The initial volume was 0.65 ml. The initial GK concentration was 0.63 mM. The concentrations of ATP and GMP stock solutions were 27.5 and 11.2 mM respectively. ID proton NMR spectra were acquired at 25 °C on a... 

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