Field lock

MHz) for spin decoupling and H (61 MHz) for field locking. The container for liquid samples is shown in Figure 2.5. It consists of a normal high precision 5 mm NMR tube cut to a length of 60 mm and closed with a piston and a cap made from the machinable ceramic Macor. These probes were successfully used to study solvent exchange on solvated metal ions and metal ion complexes [14, 15]. 

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. 

Many biofiuids are not chemically stable and for this reason care should be taken in their collection and storage. For example, cell lysis in erythrocytes can easily occur. In addition, if the biofiuid has been reconstituted into D2O after fireeze-drying or if a substantial amount of D2O has been added to provide an NMR field lock, then it is possible that certain H NMR resonances will be lost. These include not only NH and OH protons as expected but CH groups where the C-H bond is labile such as H2 of imidazole moieties (as in histidine or histidinyl-containing proteins such as... 

A Varian XLFT-100 Fourier Transform nmr Spectrometer interfaced with a Varian 620-L minicomputer with magnetic tape storage provided high-resolution, proton-decoupled spectra of natural abundance carbon-13 at 25.2 MHz. For identification of carbon peaks, chloroform-d solutions of surfactant (solubility about 20 wt%) were prepared. Chloroform-d also served for a deuterium field lock. Samples of surfactant in water or decane were placed... 

Lock. Syn. field lock. The maintenance of a constant applied field strength through the use of an active feedback mechanism. 

The field lock is established by slightly varying the strength of the applied field Bq until the frequency being generated in the NMR console is the same as the Larmor frequency of the H s of the solvent in the sample. At this point, a phase-locked loop circuit is used to lock onto the frequency. From then on, magnet drift is compensated for, unless the limit of the ability of the instrument to adjust the applied field strength is reached. 

Field lock. Syn. deuterium field lock, lock, lock. The holding constant of the strength of the applied magnetic field through the monitoring of the Larmor frequency of one nuclide (normally H, but possibly F) In the solution and making small field strength adjustments. 

Fig. 2.11 (a) ESR, (b) ENDOR, and (c) ENDOR-induced ESR (EIE) spectra of a single crystal of HC02Li-H20, X-irradiated to a dose of about 25 kGy and measured with the external magnetic field oriented along crystal axis . The ENDOR spectra are recorded with the external magnetic field locked at the positions indicated by the arrows and the labels in the EPR spectrum. The EIE spectra are recorded with the ENDOR frequency locked to the ENDOR transitions li2 and Li5, as indicated by the labels. The figure is reproduced from [29(c)] with permission from Dr. T.A. Vestad... 

Fig. 3.26 Schematic powder ENDOR spectra of an S = Vi species with axially symmetric g and H hyperfine structure. ENDOR spectra with the magnetic field locked at g and gi, respectively, are single-crystal like due to angular selection. The lines for electronic quantum numbers ms = Vi and -V2 are separated by distances equal to A and Aj, the principal values of the hyperfine coupling tensor as indicated in the figure...

The ENDOR signal is assumed to be proportional to the ESR absorption with the field locked at the ESR transition - /2, i> -> /2, j>, i.e. to s(B - By)V as indicated in Eig. 3.38 [53]. This factor affects the shape of the calculated ENDOR spectra, and gives rise to angular selection even when the g- or other anisotropy is not completely resolved in the corresponding ESR spectra. 

Fig. 3.38 Mechanism of angular selection in powder ENDOR. The ENDOR signal for the transition between the states M, J>. o- M, k>, M = V2 with the field locked at the ESR transition I-V2, i>...

Because liquid-liquid chromatography can be considered as an extension of gas-liquid clu-omatography, it warrants mention in this context. The two techniques differ only in the density of the respective eluants and in the degree of solute-mobile phase interactions. Unfortunately only activity-coefficient ratios can be determined directly, i.e. the ratio of activity coefficient of the solute at infinite dilution in the mobile phase to the activity coefficient of the solute in the stationary phase. Therefore, to obtain an activity coefficient at infinite dilution one of the coefficients must be determined by another method. In many cases, g.l.c. could supply this, thus making the two techniques complementary. The U.c. method is rapid and easy to operate and could make valuable contributions to this field. Locke has pioneered this work which he reviewed in 1969. ... 

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