Lock, field/frequency

Fast Fourier transform (FFT) A procedure for carrying out Fourier transformation at high speed, with a minimum of storage space being used. Field frequency lock The magnitude of the field Bq is stabilized by locking onto the fixed frequency of a resonance in the solution, usually of the solvent. 

As mentioned above, in an ENDOR experiment the rf field is swept while the static magnetic field is held at a constant position in the EPR spectrum. For slow sweep rates and narrow EPR lines a device would be desirable which is able to stabilize the ratio of the microwave frequency to the static magnetic field. The applicaiton of a commercially available field/frequency lock system is restricted to a region of 6 mT about the DPPH resonance field33). In metal complexes with strongly anisotropic EPR spectra, however,... 

For quantitative estimation, a sealed reusable capillary tube, with a known quantity of sodium salt of trimethylsilyl propionic acid (TSP) dissolved in 35 pi of D20, is inserted into the NMR tube while obtaining NMR spectra. The internal standard TSP is used as a chemical shift reference as well as a quantitative standard for the estimation of metabolites, and D20 is used as the field-frequency-lock . Spectra are acquired at room temperature. Typical spectra acquired at room temperature of human bile and standard glycine- and taurine-conjugated BAs are shown in Fig. 5.4.16. 

One aspect of the experimental conditions under which these spectra were obtained is important to understand so that the spectra can be rationally interpreted. For solubility and stability purposes, peptides are generally dissolved in buffered water. Recall from Chapter 3 that compounds prepared for NMR experiments are almost always dissolved in deuterated solvents. The need for deuterated solvents is so that the spectrometer can remain stable for the duration of the experiment by way of the field/frequency lock. The lock signal comes from the deuterium NMR signal of... 

A sample for NMR spectroscopy can be taken from several stages of the sample preparation path (see Chapter 9) (14-19-28). Preferably, this would be a 5-10-mL portion of extract or of aqueous or organic liquid. Common to all these solutions is the large molar excess of H in the solvent compared to the amount of 111 in the possible target chemicals. This yields an intense solvent (e.g. H20, CH2C12) resonance in the 111 NMR spectrum, making the trace analysis difficult or even impossible. A usual procedure to avoid this problem is to replace the protonated solvent (e.g. H20) with the corresponding deuterated solvent (D20). Deuter-ated solvents are also used for the field-frequency lock of the spectrometer. 

The 500-MHz, H-n.m.r. spectra were recorded with a Bruker WM-500 spectrometer operating in the pulsed, Fourier-transform mode and equipped with a Bruker Aspect2000 computer having an 80k memory-capacity. The D resonance of D20 was used as the field-frequency lock-signal. The spectra were obtained by using a 90° pulse-width, and accumulated into 16k addresses with an acquisition time of... 

Nitrogen-15 nmr spectra were obtained at 9.12 and 20.72 MHZ on spectrometers using external time-share, field-frequency lock. Details of the spectrometer modifications are published elsewhere (19). 

Suitable solvents are those which dissolve lignin to the maximum extent possible without interfering significantly with the lignin NMR signals. Usually the solvent is deuteriated. The solvent deuterium NMR resonance is usually used for the field-frequency lock signal. The most common lignin solvents and their relevant... 

It might appear that a magnetic susceptibility correction would be needed if the susceptibilities of sample and reference differ, but this is not the case. With the field/frequency lock established via the deuterated solvent, the applied magnetic field (H0) simply shifts slightly to maintain the magnetic induction (B0) inside the sample tube constant so as to keep the 2H on resonance. If different deuterated solvents are used for sample and reference, a simple correction must be made for the difference in their 2H chemical shifts. 

A Varian Unity Inova 600-MHz NMR instrument (Palo Alto, CA) equipped with a H C/ N pulse field gradient triple resonance microliow NMR probe (flow cell 60pL 3mm O.D.) was used. Reversed-phase HPLC of the samples was carried out on a Varian modular HPLC system (a 9012 pump and a 9065 photodiode array UV detector). The Varian HPLC software was also equipped with the capability for programmable stop-flow experiments based on UV peak detection. An LCQ classic MS instrument, mentioned in the previous section, was connected on-line to the HPLC-UV system of the LC-NMR by contact closure. The H resonance of the D2O was used for field-frequency lock, and the spectra were centered on the ACN methyl resonance. Suppression of resonances from HOD and methyl of ACN and its two C satellites was accomplished using a train of four selective WET pulses, each followed by a Bo gradient pulse and a composite 90-degree read pulse [41]. 

Heteronuclear NMR experiments, which can be performed with the standard equipment of practically all modern spectrometers, require in general three separate radiofrequency (RF) channels for both spectrometer and probe head. The first two channels deliver the H (for decoupling) and "X frequencies to the sample, and the third channel is commonly tuned to D and operates the field frequency lock. In most standard probe heads, these three frequencies are delivered via two concentric coils. The inner coil with the higher Q factor is generally used for detection, the outer one only for the application of pulses and decouphng. TWo general designs are in use in normal or forward probe heads, which are optimized for direct detection of X nuclei, the inner coil is a tuneable X coil and the outer coil is normally double tuned to and the lock frequency, while in inverse probe heads which are optimized for indirect detection of "X resonances via H, this order is reversed. 

Alkali metal NMR spectra were observed at the appropriate resonance frequencies listed in Table I, using 12-mm tubes and a Varian XL-100 spectrometer with Gyrocode Observe capability. External 19F or internal H field-frequency lock was used. Depending on the linewidth of the resonance being observed, spectral widths of 256 Hz to 12 kHz in 8192 frequency domain points were used. For 23Na and 85Rb, 90° pulses of 50/isec and a 0.1-sec repetition rate were used. For 6Li and 133Cs, the approximately 55° pulses were 30 /xsec, and the pulse repetition rates for the Nafion samples were 60 sec and 1 sec, respectively. 

Maciel et have observed an isotope effect of —0-28 p.p.m. on the chemical shift of the carbonyl group in acetone- /g with respect to that of the carbonyl shift of acetone. The authors discuss the results in terms of vibrational zero-point energies, and in addition the paper describes a useful method of observing resonances using a direct field—frequency lock system. 

The ideal solvent should contain no protons and be inert, low boiling, and inexpensive. Since pulsed instruments depend on deuterium in the field-frequency lock, deuterated solvents are necessary. Deuterated chlo-... 

Solvents must have no resonances of their own in Ihe spectral region of interest. Usually. NMR solvenis are deulcrated to provide the field-frequency lock signal (sec Section 19C-2), I hc most commonly used... 

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