The field-frequency lock

Despite the impressive field stability provided by superconducting magnets they still have a tendency to drift significantly over a period of hours, causing NMR resonances to drift in frequency leading to a loss of resolution. To 

The lock channel regulates the field by monitoring the dispersion mode deuterium resonance rather than the absorption mode signal that is usually considered in NMR, and aims to maintain the centre of this resonance at a constant frequency (Fig. 3.45). A drift in the magnetic field alters the 

The first procedure for locking is to establish the resonance condition for the deuterium signal, which involves altering either the field or the frequency of the lock transmitter. Of these two options the latter is preferred since it avoids the need for changing transmitter frequencies and is now standard on modem instruments. Beyond this, there are three fundamental probe-dependent parameters that need to be considered for optimal lock performance. The first of these is the lock transmitter power used to excite the deuterium resonance. This needs to be set to the highest usable level to maximise the signal-to-noise ratio but must not be set so high that it leads to lock saturation. This is the 

Despite the impressive field stability provided by superconducting magnets, they still have a tendency to drift significantly over a period of hours, causing NMR resonances to drift in frequency leading to a loss of resolution. To overcome this problem, some measure of this drift is required so that corrections may be applied. On all modem spectrometers, the measurement is provided by monitoring the frequency of the deuterium resonance of the solvent. The deuterium signal is collected by a dedicated observe spectrometer within the instrument that operates in parallel with the principle channels, referred to as the lock channel or simply the lock. 

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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... 

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. 

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. 

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... 

Figure 1 (A) 32.44 MHz spectrum of the alcoholysis products of P4S10 with methanol, ethanol, and isopropanol. The sample is dissolved in toluene, with external CeDe as the field frequency lock. (B) The P APT spectrum of the same mixture, showing the encoding of functional group and multiplicity information into the intensity and phase of the resonances. (Reprinted from Jancke H, Radeglia R, Neels J, and Porzel A (1984) Application of the attached proton test technique in P NMR spectroscopy. Organic Ma etic Resonance 22 376-378 Wiley.)...

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