Lock signal

The deuterium line of the deuterated solvent is used for this purpose, and, as stated earlier, the intensity of this lock signal is also employed to monitor the shimming process. The deuterium lock prevents any change in the static field or radiofrequency by maintaining a constant ratio between the two. This is achieved via a lock feedback loop (Fig. 1.10), which keeps a constant frequency of the deuterium signal. The deuterium line has a dispersion-mode shape i.e., its amplitude is zero at resonance (at its center), but it is positive and negative on either side (Fig. 1.11). If the receiver reference phase is adjusted correcdy, then the signal will be exactly on resonance. If, however, the field drifts in either direction, the detector will... 

Deuterium ( H) is the most common lock signal. H, Li, and F have also been employed for this purpose. Can we use a nucleus as lock signal while recording a spectrum of the same nuclei ... 

Adjust the lock-phase and check for 2H lock signal saturation. 

Figure 15.7. XH HR-MAS NMR of a forest soil. (Top) Sampled and analyzed as is after the addition of 10 pi of D20 as a lock signal. Resonances in the top spectrum are those that are in contact with water, and thus at the soil-water interface. (Bottom) Same sample as top, but freeze-dried and swollen in DMSO-d6. Note that DMSO is an excellent swelling solvent and penetrates into both the polar and hydrophobic domains in NOM (Simpson et al.,2001b). See color insert.

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... 

In their works on S NMR spectroscopy in the gas phase, Jackowski et al. have used 4-8 mm o.d. glass tubes filled with gaseous mixtures and sealed. These tubes were fitted into a standard 5-10 mm o.d. tube filled with a deuterated solvent (toluene-d8, benzene-d6, acetone-d6) in the annular space to generate the lock signal. 

When solubility dictates use of different deuteriated solvents for reference and measured samples, the chemical shift (5corr measured against the external reference and corrected for susceptibility difference must be also corrected for the difference in the chemical shifts of the lock signals, A. With a deuterium lock this difference is just the difference in 2H NMR chemical shifts of the two deuteriated solvents, which equals the difference in XH shifts of their pro tic isotopomers (equation 4) ... 

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... 

Before we acquired the spectrum to the right, the field homogeneity was carefully adjusted by maximizing the deuterium lock signal but the methyl singlets are split into distorted doublets. In fact, every line in the spectrum is identically split. What is producing this effect ... 

Answer The spectrometer was sweeping the magnetic field to find a lock signal, so the resonant frequencies were changing rapidly during the collection of the FID. One observes the same ringing pattern when continuous wave spectra are swept too fast. If one tries to collect several transients under these conditions, one obtains many copies of the spectrum superimposed at different frequencies. The field sweep can be turned off if a lock is not going to be used. 

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... 

EXAMPLE 3.2 Suppose the lock signal frequency is found to be slightly less than the constant reference frequency of the rf oscillator. Should the magnetic field be increased or decreased to bring it back to the nominal value ... 

Solution Remember Eq. (3.1)] that the frequency of any signal increases in direct proportion to the field strength. Thus, to increase the lock signal frequency, we need to increase the field strength. ... 

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