Stabilization Channel Lock

Following from eq. (1.8) and Fig. 2.35, the field/frequency ratio must not necessarily be stabilized for the nuclei to be observed (homonuclear lock). It can also be set using any other nucleus (heteronuclear lock). 

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The search for suitable borides of alkali metals recently led to a new lithium boride with hexagonal Bs-octahedra framework unknown until now. Strong evidence for the existence of planar LiaB molecules in the wide channels of the framework is given by the structural analysis ( ). This compound points out another aspect involving reactions of solid phases namely, the possible release of small molecules, which are clathrate-like, locked in the structure and are therefore stabilized. 

The ideal solvent should contain no protons and be inert, low boiling, and inexpensive. Deuterated solvents are necessary for modem instruments because they depend on a deuterium signal to lock or stabilize the B0 field of the magnet. Instruments have a deuterium channel that constantly monitors and adjusts (locks) the B ) field to the frequency of the deuterated solvent. Typically, H NMR signals are in the order of 0.1 to several Hz wide out of 300,000,000 Hz (for a 300 MHz system), so the B ) field needs to be very stable and homogeneous. 

Efficiency of spin-locks. Examine the efficiency of Zj spin-locks on / and S rf channels using pulse sequences discussed elsewhere. The longest time for which the magnetization can be spin-locked on both channels depends on the strength of the rf power, stability of rf amplifiers, rf field inhomogeneity, effects of phase transients, span of the chemical shift, T p of the nuclei (assuming no spin-lock on the other channel), and the quality of the probe. In addition, on the /-spin channel, it also depends on the degree of suppression of the / / dipolar... 

Because a large fraction of the total detected signal must be removed to leave the small fraction of the signal of interest, RE stability is critically important. The use of bandpass RE filters in the ff and C channels improves the quality of the data by preventing the ff lock channel and other RE sources from disturbing the ff and C spin populations. 

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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