Lock channel

TD-NMR and HR-NMR spectrometer systems have a majority of components in common. All spectrometers consist of a magnet, magnet temperature sensors, magnet heater power supply, RF frequency synthesizer, pulse programmer, transmitter/amplifier, sample probe, duplexor, preamplifier, receiver, and ADC, all controlled by a computer. In addition to these items a HR-NMR has several other requirements which include an electromagnetic shim set, a shim power supply, and a second RF locking channel tuned to the resonance frequency of Li. The second RF channel is identical to that of the observed H channel. Figures 10.9 and 10.10 show the basic setup of TD-NMR and HR-NMR spectrometers, respectively. 

These types of experiments call for efficient doubly tuned coils, ideally with a separate deuterium lock channel. For more complex molecules, such as proteins, considerably more intricate NMR pulse sequences, such as (HNCO) [32,33], require the probe to operate at three or four distinct frequencies. High efficiency is demanded from the proton observe channel. Ideally, the additional circuitry allowing multiple tuning should not interfere with the proton efficiency when compared to a singly tuned proton coil. In practice, some reduction is tolerated. The two most important design criteria for such... 

THE DEUTERIUM LOCK FEEDBACK LOOP 3.3.1 The Lock Channel... 

If decoupling of S is needed in F2 then add a delay d2 after p4 and nse GARP decoupling. A filter to block the FI deconpling irradiation is essential in the lock channel. 

A filter to block the F1 decoupling irradiation is essential in the lock channel. 

Lithium-6 edited C-spin-echo methods have been described and can be used with success in an investigation of aggregate formation of lithiumorganyls [12]. Since Li resonates close to deuterium, the experiments can in principle be performed in a standard multinuclear probehead by employing the lock channel as the third pulsed radiofrequency channel. 

C, Li3 spin systems). The HMQC experiment, which also allows a straightfoward determination of the involved C, Li coupling constants, is especially easy to perform with a triple resonance probehead which has, aside from the H and the lock channel, a fixed frequency for and a variable X frequency (see Chapter 2). It is of interest to note that Li, C cross peaks can also be observed in cases where the corresponding Li, C coupling is not resolved in the ID spectrum [14]. An example for an experiment using sequence (v) is shown in Figure 14 with the spectrum of isopropyllithium, where in hydrocarbon solvents solvation leads to the formation of tetramers and hexamers [76]. 

It is important to avoid lock instability due to either saturation or suspended particles, because instability interferes with magnet field regulation by the lock channel and, in severe cases, can result in loss of the lock signal altogether. When a stable lock level is achieved somewhere around midrange, the lock phase (see next subsection) should be maximized. Only an approximately maximum lock phase, however, is desired at this point, because the lock phase is dependent on the homogeneity of the magnetic field. 

The deuterium nucleus, used normally for the lock channel, can be employed as an internal NMR thermometer." Using two deuterated compounds, a power pseudo-FID can be constructed from either a conventional continuous wave (CW) deuterium sweep or a pulsed deuterium FID. The Fourier transform of this signal gives a Lorentzian line with double the natural... 

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 deuterium lock channel is the part of the NMR instrument that monitors the frequency of the H s in the sample and adjusts the strength of the applied field so that the frequency of whatever nuclide is being observed is known. The frequency of the NMR signal is monitored every 500 ns and applied field strength is adjusted to maintain a constant value. 

Shimming is normally carried out by maximizing the amplitude of the detected signal in the lock channel. Because the number of H s in the detected region is constant, the area of the signal from the solvent is also constant as long we do not apply too much RF power that is tuned to the Larmor frequency. 

RF channel. The portion of the instrument devoted to generating RF with a specific frequency. There are four types of RF channels that may be found in an NMR instrument a highband channel (for H and % and maybe H), a broadband channel, for all nuclides with Larmor frequencies at that of and lower, a lock channel (devoted exclusively H), and a fullband channel (any nucleus). Most instruments have one of the first three channels listed above. 

Not counting the lock channel, an NMR probe will typically have two or possibly three RF channels. One of the channels is almost always a highband ( H and or H-only channel, whereas the remaining channel or channels will be tunable to the NMR frequency of one or more of the nuclides that fall in to the broadband frequency range to N or lower—this range includes C). 

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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