Probe, spectrometer tuning

Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to H is to crudely shim on H2O. If there is no proton channel most multinuclear probes will tune to D, so D2O can be used. For CP-MAS probes that tune to - C, adamantane is a useful compound which should be shimmed under spinning and H decoupling conditions. A typical resolution for in admantane of 3-4 Hz at 7.05 T and 10 Hz at 11.7 T should be achieveable. 

Any definition of a low-y nucleus will be somewhat arbitrary, but a convenient practical definition is based on the tuning frequency ranges of commercial MAS probes. Since standard probes normally tune down to N, nuclei which resonate below this frequency (40 MHz on a spectrometer equipped with a 9.4 T magnet) will be defined as low-y for the purpose of this Chapter. The lower limit of this frequency range is taken to be 10 MHz, since this is the lowest frequency at which most commercial spectrometers will operate. 

With the test sample in the magnet, the probe is tuned to H and the magnet homogeneity maximized. Next, a test spectrum is determined in which the tp used is unimportant. This spectrum then serves as a starting point for another spectrum, which has a reduced spectral width. The original sw is now reduced to about 500 Hz, and the transmitter offset is adjusted so that it is in the middle of the reduced sw. Many spectrometers have programs that do both operations with one command. Since sw has been considerably reduced, the number of data points should also be decreased, to approximately 4,000, so as to maintain an acquisition time of around 4 s. 

When macroscopic oscillations occur (as monitored, e.g., with a Kelvin probe or by a mass spectrometer tuned to CO2), the partial pressures change periodically with time. Since the mean free path of the gas molecules is at the applied pressures considerably larger than the vessel diameter, this gives rise to a global coupling mechanism [71]. The coupling is essentially mediated by CO because O2 is present in excess (and CO2 is just an inert product). As... 

The heart of an NMR spectrometer is the probe, which is essentially a tuned resonant circuit with the sample contained within the main inductance (the NMR coil) of that circuit. Usually a parallel tuned circuit is used with a resonant frequency of coq = The resonant frequency is obviously the most important probe... 

An automatic probe tuning and matching (ATM) accessory allows one to automatically tune the NMR probe to the desired nuclei s resonant frequency and match the resistance of the probe circuit to 50 Q [7]. Traditional NMR instruments are designed so that one must perform these adjustments manually prior to data acquisition on a new sample. The advent of the ATM accessory allows the sampling of many different NMR samples without the need for human intervention. The ATM in conjunction with a sample changer enables NMR experiments to be conducted under complete automation. The sample changers are designed so that once the samples are prepared, they are placed into the instrument s sample holders. Data are then acquired under software control of both the mechanical sample delivery system as well as the electronics of the spectrometer. 

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. 

More information about the sequential pathway versus one in which the 0—0 and C C bond homolyses occur conceitedly was obtained by means of picosecond IR absorption spectroscopy. While IR spectroscopy, in principle, can provide more structural information, hmitations stemming from the weakness of some IR intensities can be detrimental to the detection of a reactive intermediate. In this case, the CO2 photoproduct was monitored (near 2335 cm ) instead of radical intermediates 2 or 3. It was argued that neither 1, 2, nor 3 absorbed in this region. Excitation of 1 in CCLi was achieved with a 308-nm, 1-ps pulse. The 1.3-ps probe pulse may be tuned to cover a range from 2000 to 4300 cm with the spectrometer that was used, but in these specific experiments the interrogation window was from... 

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

The second RF field may be introduced into the high-resolution NMR spectrometer probe either by winding an extra transmitter coil (11) or by double-tuning the existing transmitter coil. (12, 13)... 

Signals from different elements cannot normally be acquired in a single NMR experiment since they require changes in spectrometer hardware. However, by using an NMR probe which is tuned to several different resonance frequencies, it is possible with modem spectrometers, to acquire signals concurrently from more than one nuclear isotope, e.g., 31P, H, and nC (Styles et al., 1979), and thus markedly increase the information content of the experiment. It is also possible to detect one or more nuclei indirectly via another nucleus (see below). 

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