NMR probe designs

The magnets utilized in these systems are typically neodymium-iron or samarium-cobalt disks with appropriately designed pole shoes that are placed with a variable air gap between them to accommodate an NMR probe designed to allow sample tubes (or flow probes) of 5 0 mm diameter. The magnet disks are typically wrapped along their sides with double wound wire to heat magnets to maintain a stable 35 °C operating... 

Figure 2.1 Schematic drawing of a high pressure NMR probe designed for wide-bore cryo-magnets and a sample cell [10].

Luckily due to recent developments in NMR probe design, there is now a third option, which can, to some extent, be used to address key questions as to the... 

Fig. 4.19 The pressurizabie NMR probe designed by Benedek and Purcell. (Reprinted with permission from J. Chem. Phys. 1954, 22, 2003 American Institute of Physics.)...

Fig. 4.29 A high-pressure NMR probe, designed for temperatures up to 700°C. (Reprinted with permission from J. Magnet. Res., 35, 11, (1979).)...

A new development in high-pressure NMR probe design is a multipurpose high-pressure autoclave made from the thermoplastic polyetheretherketone (PEEK) (67). This NMR autoclave was used for in situ NMR imaging of a compressed gas system, namely, the exchange of methanol for liquid CO2 in nanoporous silica-alcogels, reported for the first time. 

With recent advances in NMR probe design, sensitivity has become less of a problem and low sensitivity experiments, such as 2D INADEQUATE, are becoming routine. Indeed, the latest generation cryogenic probes allow recording INADEQUATE [32, 33] spectra in as httle as 10-15 mg of sample within just a few hours of measurement time. On the other hand, as discussed above, multi-receiver experiments provide new ways of obtaining more information from a single measurement. 

The significant improvements in sensitivity achieved during the last 5 years have been because of improved probe design and radiofrequency circuits. Since the probe needs to be located very close to the sample, it must be made of a material with a low magnetic susceptibility, for otherwise it would cause distortions of the static magnetic field thereby adversely affecting line shape and resolution. Much research has therefore been undertaken by NMR spectrometer manufacturers to develop materials that... 

Fig. 2.6.6 Remotely reconstructed high field NMR images of laser-polarized 129Xe gas in the hollow CAL pores. Owing to the flow pattern where the spins have to flow around two corners [see the probe design in...

M. W. Hunter, P.T. Callaghan, R. Dyk-stra, C. E. Eccles 2003, (Design and construction of a portable one-sided access NMR probe), in Book of Abstracts 7th International Conference on Magnetic... 

A disadvantage in the utilisation of 13C NMR is the intrinsic low relative sensitivity (Table 5.14). However, higher magnetic fields and better probe design have... 

FIGURE 4.5 Schematic design of a microcoil NMR probe. (From Rehbein, J. et al., Characterization of Bixin by LC-MS and LC-NMR, John Wiley Sons Ltd., 2387, 2007. With permission.)... 

Get an NMR tube. They are about 180 mm long, 5 mm wide, and about a buck apiece for what is euphemistically called the inexpensive model. The tubes are not precision ground, and some may stick in the NMR probe. This should not be your worry, though. They also have matching, color coordinated designer caps (Fig. 134). 

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

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. 

While the early days of LC-NMR and LC-NMR-MS were plagued by the poor sensitivity of the NMR spectrometer, the recent probe design advances have provided a means to potentially overcome this hurdle. As reported in the literature, it is possible to get both ID and 2D homo-nuclear and heteronuclear correlation data on sub micrograms of materials in quite complex mixtures utilizing cryogenic flow-probes in tandem with SPE peak trappings [98]. While these technologies are still in their infancy, they have the potential to revolutionize LC-NMR as a structure elucidation technique. 

As described in Section 7.4.2, one of the challenges of online LC-NMR is the need to match the chromatographic peak to the active volume of the CapNMR flow cell. An excellent discussion of the comparison of CapLC-NMR with other NMR probe types has been provided by Lewis et al. [17] Table 7.1 shows the sensitivity comparison of the CapNMR probe with larger volume probes. It is important to note that the experimental design used by these authors adjusted the concentration of analyte such... 

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