Cryomagnetic

In a series of crystallographic and cryomagnetic studies, Sinn et al. uncovered yet new variations in the magnetic properties of the Fe(R2Dtc)3 complexes (93a). 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

Thus, in conventional installations the HPLC instrument is located at distances between 1.0 and 2.0 m from the cryomagnet, whereas with new available shielded cryomagnets the HPLC instrument can be directly hooked to the cryomagnet. 

This experimental design has the big advantage that it can be easily accomplished. The operation mode of the NMR instrument from routine NMR data acquisition to the LC-NMR mode can be easily changed by removing the routine probe from the room-temperature bore of the cryomagnet and inserting... 

Therefore from a practicle viewpoint it is advisable to use only two proto-nated solvents in a HPLC separation, e.g. H2O and CH3CN. To obtain field/ frequency stabilization of the cryomagnet it is advisable to exchange H20 versus D20, but even here the intense HDO signal has to be suppressed. 

Figure 7.3.2.1 shows the current experimental arrangement employed in Tubingen for capillary separations. A capillary HPLC pump is placed at a distance of about 2.5m from the 14T (proton resonance frequency, 600 MHz) cryomagnet,... 

The HPLC is connected by red polyether ether ketone (PEEK) tubing to the NMR flow cell which is inside the magnet. With shielded cryomagnets or ultra-shielded magnets the HPLC can be as close as 30-50 cm to the magnet versus... 

As mentioned in the section of modes of operation for LC-NMR (Section 20.3.2), with the use of shielded cryomagnets, the location of the MS instrument will follow the same rule as for the HPLC. The most common modes of operation for LC-MS-NMR are on-flow and stop-flow. With stop-flow, the MS instrument can also be used to stop the flow on the chromatographic peak of interest that is to be analyzed by NMR. These two modes are presented here with an example. In the loop collection mode, the MS of the LC-MS-NMR system may also monitor the trapping of the chromatographic peak inside the loop. 

Working with an NMR spectrometer is slightly more difficult than, for instance, with an electron microprobe. As the NMR frequencies are all in the radio frequency range, NMR spectrometers are computer operated radio transmitting and receiving systems controlled by a computer. A receiver and transmitter antenna surrounds the sample, which is located in the cavity of a super-conducting cryomagnet. 

Since the B-field must be parallel to the ion beam, the cryomagnet system requires a horizontal setup in contrast to that of NMR spectrometers. The magnetic fields of all... 

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