Sample Helmholtz coil

In our final realization (Fig. 18), the probes use the Helmholtz coil geometry, favoring ease of use and efficient sample temperature control over a wide range of temperature values. The tunable, broad-band probe is inserted into the magnet from below and fixed to the bottom part of the magnet assembly in a simple way reminiscent of most high-resolution NMR systems. Thanks to this design, it is possible to use standard 10 mm NMR sample tubes which are inserted comfortably from above without any need to manipulate the probe. 

In the conventional measuring mode the sample stays in the NMR tube, and thus in the radiofrequency Helmholtz coil all of the time. In the continuous-flow mode it resides within the NMR detection coil only for a distinct time of some few seconds (Figure 1.2). This residence time t is dependent upon the volume of the detection cell and the employed flow rate (Table 1.1). For example, a detection volume of 120 jjlI, together with a flow rate of 0.5ml/min, results in a residence time of 14.4 s, while with a detection volume of 8 jjlI the residence time is only 0.96 s. A shorter residence time t within the NMR measuring coil results in a reduction of the effective lifetime of... 

The equipment to carry out this test consists of a pair of Helmholtz coils, which creates the non-uniform magnetic field, and a small, nonmagnetic sample holder, included between the coils and... 

One of the first things one notices about an EPR spectrum is that it is a first-derivative spectrum rather than the more typical absorption presentation. This is due to an instrumental artifact. To enhance the sensitivity of the EPR spectrometer, the magnetic field is modulated. To obtain field modulation, a small set of Helmholtz coils are place about the sample in line with the external field. These coils allow the amplitude of the external field, to change by a small amount ( 0.01 20G) at a frequency of 100 kHz (smaller frequencies can also be used, but are less sensitive). Because the spectrometer is tuned to only detect signals that change amplitude with field changes at... 

A typical experimental arrangement for measuring RF transitions between Zee-man levels in the upper state of the optical transition is shown in Fig. 5.8. A coil around the sample cell provides the RF field, while the dc magnetic field is produced by a pair of Helmholtz coils. The fluorescence induced by a polarized dye laser beam is monitored by a photomultiplier through a polarizer as a function of the radio frequency co f [514]. 

For FDMR experiments it is desirable to be able to apply an external magnetic field. In one of the arrangements the field is provided by a pair of Helmholtz coils outside the cryostat which can be rotated with respect to the sample to obtain the directions of the principal axes of the zero-field tensor. With this approach the exciting laser can be kept focussed on the same sample volume while orienting the field, but the fields are limited to lOmT. In the other set-up the sample holder is inserted into the bore of a fixed superconducting Helmholtz-type magnet immersed in liquid helium. This allows the application of fields up to 1.3 T, but the direction of the magnetic field with respect to the zero-field tensor cannot be varied. 

Aerogels thus allow a perfect control of the solidification process and an analysis of the intensity curves allows to extract further information, like the solid fraction or the eutectic fraction [31]. The photo of such a facility shown also in Figure 34.26, shows an additional feature easily build in close to the sample three pairs of Helmholtz coils are inserted, allowing to induce rotating magnetic fields and thus to stimulate fluid flow in the samples for mixing [32]. With conventional Bridgman furnaces this is only possible with... 

The experimental setup is shown schematically in Fig. 23(a). Uniformly sized microspheres with diametey ther 10, 25 or 96 pm, were dispersed in kerosene-based ferrofluid " and confined between two glass plates. The spacing between the plates were several times the diameter of the spheres. Two pairs of Helmholtz coils were used to produce two sinusoidal fields, H sinui t and H sin(w t+i/2) in the plane with amplitude H. The sample cell"(20x20 mm ) contained a very dilute dispersion of polystyrene spheres. This produces only a few pairs of spheres which were far apart and thus not interacting. The frequencies of the various modes were low (< Hz) and could easily be measured manually using a stop-watch. 

Figure 2 (A) A cylindrical EPR cavity fitted with anti-Helmholtz coils and a PTFE sample holder. The test sample comprises two portions of DPPH powder separated by a 50 pm mylar film. (B) EPR spectra of DPPH for currents ranging from zero to 5 A in the gradient coils. The signal separation of about 1 mT oorresponds to a field gradient of 20 T m". Reproduced with permission from Ikeya M and Miki T (1987). Japanese Journal of Applied Physics 2S L929.

To date, two arrangements of NMR coils, the solenoidal radio frequency (RF) coil (Figure 2A) and the saddle-type (Helmholtz) RF coil (Figure 2B) have been employed as on-line NMR detectors with CE and CEC. Theoretical studies have shown that reduction of the diameter of the RF coils increases the coil sensitivity [32], The miniaturized versions of saddle types are commonly used in commercial probes. As a major development, a saddle coil which houses 1.7-mm-diameter sample tubes has been introduced by Varian. Another significant contribution is the designing of an inverse coil to accommodate 3-mm-diameter sample tubes with a detection volume of 60 pL and a total volume of 140 pL [33], Fabrication procedures hinder further reduction of diameter of saddle-type coils that are optimized for sample volumes smaller than —1 pL. 

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