Saddle coil

Fig. 2.1.3 A pair of saddle coils creates an x or y gradient (left), a pair of Maxwell coils creates a z gradient (right). The arrows on the coils indicate the current direction, the circles with...

The whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

Providing that hid remains constant, the RF coil sensitivity is inversely proportional to the coil diameter for solenoidal coils of diameter greater than about 100 p,m. It can be shown theoretically and has been demonstrated experimentally that the sensitivity of a solenoidal coil is about three times that of a saddle coil of the same size [3], In order to translate the coil sensitivity into measurement of SIN defined by Equation 7.6, we also have to consider the noise characteristics of NMR microcoils. For coils of diameter less than 3 mm, the major noise source is from the resistance of the coil itself, and the noise from even lossy biological samples can be neglected [10]. 

One of the disadvantages concerns the fact that in axial magnets it is rather difficult to use probes with solenoid RF coils. The difficulties are related to sample insertion/removal complications and to numerous spatial constraints, exacerbated by the presence of a glass dewar for sample-temperature control (see Section VI). This is unfortunate because the alternative saddle coils are substantially less efficient, especially at the relatively low excitation/detection frequencies used in FFC NMR. 

An alternative to a saddle coil would be a solenoid coil which, however, would have to be oriented perpendicularly to the magnet bore and thus to the physical axis of the probe assembly. Due to spatial constraints, such an arrangement complicates considerably sample insertion, especially when the sample temperature has to be controlled and the assembly has to include an enveloping dewar for temperature control of the sample. ... 

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. 

Because of the difficulty of fabrication, saddle coils tend to be larger than solenoids, although a 2.5-pL saddle coil probe for static samples has recently been... 

A cylindrical structure fitting the geometry of the magnet bore is the saddle coil. It can provide homogeneous B fields at moderate NMR frequencies (25 MHz) for larger volumes (up to 30 cm diameter). However, its sensitivity is lower by a factor of /3 compared to a solenoid. But a factor of >/2 can be recovered, if two saddle coils are arranged in quadrature for independent detection of the jc- and y-components of the induced signal. 

The saddle coil is an approximation of a long cylinder which carries a current distribution I [Pg.59]

Fig. 2,3.6 The saddle coil is an approximation of a cylinder with a sinusoidal current distribution, (a) Sinusoidal current distribution on a cylinder, (b) Discrete approximation of the cylinder by six parallel wires carrying current /. (c) Saddle coil obtained from (b) by connecting the current bearing wires. Adapted from [Krel] with permission from publicis MCD.

The standard LC-NMR detector is a saddle coil which is wound on the flow cell with a diameter of 2-4 mm and a length of 12-14 mm (Fig. 9.3.5) [Webl], The cell consists of a glass tube. Inflow and outflow are provided by tubings from FIFE with an inner diameter of typically 0.25 mm. The associated chromatography column is positioned outside the magnet. Because of time requirements 2D-NMR spectra can be acquired only in stopped flow mode, while in continuous flow mode ID-NMR spectra are acquired as a function of the elution time. 

Fig. 9.3.5 [Webl] Schematic drawing of an LC-NMR flow cell. A saddle coil is wound on the cell. The cell is centred in the magnet and the chromatography column is positioned outside.

Fig. 8 Design features of a wide bore probe head for HPNMR (400 MHz) measurements. 1 O-ring 2 probe jacket 3 thermal insulation 4 polyvinyl chloride 5 O-ring 6 O-ring 7 semi-rigid coaxial cable 8 connection to thermostat 9 titanium tube 10 lid 11 screw 12 capacitor 13 capacitor holder 14 aluminium tube 15 upper plug 16 sample tube 17 saddle coil 18 -Macor 19 TiA16V4 vessel 20 lower plug 21 lower pressure screw 22 capacitor 23 coaxial cable and 24 capacitor holder.

NMR imaging can be applied for different problems of network characterization. The following experiments were done with a VARIAN unity 200 MHz (wide bore) equipped with a homemade imaging probe of following specifications active-shielded design, maximum gradient 5 mT/cm RF part resonance frequency 67 MHz (deuterium), 200 MHz (protons) saddle coil or solenoid with different inner diameter (5 mm, 7 mm, and for deuterium coil 26 mm) temperature control between 0 °C and 120 °C. 

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