Detection coil

When a toroidal ferromagnetic sample is subjected to shock loading, a pressure wave of pressure P moves through the sample with a velocity U and produces a change in magnetization AM. An N-turn detection coil with inductance L is wound around the sample and connected to a resistive circuit in which the L/R time constant is longer than the time required for the shock wave to traverse the sample thickness. The current i in the coil is then... 

The alternative method is continuous-flow , in which the reactants flow through the detection coil during data acquisition. Continuous-flow NMR techniques have been used for the direct observation of short-lived species in chemical reactions [4—6]. The main difference between stopped- and continuous-flow NMR is that in the latter the sample remains inside the detection coil only for a short time period, termed the residence time, x [7], which is determined by the volume of the detection cell and the flow rate. The residence time alters the effective relaxation times according to the relationship in Eq. (2.5.1) ... 

Most of the commercially available NMR imaging probes have an accessible clear bore above and below the coil, which can be used for remote signal encoding. A detection-only probe can be inserted from the top into the bore of the magnet in such a way that the detection coil sits immediately above the imaging coil [Figure 2.6.5 (c)]. Similarly, an rf probe with a narrow body can be built that can be inserted into the... 

Because the excitation/detection coil is in the x-y plane and the longitudinal component relaxes along the z axis, T cannot be measured directly from an NMR spectrum, but must be obtained using a pulse sequence. The most commonly used pulse sequence to measure T is an inversion recovery pulse sequence (Kemp, 1986). Other commonly used pulse sequences for measuring 7j are given in Ernst et al. (1987). 

There have also been recent improvements to reduce background noise, which is caused by random thermal motions of electrons in the detection coil and in the first amplifier range.15 Cryoprobes in which the coil and first amplifier are cooled to 30 K and the size of sample coil reduced lower electrical resistance and noise. A further benefit noted by Grivet et al.15 is that "the smaller the probe, the closer the conductors are to the sample and the higher the measure of the coil s [efficiency], as felt by the nuclei."... 

The most commonly used probehead is the inverse dual probehead, which contains an additional coaxial coil that is tuned to resonance frequency, surrounding the detection coil for hetero-... 

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... 

Another CE separation method that has been adapted to on-line NMR detection for trace level separations is capillary isotachophoresis [22]. In this case, after the separation, the analyte bands are slowly moved through the capillary until they lie directly within the coil. Precise positioning of the analyte bands in the NMR detection coil can be difficult. A recent enhancement is the use of several NMR detection coils on a single separation capillary [23], In this way, the first coil acts as a scout coil and is optimized for sensitivity (not necessarily linewidth) to locate the analyte band as it moves through the coil. After an analyte band is detected, the flow is stopped after the appropriate time-interval so that the analyte bands are now located in the second coil, which is used to acquire high-resolution NMR spectra. 

Fig. 5. Basic electronic circuit for a nuclear quadrupole resonance (NQR) probe L is the detection coil C is the tuning capacitor Xra is a component to match the probe impedance to the transmitter/receiver impedance.

Figure 20 In situ NMR probe system. The direction of the magnetic field is through. The sample was set at the centre of a detection coil. Heated air was blown perpendicular to the drawing direction in the probe through a polyftetrafluoroethylene) channel.

Figure 6 Instrumental schematic of CE with two-microcoil NMR detection showing the arrangement of the separation capillary, the two outlet capillaries, and the two NMR detection coils. (Reprinted with permission from Ref. 43. Copyright 2002 American Chemical Society.)...

Figure 20 shows how flow changes the signal when a thin slice of excited spins is moving through the detecting coil. The frequency of the weak modulation is determined by the product of the first moment of Siq) and the flow velocity. 

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