Radiofrequency coils

Fig. 5.3.8 Photograph of the detection region of the NMR probe with radiofrequency coil. A methane—air mixture was ignited above the zeolite pellets. The mixture also contained xenon for NMR detection. Hp-129Xe NMR spectra with 30% xenon (from high-density xenon optical pumping) in 70% methane is depicted. (1) The spectrum in the absence of combustion and (2) the spectrum during combustion. Adapted from Ref. [2],...

Because sensitivity depends on so many different experimental factors, NMR spectroscopists generally use the signal-to-noise ratio, SIN, as a figure of merit for sensitivity comparisons. For example, in a comparison between NMR probes or spectrometers from two vendors, the spectral SIN measured for a standard sample acquired with specified acquisition parameters and probe geometry would provide a direct indication of relative sensitivity. The SIN is calculated for an NMR experiment as the peak signal divided by the root mean square (RMS) noise, given by Equation 7.6, and is directly related to the performance of the radiofrequency coil [3,6]... 

This experiment presents the measurement of uranium with an inductively coupled plasma mass spectrometer (ICP-MS). In this system, a nebulizer converts the aqueous sample to an aerosol carried with argon gas. A torch heats the aerosol to vaporize and atomize the contents in quartz tubes. The atoms are ionized with an efficiency of about 95% by an RF (radiofrequency) coil. The plasma expands at a differentially-pumped air-vacuum interface into a vacuum chamber. The positive ions are focused and injected into the MS while the rest of the gas is removed by the pump. The ions are then accelerated, collected, and measured as a function of their mass. Losses at various stages, notably the vacuum interface, result in a detection efficiency of about 0.1 %, which is still sufficient to provide great sensitivity. The amounts of uranium isotopes in the sample are determined by comparisons to standards. Because different laboratories have different instruments, the instructor will provide instrument operating instmctions. Do not use the instrument until the instructor has checked the instrument and approved its use. 

Fig. 12.41. Schematic of an NMR imager showing the approximate size and relative position of the patient, magnet, and radiofrequency coils in one of several arrangements possible.

Allis and co-workers54 used 87Rb NMR to measure the K+ flux into cardiac muscle. Hearts from Wistar rats were perfused in the Langendorff mode91,92 with modified Krebs-Henseleit buffer (KHB).93 The heart hung freely within the radiofrequency coil, thus eliminating the problems associated with Rb+ accumulation within any enclosing perfusion chamber. The temporal resolution between successive spectra was 250 s. Quantification of absolute amounts... 

The tubing is used for sample delivery to the coil. This radiofrequency coil is intended for both concentration measurements and viscosity measurements. The coil is approximately one order of magnitude smaller than standard spectroscopy/ imaging coils. 

The performance of the spectrometer is impacted by changes in the dimensions of the radiofrequency coil. The signal-to-noise ratio (S/N) of an NM R experiment is increased by decreasing the size of the radiofrequency coil. Decreasing the size of the receiver coil results in increasing the S/N ratio since this ratio is proportional to the inverse of the coil diameter (for saddle and solenoid geometries). This functionality holds until the diameter of the coil is approximately 100 pm and... 

NMR spectra of liquids are routinely acquired in 5-mm NMR glass tubes inserted into the coaxial radiofrequency coil of an NMR probe in the room-temperature bore of a ciyomagnet (Figure 7-1). 

In 1987 Gahler and Golub suggested a new type of spectroineter designed as "neutron resonance spin echo" or on "zero field neutron spin echo" that uses radiofrequency coils directly inspired from nuclear magnetic resonance.Instruments of this type have been built at HMI and LLB or are under development (Munich). 

Schenck, J. F., Radiofrequency Coils Types and Characteristics, 1993, in American Association of Physicists in Medicine (AAPM) Monograph No. 21, The Physics of MRI, P. Sprawls and M. Bronskill (eds.), American Instimte of Physics, New York, pp 98-134. 

The probe contains the sample holder and the radiofrequency coil, as well as the sample spinning device, temperature control and part of the detection system. Commercial probes can hold samples from 5 to 25 mm in diameter, but, as we have mentioned, much larger sample holders are being built experimentally. 

A pulse consists of a very short and intense discharge of radiofrequencies resulting from the superposition of a whole frequency spectrum. When the radiofrequency coil emits a pulse, all the resonances in the sample are excited simultaneously. After the pulse is switched off, the coil, now acting as a receiver will receive a signal from the excited nuclei the signal will decay as relaxation takes place. The signal, known as free induction decay (FID), is a sine wave its frequency is related to the resonance frequency of the nuclei, and its decay rate is related to T2. 

Figure 9.20 A diagrammatic representation of the main components of an NMR spectrometer (1) magnet (2) probe (sample holder + radiofrequency coil) (3) radiofrequency amplifier and pulse generator (4) computer (5) recorder (6) main console (7) lock and shimming systems (8) magnet power source.

CW ENDOR is considered in this section pulse ENDOR is dealt with below. The instrumentation is usually based around a computer-controlled CW ESR spectrometer and is commercially available. A radiofrequency coil, capable of handling up to 1 kW poweg is used to introduce the NMR frequencies. The coil is contained in a special resonant cavity. Various modulation strategies are employed to improve the signal-to-noise ratio. In order to carry out the ENDOR experiment, the spectrometer is set for a given line in the ESR spectrum. The microwave power is increased to just beyond the saturation level, and then the selected NMR frequency is swept. Two NMR transitions are observed at the frequencies ... 

---