Continuous wave radiofrequency

Greatly enhanced sensitivity with very short measuring time is the major advantage of PFT (pulse Fourier transform) experiments. In the CW (continuous wave) experiment, the radiofrequency sweep excites nuclei of different Larmor frequencies, one by one. For example, 500 s may be required for excitation over a 1-KHz range, while in a PFT experiment a single pulse can simultaneously excite the nuclei over 1-KHz range in only 250 jits. The PFT experiment therefore requires much less time than the CW NMR experiment, due to the short time required for acquisition of FID signals. Short-lived unstable molecules can only be studied by PFT NMR. 

Continuous wave A method of recording an NMR spectrum in which the field B] is applied continuously and either the magnitude of Bo or the radiofrequency is varied so that the nuclei are brought successively into resonance. 

Continuous Wave (CW) Technology used initially in the acquisition of NMR data. The radiofrequency or the magnetic field was swept and nuclei of different chemical shift were brought to resonance sequentially. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

The radiofrequency energy p, can be introduced either by continuous-wave (CW) scanning of the frequency range or by pulsing the entire range of frequencies with a single burst of radiofrequency energy. The two methods result in two distinct classes of NMR spectrometers. 

We have been concerned with the precision and accuracy of NMR data of polymers since we first started NMR studies on polymers.1-4 Using continuous-wave (CW) spectrometers, the effects of measurement conditions including temperature, sample concentration and radiofrequency (rf) field strength, were examined using several polymer and copolymer samples. Since our research group have been deeply involved in stereospecific polymerization of methacrylates, one of the main concerns about NMR measurement was the precision of tacticity determination by NMR. The errors in determining the tacticity of poly(methyl methacrylate) (PMMA) and those in the results of polymerization of methyl methacrylate (MMA) by radical and anionic initiators were examined and found to be satisfactorily small.4 Although there... 

The radiofrequency vx can be introduced either by continuous-wave (CW) scanning or by a radiofrequency pulse. 

Multi-resonance and pulsed ESR techniques can provide better spectral resolution than conventional ESR. Multi-resonance involves ENDOR, TRIPLE and ELDOR. In an ENDOR experiment a radiofrequency (RF) field is applied in addition to the microwave (MW) employed in standard continuous wave (CW) ESR. ENDOR is mainly used to increase the spectral resolution, so that overlapping or unresolved hyperfine structure in the ESR spectra can be detected. In the classical work by Feher [1] the radiofrequency was continuously swept. CW X-band spectrometers with an ENDOR attachment have been commercially available for a long time. Accessories for other frequency bands and for pulsed ESR have been developed more recently. In a TRIPLE experiment two RE fields are applied [2, 3]. A theoretical application has been to determine the relative signs of two hyperfine couplings. In an ELDOR experiment two MW frequencies are applied. Early applications using... 

In continuous wave (CW) NMR, one of the problems is the separation of the Overhauser effect from the coherent effects of radiofrequency (RE) irradiation. For example, even at very low RE field ( 0.02 Hz) CW double resonance experiments, the observed intensities can only be explained by including coherent effects, as splitting or broadening of directly connected transitions. In higher power experiments, the lines are split and several coherent... 

In pulsed FT NM R spectroscopy a radiofrequency (rf) pulse instead of a varying magnetic field is employed to excite nuclei. A number of pulses could be applied to a sample in the same time normally used in a continuous wave recording. In this way up to a tenfold increase in sensitivity can be obtained. 

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