Radio-frequency excitation pulses

NMR method of analysing the spin polarization of a non-equilibrium system of coupled nuclei in CIDNP experiments. Using Fourier analysis of the variation of NMR line intensities as a function of the radio frequency excitation pulse length, they have established a relationship between the spectral components of various harmonic order and the alignment in the spin multiplet. 

The radio frequency excitation pulse and resulting NMR signals are sent through cables between the probe coils in the magnet and the computer. 

Similarly to non-selective experiments, the first operation needed to perform experiments involving selective pulses is the transformation of longitudinal order (Zeeman polarization 1 ) into transverse magnetization or ly). This can be achieved by a selective excitation pulse. The first successful shaped pulse described in the literature is the Gaussian 90° pulse [1]. This analytical function has been chosen because its Fourier transform is also a Gaussian. In a first order approximation, the Fourier transform of a time-domain envelope can be considered to describe the frequency response of the shaped pulse. This amounts to say that the response of the spin system to a radio-frequency (rf) pulse is linear. An exact description of the... 

SateUite Transition MAS (SATRAS or STMAS), developed by Gan in 2000 [25], is an alternative approach to MQMAS for the acquisihon of high-resolution NMR spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS is that it is not dependent upon an efficient transfer of multiple-quantum coherences. like MQMAS, SATRAS is a 2D experiment performed under MAS conditions. The technique involves exciting the sateUite transitions in the spin manifold of quadmpolar nuclei using short radio frequency (rf) pulses. The second-order... 

At the present time it is unclear whether the spin echo decay spectroscopy approach successfully demonstrated for Na is generally transferable to other alkali ion nuclei such as Li and Cs. The latter two isotopes have moderately small electric quadrupole moments causing first order quadrupolar splittings that are comparable to the radio frequency excitation window. As a result, the 7i-pulse length is ill-defined in many situations and the contribution of dipolar coupling of the observed spins to nuclei in the outer Zeeman levels is difficult to quantify. Nevertheless, some promising initial results on cesium borate... 

Fig. 3.6. Schematic representation of an ion cyclotron resonance ion trap. Ions are trapped in the x-y plane by the magnetic field B) and held in the z plane by a trapping potential applied to the end plates. Excitation occurs by applying broadband radio frequency voltage pulse to the transmitter plates, followed by detection of the image current on the same plates. Figure adapted from Fig. 7 in Marshall etal. (1998).

The principle of Fourier transform (FT) NMR spectroscopy is the observation of the so-called free induction decay (FID) after the application of radio frequency (rf) pulses to the resonating nuclei. The carrier frequency of the rf-pulses is the Larmor frequency. In many cases, the FID is observed after single-pulse (SP) excitation, e.g., after application of a so-called 7r/2-pulse which rotates the magnetization by 90° from the direction of the external magnetic field (z-direction) into the x,y-plane. The characteristic time constant for the free induction decay is the transverse relaxation time, T2, which is given by T2=(2/M2) =0.53 (A Vi/2)" for Gaussian lines. Fourier transformation of the FID yields the common absorption spectrum. 

The SFC is a critical parameter for the fats and oils industry. The official American Oil Chemists Society (AOCS) wet method is dilatometery. Alternative wet methods are differential thermal analysis and differential scanning calorimetry (DSC). LR NMR was proved to be an alternative method for SFC determination in late 1950s. The early continuous wave LR NMR spectrometers rapidly found their way into the fats and oils industry, the method being accepted by the Instrumental Techniques Committee of the AOCS as early as in 1972. Presently the technical choice is radio frequency (RF) pulsed LR NMR. Pulse NMR spectrometers are more compact, very efficient, and relatively cheap. They have the advantage of exciting the protons in the whole sample at once. 

Fourier Transform NMR. In Fourier transform NMR (FTNMR), a repetitive radio frequency (RF) pulse is applied in order to excite all of the nuclei of the particular nuclear species being studied. The sum of the free induction decay (FID) curves from each pulse is analyzed by a Fourier transform method in order to generate the familiar frequency domain spectra. Fundamentally, parameters such as the frequency, intensity, application time of the appropriate RF pulse, and time intervals between these pulses are important variables when using this technique. The principle of the pulsed Fourier transform technique can be found in books covering the fundamental concepts of NMR spectroscopy (58,59). 

The sample is again subjected to a constant magnetic field but all the nuclei are excited by a very short radio frequency pulse. The frequency e (e.g., 400 MHz for a proton at 9.4 tesla) is applied over a period of several... 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

FID Free induction decay, decay of the induction (transverse magnetisation) back to equilibrium (transverse magnetisation zero) due to spin-spin relaxation, following excitation of a nuclear spin by a radio frequency pulse, in a way which is free from the influence of the radiofrequency field this signal (time-domain) is Fourier-transformed to the FT NMR spectrum (frequency domain)... 

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