Radiofrequency domain

Going back to the electromagnetic spectrum, one can easily note that the energy associated with the NMR phenomenon is very weak in comparison with the other common types of spectroscopy. The radiation generating the NMR transitions is situated in the radiofrequency domain. 

NMR collects information concerning interactions between the nuclei of certain atoms present in the sample when they are submitted to a static magnetic field which has a very high and constant intensity and exposed to a second oscillating magnetic field. The second magnetic field, around 10 000 times weaker than the first is produced by a source of electromagnetic radiation in the radiofrequency domain. 

There is a plasmonic analogy of the well-known Yagi-Uda antenna. It can be fabricated by placing a resonant nanorod antenna between a reflector nanorod and a group of director nanorods [315]. Similar to such antennas used in radiofrequent domain, it ensures a good directivity. 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

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)... 

Free induction decay A decay time-domain beat pattern obtained when the nuclear spin system is subjected to a radiofrequency pulse and then allowed to precess in the absence of Rf fields. 

Nuclear magnetic resonance spectroscopy Interaction magnetic fields - nuclei Resonance of radiation quanta, h v Radiofrequency pulses Spectrum in time or frequency (FT) domain ... 

A. Kastler, Quelques reflexions a propos des phenomenes de resonance magnetique dans le domaine des radiofrequences, Experientia, 1952, 8, 1-44. 

The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

Fig. 10. Radiofrequency and gradient pulse sequence for diffusion-ordered spectroscopy (DOSY) owing to Morris and Johnson [1992] in which a PGSE pulse pair (Gj) represents the first (t,) domain and the acquisition time provides the second (tj) domain. Note the storage period 7], which allows for eddy current decay prior to data collection.

Fig. 12. Radiofrequency and gradient pulse sequence for velocity exchange spectroscopy (VEXSY) in which successive PGSE pulse pairs (G, and G2X separated by a delay time t , are applied. For an unambiguous correlation, Gj and Gj are required to be collinear. Note the two orthogonal Fourier domains represented (schematically) by r, and tj- Fourier transformation with respect to the acquisition time leads to a third spectral dimension.

Fig. 14. Radiofrequency and gradient pulse sequence for velocity and diffusion imaging in which the molecular motion is measured in the domain of two spatial dimensions (x, y) of a slice selected normal to the z axis. Note that the PGSE pulse pair (g) provides the third dimension—that of motion—while the phase encode (Gj,) and read gradients (G ) provide the first and second spatial dimensions.

Modern Fourier transform (FT) NMR spectroscopy excites all the nuclei of interest at once with a radiofrequency (rf) pulse and detects the alternating current produced by the precessing spins in a coil surrounding the sample. This signal, or free induction decay (FID), is collected in the time domain and then processed to the frequency domain to generate the NMR spectrum. 

The core of a two-dimensional NMR experiment is the sequence of radiofrequency pulses used to obtain the matrix of time-domain signals S(tj,t2), since it is this which governs the signal distribution in the final spectrum. In the preceding section, one of the earliest high-resolution 2D NMR experiments was used to exemplify the basic features of two-dimensional methods. The details of the experimental methods and data manipulations used are deferred to the next section this section sets out to make a brief summary of the many different types of 2D spectrum that may be produced, and the many and various pulse sequences used to generate them. 

One method for temporal precnrsor isolation is stored waveform inverse Fourier transform (SWIFT) [40]. In this method, the desired freqnency domain profile (all frequencies except that of the ion of interest) is inversely Fonrier transformed to a time domain waveform. This waveform is then applied to the excite electrodes in the ICR cell and, thns, the precursor ions are isolated in the cell. An alternative techniqne for in-cell isolation is correlated sweep excitation (COSE) [41], also known as correlated harmonic excitation fields (CHEF) [42]. This method involves application of radiofrequency pulses to the excite electrodes. The technique correlates the duration and frequency of the RF-pulses with those appropriate to the ions to be isolated. Both SWIFT and COSE are capable of isolating single isotopomers in peptide and protein ions [43-45]. 

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