RF pulses

The bulk magnetization is stimulated into processional motion around Bq by a radiofrequency (RF) pulse at Vj, applied tlirough a solenoid-like coil whose axis is perpendicular to Bq. This motion amounts to a nuclear... 

Many other pulsed NMR experiments are possible, and some are listed in the final sections. Most can be canied out using the standard equipment described above, but some require additions such as highly controllable, pulsed field gradients, shaped RF pulses for (for example) single-frequency irradiations, and the combined use of pulses at several different frequencies. 

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

Figure Bl.12.2. Power distribution for an RF pulse of duration applied at frequency v...

Nimieroiis schemes exist that are used to obtain 2D MQMAS spectra [24]. The simplest fomi of the experiment is when the MQ transition is excited by a single, high-power RF pulse, after which the MQ coherence is allowed to evolve for a time (figure B 1.12.12). After the evolution time, a second pulse is applied which converts the MQ coherence into ap = coherence which is observed during /2- The signal is then acquired inunediately after the second pulse and the echo will fomi at a time /2 = Both pulses are... 

In many instanees, it is important tiiat some fonn of ehemieal seleetivity be applied in magnetie resonanee imaging so as to distinguish nuelei m one or more speeifie moleeular enviromnent(s). There are many ways of doing this and we diseuss here just three. The first option is to ensure that one of the exeitation RF pulses is a narrow bandwidth, frequeney seleetive pulse applied in the absenee of any gradient [22]. Sueh a pulse ean be made speeifie to one partieular value of the ehemieal shift and thereby aflfeets only nuelei with that ehemieal shift. In praetiee this ean be a reasonable method for the speeifie seleetion of fat or oil or water in a mixed hydoearbon/water system. 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

Time delay ber / een laser puJse arid RF-pulse. ms... 

Time delay betzcecn laser poise a.nd RF-pulse, us... 

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. 

A computer-controlled bandpass filter system controls the size of the acquired spectral window. Typically, this is set to about 120% of the desired sweep width. Only frequencies within these limits are allowed to reach the ADC. Those frequencies outside the limits would only contribute to the noise in the final spectmm. The need for this system is dictated by the nonselective nature of the excitation rf pulse. 

All these experiments involve at least three distinct time periods preparation (tp), evolution (tl), and detection (t2) these periods are usually separate by rf pulses. Some experiments (e.g., NOESY, RELAY) further contain an additional "Mixing period, tm, between the evolution and detection periods. 

The spin system is perturbed by rf pulses during the preparation period to create the desired coherences which are then allowed to evolve during the time tl. 

Fourier transformation of Rf pulses (which are in the time domain) produces frequency-domain components. If the pulse is long, then the Fourier components will appear over a narrow frequency range (Fig. 1.24) but if the pulse is narrow, the Fourier components will be spread over a wide range (Fig. 1.25). The time-domain signals and the corresponding frequency-domain partners constitute Fourier pairs. 

Figure 1.24 Fourier components of a long Rf pulse ( soft pulse) are spread over a relatively narrow frequency range. (Reprinted from S. W. Homans, A dictionary of concepts in NMR, copyright 1990, pp. 127-129, by permission of Oxford University Press, Walton Street, Oxford 0X2 6DP, U.K.)...

The probe contains the electronics designed to detect the tiny NMR signal. The central component of the probe is a wire that receives the Rf pulse from the transmitter and dissipates it into the sample. It also receives the signal from the sample and transfers it to the receiver... 

To understand how nOe occurs, we have to consider the following situations (a) the populations of the nucleus I prevailing at thermal equilibrium before the application of the Rf pulse on nucleus S, (b) populations immediately after the pulse is applied to nucleus S, and (c) populations after the system has had some time to respond, with either 1% or being the predominant relaxadon pathway. 

Pulse sequence A series of Rf pulses, with intervening delays, followed by detection of the resulting transverse magnetization. 

Figure 6. The sequence of events in a laser desorption FTMS experiment, (a) The laser beam enters the cell and strikes the crystal, (b) Some of the desorbed molecules are ionized by an electron beam, (c) Ions are trapped in the analyzer cell by the magnetic and electric fields, (d) Ions are accelerated by an RF pulse and the resulting coherent image current signal is detected. Reproduced with permission from Ref. 18. Copyright 1935, North-Holland Physics Publishing.

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