Pulse sequence generator

Each switch is controlled separately. This guarantees a maximum of flexibility in pulse sequence generation. Switching and non switching case can be investigated for different amplitudes, pulse widths, and delay times. Figure 3.8 (b) shows the typical rise time of the setup (1 ns). 

ID INADEQUATE LBR pulse sequence generates a theoretical sensitivity enhancement of 41% compared to a regular ID INADEQUATE. The same gain is also provided by ID INADEQUATE CR.5 As the latter pulse sequence contains more delays, ID INADEQUATE LBR is more robust when it comes to the mismatch between the set and actual / values. An experimental comparison, based on 11 sucrose signals, of all three ID INADEQUATE pulse sequences... 

The rf part of the pulse sequence generates a Hahn spin echo at time 2x. In imaging jargon the time from the center of the 90° pulse to the center of the echo is called the echo time (or time to echo) TE = 2x (where x is the spectroscopist s usual symbol for the time from the 90° pulse to the 180° pulse). The time from the center of the 90° pulse to the center of the next 90° pulse is called the repetition time (or time to repeat) TR. Spectroscopists know TR as the time equal to the recycle delay plus the time taken by the pulsing and data sampling. 

Some modern (new in 1981) commercial NMR spectrometers are extremely versatile with computer controlled pulse sequence generators which are easily programmable even for complex sequences. Thus, even for a homemade machine, you should seriously consider purchasing the operating system software with a computer and a pulse sequence generator. 

Figure 5.17 Cartoon diagram to represent general structure of 4D correlation experiments. This is the same as for 3D correlation experiments (Fig. 5.14) except that an extra resonant population of heteroatom nuclei are involved in generation of transverse magnetisation (in time ts) and magnetisation transfer (during M3). Final pulse sequence generates transverse magnetisation in the Destination Nuclei S that is observed, acquired and digitised in time t/,. Fourier series transformation is used to transform time domain signal information Sfid (ti, ta, ts, 4) into frequency domain (spectral intensity) information, /NMR(fi, F2,...

A fourth component of the FTMS is the data system. While the mechanical assembly of a FTMS system is relatively simple compared to other mass spectrometers, the data system is quite sophisticated. The FTMS data system consists of a pulse sequence generator to control the timing of the various events in a measurement cycle, a frequency synthesizer, a wideband excitation amphfierfor ion isolation and ion detection, and a transient digitizer. A computer controls all of these components and is used to acquire and process the data. 

Figure Bl.14.2. Gradient-recalled echo pulse sequence. The echo is generated by deliberately dephasing and refocusing transverse magnetization with the readout gradient. A slice is selected in the z-direction and v- and y-dimension are frequency and phase encoded, respectively.

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

The measurement procedure is known as the pulse sequence, and always starts with a delay prior to switching on the irradiation pulse. The irradiation pulse only lasts a few microseconds, and its length determines its power. The NMR-active nuclei (here protons) absorb energy from the pulse, generating a signal. 

One last comment about pulse widths it is important that we know what the 90° pulse width is for the nuclei that we observe as accurate pulse widths are required for many pulse sequences (as mentioned previously). Failure to set these correctly may give rise to poor signal to noise or even generate artifacts in the spectrum. When instruments are serviced, these pulse widths are measured and entered into a table to ensure that the experiments continue to work in the future. 

Siegel et al. showed that enhancement of the CT can also be obtained using hyperbolic secant (HS) pulses to invert selectively the STs [74], Unlike the DFS waveform, whose frequency sweep is generated by a constant rf-pulse phase while modulating the amplitude, the HS pulse utilizes both amplitude and phase modulation, yielding an enhancement exceeding that obtained by DFS or RAPT [61, 74, 75]. Most recently, the pulse sequence called wideband uniform-rate smooth truncation (WURST) [76] was introduced to achieve selective adiabatic inversion using a lower power of the rf-field than that required for the HS pulses [77,78]. One of its applications involved more efficient detection of insensitive nuclei, such as 33S [79]. 

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