Practical Aspects Of Pulsed Field Gradients And Shaped Pulses

8.8.1 Gradient Hardware

The gradient coils are located in the NMR probe, surrounding the sample. Only the RF send-receive coil is closer to the sample. Gradient amplifiers in the console provide direct currents up to 10 A to create the gradient magnetic field. A heavy cable attached to the probe delivers these currents to the gradient coils. It is important to keep in mind that the gradients are driven by direct currents (DC), not the MHz oscillating signals (RF) that we use for pulses. When not actually producing the gradient, the amplifiers must be either "blanked" (blocked from introducing any current into the gradient coils) or adjusted to a zero current value with very low noise. Some spectrometers have the capability to deliver pulsed field gradients in all three directions: x, y, and z (the NMR tube and the Bo field are aligned with the z axis). This is achieved with three separate gradient coils in the probe, each driven by a separate current source. This is standard in MRI, but for NMR the three-axis gradient capability is used mainly for improved water suppression in DQF-COSY of biological (proteins and nucleic acids) samples, and the crowded three-coil design actually sacrifices some sensitivity.

The gradient not only "twists" the magnetization of the observed nucleus (XH,13 C, etc.), but also twists the 2H magnetization of the lock channel. You will see that the lock signal drops sharply when a gradient pulse is executed, and then recovers gradually to its former level. This is evident with the dipping down and bouncing back of the lock level on the meter in the Varian remote status unit or the graphic display in the lock window of the Bruker. This gives a convenient "heartbeat" of the gradient experiment, so you know what is going on. The Bruker lock system has a "sample-and-hold" feature that allows it to sample the lock signal before the gradient pulse and then hold onto this value until the lock has recovered fully. During a gradient experiment, you will see the message "Lock Sample and Hold Activated" on the LED display of the shim keyboard.

8.8.2 Gradient Parameters

Gradient pulses can be simply turned on and off like high-power RF pulses ("rectangular pulses") or they can be shaped so that they turn on and off more gradually. The shaped gradient pulses are less troublesome because they do not create a big transient response from the sharp rise and fall times of rectangular gradient pulses. Bruker uses almost exclusively the "sine"-shaped gradient pulse, which has the shape of the first 180° of the sine function. Varian generally uses rectangular (simple on/off) gradients. Parameters related to pulsed field gradients include the (z axis) gradient strength (Bruker gpzl, gpz2, etc., in percent of maximum gradient current, and Varian gzlvll, gzlvl2, etc., in arbitrary units -32,768 to +32,767), the time duration of the gradient pulse (gtl, gt2, etc. for Varian and p16 for Bruker, typically 1-5 ms), the shape of the gradient pulse (Bruker gpnaml, gpnam2), and the duration of the recovery delay, which allows the magnetic field to go back to homogeneous after the gradient pulse (Varian gstab, Bruker d16, typically 200 jxs).

8.8.3 Shaped Pulse Hardware and Software

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like "Wurst", "Sneeze", "Iburp", and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed.

For each shaped pulse you must select the pulse width (duration in ^s: Bruker p12, p13, etc., or Varian selpw), the name of the text file that contains the shape function (Bruker spnam1, spnam2, etc., Varian selshape), the maximum power (B1 amplitude) at the top of the pulse shape (Bruker sp1, sp2, etc., or Varian selpwr), and the offset frequency in hertz if you want to excite a peak that is not at the center of the spectral window (Bruker spoffsl, spoffs2, etc., in hertz relative to the center of the spectral window, or Varian selfrq in hertz).

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