PPT Slide

Alternative Methods: Non-excitation of water

Example:

Jump and Return (90x - t - 90-x)

P. Plateu, M Gueron, JACS 104,7310 - 7311 (1982).

90x

t

The center of the spectrum (tof) must be right on the water frequency= on resonance (wo). Then, the water is stationary in the rotating frame and will experience a 90x-90-x pulse = no excitation.

The only detected signal will be the projection of all magnetization vectors on the x-axis, only magnetization which precesses. The projection is a sin (w-wo )t modulated signal, thus one obtains a sinusoidal excitation profile with .
• zero excitation at (w-wo) = np
• maximum excitation (offset) at = 1/4t

The spectrum has a phase step (180 deg) at the center, but no frequency dependent phase shift and thus, in principle, yields a perfectly flat baseline

90 -x

x

y

x

x

y

x

y

x

B

A

1/4t

no

Acquire

A

B

Previous slide

Next slide

Back to first slide

View graphic version

In practice, the Jump-Return sequence is difficult to set up and to achieve the supression factor needed for obtaining good results. Usually supression of 50 to 300 can be obtained, however, phase cycling improves the result substantially. The limitation of this sequence is the finite region of zero excitation: its very sharp, basically one frequency, and much narrower than the signal to be supressed. So, the broader water is only partially supressed, leaving the sides of the water signal partially excited. Due to the large concentration, even this partial excitation leads to a relatively strong residual water signal , which due to phase shift at the center of the spectrum appear as “dispersive signal” or strong baseline distortion.

Further, the sequence is sensitive to pulse phase errors, pulse inequality between the two pulses. Radiation damping strongly affects the saturation, which usually leads to second pulse being shorter than an exact 90 (approx. 10 %). Therefore, the Return-pulse needs to be carefully optimized.

Advantage: No presaturation transfer = see below.