Stan leaves us clues to this with in his patent diagrams such as why does 90% of his drawings of the pulse trains have three pulses and then a gate? Let me see if I can explain this, in NMR there are two ways to apply the pulses, (1) you apply a 90° pulse and then apply a 180° pulse and then you gate for X time. (2) You apply three 90° pulses and then gate for X time.
So it looks like to me that Stan was applying the three 90° pulses and then gating. During the gated time the water will give an echo know as Spin Echo. Here is what the three 90° pulses do.
After the first 90° pulse, the magnetization vector is exchanging energy through dipole, dipole interactions and in a time ?, forms what is often referred to as a “pancake” in the x’-y’ plane. A further 90° pulse is then applied such that our “pancake” is now in the x’-z’ plane. When considering the two types of relaxation, spin – lattice and spin – spin (T1 and T2) we assume the former to take an infinite amount of time as such allowing the spin vectors to precess about the z axis. Now, the angle each spin makes with the z’ axis is equal to the angle it previously made about the y’ axis. At this point any change in angle that now takes place will require a change in energy thus implying a spin – lattice interaction is necessary. This implies a permanent memory of the state of the system as it was at time ?. After a further time ?2 a third pulse is applied and our Magnetization vector is back in the x’ – y’ plane and will lie in the same direction as for a (90 – ? – 180) spin echo sequence. Then after final delay of ? we see what is commonly referred to as a stimulated echo. This technique is commonly used when studying T1 relaxation times. This is because by measuring the magnitude of the correct echo and its decay with pulse width separation we can determine T1. The echo magnitude will depend on the relation, exp(-?2/T1).
Another think I would like to point out is something known as "Magic Angle". The
magic angle is a precisely defined angle, the value of which is approximately 54.74°. It's funny that this angle is half the angle of the water molecule which is 109°28'. Two nuclei with an internuclear vector at an angle of ?m to a strong external magnetic field, have zero dipolar coupling, D(?m)=0.
Magic angle spinning is a technique in
solid-state NMR which employs this principle to remove or reduce dipolar couplings.
This is just a part of the NMR process and there is a lot more to it. There is a lot of physics involved here. I'm still working everything out, so this is an ongoing project.
-Tony Woodside-
