Dear BCM 230,

Please read pp. 39-52 of the Class Notes before the next lecture.

Now here are some additional points about the third lecture just completed:

1) What effect does the strength of the B1 pulse have on theta, the pulse tip angle?

The relevant equation is theta = (gamma)(B1)(tp)

where tp is the length of the B1 pulse. If B1 is fixed, theta varies linearly with tp, which serves as the basis for calculating pulse times as discussed on p.37-38 of the Notes. However, varying the magnitude of B1 while keeping tp fixed will also cause theta to vary linearly. The bottom line is that altering B1 will change the calculated 90 degree pulse width. Reducing B1 by 50% means the 90 degree pulse time will double.

2) What is the best way to shim, on the FID or the lock?

That completely depends on what sort of sample you are studying. First of all, for many in vivo and imaging applications no deuterium will be present; hence shimming on the FID is the only realistic option. For studies of chemicals in tubes where we can dissolve the compounds in deuterated solvents, shimming on the deuterium lock signal is usually quicker and faster. In general people on the 7T will shim on the FID and people using the 400/500/600 MHz will most commonly shim on the lock.

3) What is the definition of linewidth?

Linewidth is the width of the NMR peak (in hertz) at one half the total height of the peak from the baseline.

4) Why must the gain be set to sample the noise? Why observe the noise at all?

The signal-to-noise (S/N) in nmr increases as the square root of the number of scans (ns). This is because S is linear with ns, while N increases with the square root of ns, but ONLY if the noise is sampled accurately in the FID!! IF the noise is sampled accurately, then N varies with square root of ns as will the ratio S/N (ns divided by square root of ns). See bottom of p.13 also.

For people in the BCM 230 LAB here are a few more points:

1) Remember that the basic series of commands to process the FID in Xwinnmr is: em (apply exponential multiplication to the FID); ft; phase. These commands can be executed more simply with the alias: efp.

2) For those in the lab: at this point we have covered all of the BASIC steps for acquiring data. There is no one absolutely correct series of steps to execute a NMR experiment, but here's what I usually do:

1. Check probe temperature, insert sample in magnet; create a new dataset & load parameters.

2. Tune the probe; lock if using a deuterated solvent.

3. Shim.

4. Get trial spectrum and (this is optional) measure linewidth to check shimming.

5. Find the 90 degree pulse length.

6. Find the optimal sweepwidth and number of data points.

7. Set all other values such as predelay to optimal values for my estimated recycle time.

8. Set the gain.

9. Collect data (zg).

10. Process data (efp).