Measuring Relaxation Time Constants 3/4/94

The decay of the FID is described by the T2 (transverse) relaxation time constant. Relaxation of the signal back to equilibrium is described by the T1 (longitudinal) relaxation time constant. For samples in solution with molecular weight £1000 a.m.u., T2 and T1 are very similar. T2 and T1 differ for samples with larger molecular weights.

Estimating T2 from the FID:

The error of this method is about ±20%. This method cannot distinguish between peaks at different chemical shifts, but determines an average T2 value for the entire sample. However, this method is very quick and easy.

1. Type df to display the FID or click on process and display FID. If the FID is not a smoothly decaying envelope, type wft and determine if the peaks are unsymmetrical, which indicate that shims are not optimized. If the peaks are symmetrical, place the cursor far away from peaks in the spectrum (especially large peaks), type movetof and collect another FID. Display the FID.

2. Expand the display to show beginning of the FID.

3. Type vpf=0.

4. Set the mouse arrow above the tallest point of the fid at the top of the display window, and click the middle mouse button. This sets the scale of the tallest point of the FID to the top of the display.

5. Multiply the current value of vf (listed at the bottom of the display window) by 16. Type vf=vf*16.

6. Expand the FID to full view and set the left cursor to the farthest right point of the FID that reaches the top of the display window.

7. Divide the value of crf (listed at the bottom of the display window) by 2.77. This is the T2 relaxation time constant.

At 3 times the value of T2, 95% of your signal is gone. At 5 times the value of T2, 99% of your signal is gone. You can use these guidelines to help set up your parameters in your experiment.

Determining T2