T₂ Hahn Spin Echo Demo

This experiment was developed by S.E. Van Bramer for Chemistry 465 at Widener University.

Introduction:

This experimental procedure is for finding the T₂ relaxation constant of an NMR sample. This constant is useful for optimizing other NMR experiments and it shows how the NMR signal dephases as the FID develops. The experimental details outlined here are in addition to the basic steps outlined in "NMR Operations for QE-300" and this should be treated as a supplement to that document.

Experimental:

Safety

DO NOT ENTER KB 421 if you have a pacemaker or other metal implant.
DO NOT place any magnetic material near the magnet.
DO NOT drop small metal material in the room.
DO NOT bring large metal objects into KB 421.
Credit Cards, Computer Diskettes, and ATM cards can be erased inside the yellow warning rope.
DO NOT work unsupervised
Follow Directions and Ask questions. The NMR costs about $150,000, be careful
Sign-in the logbook

Sample Preparation

Sample Loading

Setup the Magnet For The New Sample

Setup Computer for Experiment

Select the Nucleus.
Set the Receiver Gain.
Set Number of Acquisitions. (Start with 1 to determine approximate T₂, increase to improve S/N)
Set the Block Size (suggest 16k for 1-H, 32k for 13-C).
Set Pulse Width
1. Set π pulse: P1, 180D, <return>
2. Set π/2 pulse: P2, 90D, <return>
Turn the Spinner Air Off.

Survey Spin Echo Signal for approximate T₂.

(If you have an approximate idea for T₂ this step may be skipped.)

Select the T₂ Spin Echo Experiment pulse sequence. There are two slightly different pulse sequences available:
1. Spin Echo sequence 90-π-delay-180-π-delay-acquire: (Acquire after second delay)
  1. D1 determines the delay between π/2 and &pi pulses (in seconds). The D1 value may be a list. End the list by entering a value of 0. For initial experiment try T₂^* sec (Where FID has decayed to 30% initial intensity).
  2. D5 determines recycle time. If you know what T₁ is, set D5 > 5 × T₁.
2. Spin Echo sequence 90-π-delay-180-acquire: (This sequence shows FID develop as signal rephases after pulse.)
  1. D1 determines the delay between π/2 and π pulses (in seconds). The D1 value may be a list. End the list by entering a value of 0. For initial experiment try T₂^* sec (Where FID has decayed to 30% initial intensity).
  2. D2 is the acquisition time. DO NOT SET, change the block size to adjust the acquisition time. D2 should be greater than D1. D2 may be increased by reducing the spectral window, Zoom on spectrum and set the spectral window.
  3. D3 delay between the π pulse and the acquisition. Can be set to 0.
  4. D5 determines recycle time. If you know what T₁ is, set D5 > 5 × T₁.
Collect Data.
Process the FID
1. Baseline Correct FID.
2. Exponential Multiplication (optional to improve S/N)
3. Fourier Transform FID.
Process the Spectrum
1. Phase the Spectrum with Autophase.
Change D1 and repeat experiment.

Run T₂ Sequence

(To run a list of D1 values).

Set up a list of values for [D1] (Suggest D1 from 1 ms to 5×T₂*)
Set D5 for complete recovery between pulses (approximately 5×T₁)
Set NA to multiple of 8 that produces desired S/N (Multiple of 8 for complete phase cycling with quadrature detection) .
Determine the Total Time for the experiment sequence.
Run the experiment and save data to disk. (This will sum FID's for each value of D1. Each FID is saved with a different file name.)

Process the sequence Data

Retrieve each FID and process.

Baseline correct the FID.
Optional Processing of FID.
1. Zero Fill.
2. Exponential Multiplication.
Transform the FID.
Phase the spectrum.
For the first spectrum with autophase. (NOTE: spectra collected with the MT2HSE sequence and short D1 values may have significant sinc wiggles. Selection of an appropriate apodization function will reduce this noise.)
Save the spectrum as a new file. (Use sequential extensions. ie: filename.001, filename.002.)
Repeat processing for each FID. (One FID for each D1 value. Can use [LI] to create a list of commands. Then run this list with [AU].)

Create a Stacked Plot for the T₂ Experiment

Import the spectrum with the shortest D1 value. [GA]
Set the Y-Scale. [YS]
Check Offsets by plotting to the Video. [SV]
Adjust Offsets as necessary. [PO] (values for X & Y offsets in mm)
Plot the spectra. [SP] (enter filename of first spectrum)

Analysis of T₂ Spin Echo Data.

Obtain peak intensity values for each Delay. Measure this value from the plot, or use the peak picking routine. All spectra must be acquired and processed identically.
A graph of ln(peak height) vs total delay (2 * D1) will have a slope of -1/T₂. (NOTE: this is just 1^st order kinetics. Use 2*D1, because signal maximum is after vectors rephase.).

This rearranges to give:

NOTE: This is equivalent to the integrated rate equation for first order kinetics (See your Freshman Chemistry Textbook.). The only difference is that the relaxation time T₂ is used instead of the relaxation rate (k). These two values are related as follows:

Note the units on the two. Relaxation times are given in seconds, reaction rates are in reciprocal seconds (sec^-1) or frequency (Hz).