External Chemical Shift Referencing

When you run your proton and carbon-13 NMR spectra, you usually reference the chemical shifts to TMS at 0 ppm or to the resonances of the solvents whose chemical shifts are known with respect to TMS. What do you do if there is no internal reference in your sample? For example, if you are running a 31P, 19F or 11B NMR spectrum, how do you calibrate the chemical shift scale? One way, of course, is to put an internal standard in your sample. You can also reference the chemical shifts externally to a standard sample. Here is what you must do to reference your chemical shifts externally on any of our Bruker spectrometers.

1. Lock and shim your sample as usual. If you are not using a deuterated solvent then you should lock and shim on a sample filled to the same level as your sample containing the deuterated version of the protonated solvent you are using in your sample.

2. Push the "lock" button on the BSMS hand controller such that the "lock" light is NOT illuminated.

3. Push the "sweep" button on the BSMS hand controller such that the "sweep" light is NOT illuminated. (Steps 2 and 3 can also be carried out by clicking on the "Run Unlocked" button on the command panel)

4. Run the spectrum of your sample.

5. In another experiment, run the standard sample. Do not try to lock or shim.

6. Calibrate the chemical shift in the spectrum of the reference compound. For example for 31P NMR, the accepted reference is 85% phosphoric acid at 0 ppm.

7. Type "sr" and write down the number the spectrometer reports.

8. Return to the experiment where your sample was run.

9. Type "sr" and enter the number you recorded in step 7. Your spectrum is now referenced.