Preparing an NMR Sample

Background

      It will be necessary to obtain full characterization on all compounds you prepare during your research efforts. This includes IR, TLC, and NMR data (both proton and carbon).1 It is my hope that everyone in the group will be checked out on the T-60 NMR, which obtains 1H NMR spectra quickly and easily, which are quite satisfactory for 95% of the spectra we run for routine purposes.  The time required for checking out people on the QE-300 (300MHz) NMR spectrometer is far greater, so many of you will not get authorization to run this instrument.  Who does and doesn't get checked out on the QE-300 is determined only by the probable frequency of its need.  Group members needing 13C NMR or high-field 1H NMR spectra can just ask one of the students who have authorization. Regardless of who runs it, you'll have to prepare your own sample.

      Because FT-NMR is a fairly sensitive technique, very little sample is actually required for obtaining an excellent NMR spectrum; additionally, since NMR is a "nondestructive" analytical technique, your sample is easily recovered, unchanged, at the end of the experiment. A typical sample size for obtaining a proton NMR spectrum is about 100 mg on the T-60, and 5-10 mg on the QE-300. A carbon NMR spectrum requires considerably more sample (25-50 mg) due to the low natural abundance of 13C (1.1%), relative to 1H (ca. 100%).

      Although CDCl3 is the NMR solvent of choice, not all research samples will dissolve in it. If this is the case, find a solvent which will dissolve your sample. DO NOT use our very expen$ive deuterated solvents for your solubility studies!  Alternative solvents to examine are acetone, water, methanol, benzene, and dimethyl sulfoxide (DMSO). These solvents are also available in deuterated form, which we usually have in stock.


Preparing a Sample for NMR Analysis

      Place the appropriate amount of sample in a small test tube or sample vial.2 Then fill an NMR tube (for the QE-300, use a precision NMR tube, like a Norell XR-55) with approximately 1.5" of CDCl3, assuming that your sample is soluble (see figure below). Be sure to use a clean pipette for the transfer of the solvent into the NMR tube, otherwise there is a considerable risk of contaminating the solvent for the next person (these solvents are very expensive). Next, transfer this solvent into the sample vial which contains the compound for which you wish to obtain an NMR spectrum.

      Make sure the compound dissolves completely in the CDCl3; if there are any particulates, filter them off using a small plug of glass wool pushed to the bottom of a disposable pipette directly into the NMR tube.3 If your sample dissolves completely, transfer this solution, via pipette, into the NMR tube. Cap the end of the NMR tube with the appropriate lid.

      When finished, empty the contents of the NMR tube (save the sample if you wish), and rinse the NMR tube thoroughly with acetone (if the tube is not perfectly clean after this, use additional solvents until it is). Once the NMR tube is dried, return it to the oven clean for the next person to use.



1 If it is a new (previously unknown) compound, you will also be obtaining a combustion analysis and/or mass spectrum.
2  Do not even think about weighing it out! An approximation will do just fine.
3  The presence of solids in an NMR solution introduce anisotropic effects which will compromise the quality of your spectrum.