•  NMR-004: Sample Solvents in NMR Spectroscopy

    It isn't surprising that considerable attention is directed toward carefully choosing solvents for NMR experiments. Today, solvents must provide an effective medium in which to dissolve a sample to the required concentration and filling-factor . . . must contribute no interfering resonances . . . and must provide a lock signal to assure spectrometer stability. This presents stringent challenges for manufacturers of NMR solvents when added to the obvious need for chemical purity. To make your use of solvents in NMR easier, we provide below an explanation of the important factors to consider when choosing and handling deuterated solvents in NMR.

    Purity
    Proton and 13C NMR provide the most challenging applications for solvents, since most of the contaminants found in solvents, like water or organic chemicals, contain protons and/or 13C atoms that create impurity signals in the NMR spectrum. In addition, the isotopic purity of deuterated solvents, which easily degrade through exchange with the protons in water in the atmosphere, must be maintained at high levels.

    When you buy NMR solvents, it pays to use a reliable supplier like Sigma-Aldrich or Cambridge Isotope. Choosing the correct isotopic purity of deuterated solvent in proton NMR is also critical to avoid excessive protic solvent signals. The table below provides a useful guide to selecting isotopic purity of the NMR solvent for your samples. You should make adjustments for the molecular weight of your samples when using this table.

     

    Sample Mass 
    MW=250 
    Molar Sample 
    Concentration 
    Min. Solvent 
    Isotopic Purity 
    12.5mg 0.1M 99.0%
    6 0.05 99.5
    2.5 0.02 99.8
    1 0.005 99.95


     

    Higher isotopic purities (>99.5%) are best purchased in sealed glass containers, like ampules or the Aldrich Sure-Seal® closure. Other closures may not maintain isotopic purity as well and can sometimes contribute small amounts of organic contamination to non-aqueous solvents, especially over extended periods. Deuterium isotopic purity isn't quite as critical for solvents in 13C NMR, so 99.5 or 99.0% is usually acceptable. But with small amounts of sample, 13C concentration in the solvent becomes more critical. Then, 13C depleted solvents are an important consideration. You can apply the same table, below, to 12C purity of the solvent. But since 13C depleted solvents are usually available in only one isotopic purity, your decision is typically restricted to deciding when to use 13C depleted solvent. If you find your sample size is, for example, less than 10mg of a MW250 compound, you should consider using 12C enriched (or 13C depleted) NMR solvents or applying a more sophisticated experiments to your sample, such as DEPT or INEPT.

    When you open the container in which a solvent is delivered, the solvent will rapidly absorb moisture from the atmosphere, compromising deuterium purity and adding unwanted contaminant to the solvent. TMS bottles should be warmed to room temperature before opening to avoid condensing water onto the cold inner surfaces of the bottle and cap. Always recap TMS tightly.

    We recommend that deuterated solvents with >99.8% isotopic purity be opened only in a glovebox or glovebag under a dry atmosphere. Most solvents can be dried over molecular sieves; Linde 3A and 4A, 1.6mm are acceptable. Avoid bead-shaped sieves; they don't do as well in drying solvents.

    Make sure that all the glassware you use is dry, too. Since water chemisorbs to glass and even vigorous oven drying doesn't remove the last traces, we recommend you equilibrate residual water through D2O exchange. It's important to recognize that oven drying NMR tubes can irreparably distort precision NMR tubes. Rather, in a dry atmosphere, soak all glassware with a quantity of D2O that covers the working area of the glassware, rinse with sieve dried deuterated Acetone, and dry in vacuo or under a stream of dry nitrogen. Cap all NMR tubes and store all glass in a very dry atmosphere or in a D2O saturated atmosphere (e.g. in a desiccator). WILMAD long-tip NMR pipettes help minimize the area of glass that must be equilibrated with D2O, because they allow you to place the sample directly into the bottom portion of a preconditioned NMR tube. Large bottles of D2O can be kept isotopically pure even with a septum affixed on top. Use PTFE-sheathed Silicone septa (good resealability) and always flush a carefully dried syringe with dry nitrogen before using. Remember to inject the bottle with a volume of dry nitrogen equal to the volume of solvent to be withdrawn. Careful, firm application of a septum cap under dry atmosphere in a glovebox or glovebag will help avoid contamination when you first open a capped bottle of deuterated NMR solvent. For the study of nuclei other than 1H and 13C, most any solvent that dissolves the sample can be used. The addition of a sufficient quantity of a miscible deuterated solvent to provide a lock signal is necessary to assure spectrometer stability. Combinations of protic and deuterated solvent are frequently used to control solvent costs while providing a sufficient source of deuterium lock signal.

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    Linde is a Trademark of Linde Corp.