Solvent Selection
Once the sample has been sufficiently purified and dried, the next step is to choose a suitable solvent. Since deuterium is by far the most popular lock nucleus the sample is usually dissolved in a deuterated solvent (a deuterated solvent is one in which a large proportion, typically more than 99%, of the hydrogen atoms have been replaced by deuterium). Commonly used deuterated solvents are benzene-d6, acetone-d6, and chloroform-d though many other solvents are available. Factors to be considered when choosing a solvent are:
- 1.
- Solubility: Clearly the more soluble the sample is in the solvent the better. This maximizes the amount of sample within the sensitive volume which increases the sensitivity of the experiment. High solubility is particularly important if only small quantities of the sample are available.
- 2.
- Interference of solvent signals with the sample spectrum: The solvent itself will inevitably produce NMR signals which will obscure regions of the spectrum. These ‘residual solvent peaks’ should not overlap with signals from the sample.
- 3.
- Temperature dependence: For experiments above or below room temperature the solvent‘s melting and boiling points are also important factors. Furthermore the solubility of the sample is likely to vary with temperature.
- 4.
- Viscosity: The lower the solvent viscosity, the better the resolution of the experiment.
- 5.
- Cost: Clearly for routine NMR where many samples need to be measured, the solvent‘s cost is an important consideration. As a rule of thumb, the price increases with the number of deuterated atoms.
- 6.
- Water content: Almost all NMR solvents contain water traces. Also many are hygroscopic (they absorb water from the atmosphere) and hence the longer they are stored the more water they contain. The presence of a water (HDO) peak will only serve to degrade the quality of NMR spectra. The water level in the solvent can be greatly reduced by filtration through a drying agent or by storing the solvent by adding molecular sieves.
The choice of solvent for a particular sample will be the best compromise between the various advantages and disadvantages of each. Surge the web for precise details of specific solvents.