Desirable Characteristics of Solvents used for NMR Studies

NMR grade solvents

NMR grade solvents

NMR characterization employs a solvent for dissolution of the sample like any other spectroscopic technique. This is essential for homogenized distribution of sample molecules throughout the observation volume. The desirable characteristics of such solvents are:

Solubility

Higher solubility of sample in the chosen solvent contributes to higher sensitivity. This property assumes significance in cases where sample availability is limited.

Purity

Degree of solvent purity is a major selection criteria. Chemical impurities or water content can lead to interference signals in the NMR spectrum. Solvent purity is of even higher concern for ^1^3C NMR as the proportion of the isotope nuclei is much less than the major isotope.

High purity solvents are available in sealed glass bottles. After use the bottle should be immediately closed to prevent intake of moisture or other impurities from the atmosphere.

Solvent blanks should be run in advance so as to determine that the solvent peaks do not interfere with the sample peaks.

Solvent viscosity

Lower the sample viscosity the better will be the spectral resolution due to better homogenization of the sample.

Moisture Content

Presence of traces of water will contribute to spectral interferences. Moisture content can be reduced by filtration of solvent through a drying agent or storing the solvent over molecular sieves.

Price

The solvent price is of concern especially when large number of samples need to be analysed. The price goes up further as the degree of deuteration increases.

Deuterated Solvents

NMR solvents are distinctly different from other spectroscopic solvents as majority of hydrogen nuclei are replaced with deuterium so as to minimize the interference due to protons. The price increases with the degree of deuteration. Deuterated chloroform,CDCl_3 is most commonly used because of its low price. A reference standard such as tetramethylsilane is commonly added (around 0.03%) to most commercially available solvent grades to serve as zero ppm chemical shift reference for ^1H and ^1^3C NMR studies.

Common spectroscopic solvents are available commercially in different degrees of deuteration. Apart from CDCl3 other deuterated solvents in common use are:

  • Deuterated water-D_2O
  • DMSO-d_6
  • Methanol- d_4
  • Methylene chloride-d_2
  • Pyridine-d_5
  • THF-d_8
  • Acetic  acid- d_4
  • Acetone-d_6
  • CD_3 CN

You can select the solvent on the basis of factors already discussed. Remember to flush the sample drawing syringe with dry nitrogen before use. After use inject the sample bottle with the same volume of dry nitrogen and reseal. This will keep the sample environment dry and moisture free to provide interference free spectra.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

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