Due to the high sensitivity of the system unexpected peaks from impurities can become an issue, especially when working with low concentrations of samples in proton spectra.
Sources of common impurities include:
- Poor quality solvents.
- Leaching from plasticware. Plasticizers such as phthalates can be readily leached by organic solvents during sample prep and have a characteristic doublet of doublets at around 7.5 and 7.3ppm.
- A broadish peak at ~1.5ppm is due to water in CDCL3.
- A broadish peak at 1.26ppm and a smaller associated triplet at 0.88ppm may be due to higher molecular weight hydrocarbons which have not been removed during sample prep. This is generally caused by grease which has leached into the sample.
- A singlet at 0.1ppm may be due to silicone grease contamination.
- For biological samples, glycerol and Tris can often be observed at 3.5-4 ppm and 3.4 ppm, respectively, even after extensive desalting. We have noticed that ultracentrifugation spin filters can add glycerol to your sample unless they are washed thoroughly before use.