The SNIF-NMR is applied on pure (or purified) molecules. Therefore, some preparation steps may be required in the lab before analysis.
For example, for the SNIF-NMR of ethanol, according to official methods:
* Fermentation (for fruit juice)
* Quantitative extraction of ethanol by distillation 
* Standardized preparation of NMR samples
* NMR acquisition
* Interpretation of results and report on the authenticity
At each step of the SNIF-NMR analysis, efforts should be made to avoid parasite isotopic fractionation. Control measures such as determining the alcoholic strength of the intermediate products of the analysis (fermented juice or distillate) are performed on each sample.