Purification requires the separation of compounds and distillation is a common purification process based on the difference in boiling points between individual components in a mixture. Simply put things that boil at a lower temperature are boiled off while higher boiling point compounds remain behind. As heat and/or vacuum are applied the compounds which have the lowest boiling points are vaporized and flow to the condenser where they are condensed back into a liquid and collected. As the temperature is raised the next set of compounds with a higher boiling point distill after and are condensed, collected in a new receiver flask and so on until the target compound is distilled or is left behind in the boiling flask.
When distilling cannabis extracts the order of compounds are as follows:
(temperatures assume ~100 millitorr and are only a rough estimate)
- Solvents <50°C
- Water 50-70°C
- Terpenes 130-170°C
- Flavonoids 170-200°C
- Cannabinoids 200-240°C
When a sample needs to be separated from its components, we might simply assume that the distillation process would provide the individual components in pure form. No distillation can produce this magic. Components in a sample will exert a vapor pressure at temperatures well below their boiling point. A boiling point is merely the temperature at which the vapor pressure of the liquid is greater than the pressure of the atmosphere surrounding the liquid. It is not an absolute figure and should be considered a range that is maximum at this temperature.
The greater the difference in boiling points between the components in a sample the greater the separation or purification from the resulting distillation. If the difference in boiling points is small then much of the unwanted components will be co-distilled along with the target fraction and must be distilled multiple times to achieve the desired purity. The task of separation usually dictates the type of apparatus required.