Crude drugs containing volatile oils are mostly characterized by typical odoriferous nature. Because they are considered the ‘essence’ of herbal material and are often biologically active, they are also known as essential oils. Chemically usually composed of mixtures of monoterpenes, sesquiterpenes and their oxygenated derivatives, they are volatile at room temperature. For volatile oil-bearing crude drugs, minimum standards for the percentage of volatile oil present in a number of drugs are prescribed by many pharmacopoeias.

To determine the volatile oil content of a crude drug, the plant material is distilled with water and the distillate is collected in a graduated tube. The aqueous portion separates automatically and is returned to the distillation flask. For oils with relative densities equal to or greater than water, separation from water is assisted by placing a known volume of xylene in the receiver and reading off the combined oil and xylene. Time taken to complete the distillation of the oil varies with the nature of the drug and its state of size reduction, but about four hours is usually sufficient. Solution of the volatile oil in a fixed oil such as in powdered drugs of umbelliferae may retard distillation. Pharmacopoeial standards for volatile oil contents of powdered drugs are lower than those for corresponding whole drugs.