Essential oil is a volatile aromatic substance extracted from flowers, leaves, stems, roots or fruits of plants by steam distillation, extrusion, cold leaching or solvent extraction. It is the hormone of plants, also known as "soft gold". Essential oils can be classified into single essential oils and compound essential oils. Undiluted, extracted from a specific part of a single plant and free of any second plant or chemical substances. Compound essential oil is diluted by mixing two or more single essential oils with base oil (also known as medium oil or base oil), which can enhance each other's effects. Essential oils have the characteristics of lipophilicity, water resistance, permeability and high concentration.

The refined oil is divided into four grades, which are grade four (D), fragrance fragrance, pure dew or flower water; Grade III (C), Perfume Grade; Grade II (B), food grade; Level 1 (A), pure essential oil.

Fragrances and perfumes, as they are known, are usually made from grade C oils (perfume oils) mixed with chemical solvents.

Perfume can be divided for an index with concentration content 5 classes, distinction is shown as follows: concentration is higher than 20% call essence; The concentration is about 10-20% between called perfume; Concentrations of about 5-12% are called eau de toilette; A concentration of only 3-6% is called cologne; A concentration of only 1-2% is called a fragrance. Grade A essential oil is of high grade and belongs to pure medical grade quality. It is extracted from natural organic plants by steam distillation under appropriate conditions.

In recent years, plant essential oils have been on the rise, and their economic and social values have attracted wide attention and strong interest. How to improve the oil yield of plant essential oils has become a subject of many researches. At present, there are many extraction methods of essential oil, such as steam distillation, dry distillation, solvent extraction, water osmotic distillation, CO2 supercritical extraction, microwave extraction and so on. Among them, dry distillation can cause a scorched taste, which affects the aroma of essential oil itself. Solvent extraction method is to use organic solvents with low boiling point and raw materials to heat extraction. The essential oil extracted by this method also contains components such as lipids and waxes, which need subsequent distillation and refining, otherwise impurities may be introduced. At present, a kind of "Nikens-Mikerson" steam distillation/solvent extraction combined device is widely used, which is characterized by less sample consumption, less solvent loss and good repeatability [4]. CO2-SFE adopts supercritical extraction device, which is difficult to industrial production, and its instrument capacity and operation level need to be improved. Moreover, this method has high cost, so it has not been widely used at present. Microwave extraction was put in the microwave field will be extract with solvent, both under the action of high frequency electromagnetic wave produced intense oscillation, at this time if the environment is poor concentration, molecular can be finished in a very short time migration from inside to outside, this method can shorten the production time, lowering energy consumption of solvent, at the same time can improve the yield and purity of extract.