Essential oils are the volatile substances that a plant owes its odour and flavour to and are often referred to as the soul or the hormones of the plant. Essential oils are found in different parts of plants (leaves, flowers, bark, fruit etc.). For example Lavender essential oil is obtained from the flowering tops of the plant, rose oil from the petals, patchouli oil from the leaves, sandalwood from the wood and lemon oil from the rind. The amount of oil which can be extracted from the plant varies tremendously, for example to produce one drop of pure rose oil it takes 30 rose petals, whereas clove oil yields a much higher amount of oil and therefore less is needed. Pure essential oils can be very expensive as they are the pure oil from the plants and have been unadulterated unlike other less therapeutic oils such as fragrant oils or synthetic oils.
During distillation, only very small molecules can evaporate, so they are the ones that leave the plant. These extremely small molecules make up an essential oil. Oils containing more of the smallest and therefore most volatile of these tiny molecules are termed top notes. Those containing more of the heaviest and least volatile of the tiny molecules are called base notes. Those in-between are known as middle or sometimes heart notes.
Top notes are generally the smells that you immediately notice when smelling a blend, for example the light flowery aromas, they are quick to wear off but are delightful to the senses. Base notes are the ones that are left after wearing an aroma blend for some time, they linger and are more grounding in effect. Some essential oils can be found in more than one note.