They can be adulterated with all natural constituents. An example of this would be Lavender. True Lavender is Lavandula angustifolia . This oil is most widely presented as a Lavender oil but is not acceptable in the practice of Aromatherapy. Other forms of adulteration are more difficult to detect without the use of gas chromatography and much experience.
Some terpenes oxidize rapidly, which changes the aroma, longevity and solubility of the oil.
One should take care when encountering these terpeneless oils because the natural synergy of the oil has been changed. It now has a higher percentage of the other constituents. These oils are useful mostly in the food and cosmetic industries to provide a uniform product. These oils too, would surely lack the natural synergy found in natural oils. Using even one synthetic chemical in an otherwise natural oil renders the oil synthetic as a whole.