Essential oils are concentrated hydrophobic liquids, consisting of volatile compounds. This thesis explains and describes how to ascertain and summarise the chemical composition of essential oils. Also how to demonstrate the application of gas chromatography- mass spectrometry ( GC- MS). Furthermore; understanding how the quadruple mass analyser works in the GC- MS. All products contain allergens and impurities but the important thing is they must be in very minute amounts. This thesis recorded the detected impurities found in the samples of essentials oils used.