GC-MS is an additive-revealing technique that can help you analyze the components in essential oils.
Gas Chromatography is a technique that targets the different molecular volume of the constituents of essential oil and separates them. It provides a fingerprint of the oil, and as it is mentioned above, it can also offer information on additives, but it is not certain to be conclusive. At most, it tells you about the presence and relative quantities of chemical components within an essential oil.
The relative concentration of the chemical compounds based on the retention time is recorded in the chromatogram. However, you can gather a more conclusive reading and even find out the presence of adulterants. This can be achieved through the process of Mass Spectometry.
In a Mass Spectrometer, molecules are ionized and the accelerated ions are separated w.r.t. their mass-to-charge ratio. An ion is an electrically charged particle or group of atoms. All the positive and negative ion currents that determine the molecular weight can be measured in the MS and recorded as a computer output. Each peak is compared with a database, which contains about 75,000 spectra. Therefore, the MS can measure the presence and quantity of chemical constituents.
The entire process of GC-MS is quite expensive and in an ideal case, it should be conducted for each batch of essential oils because the natural factors can alter the state. Also, you need a skilled professional for the readings to be taken accurately in a GC-MS testing. Please note that the process is not an absolute guarantee of quality. However, it proves to be a good start in most of the cases.