## 4 Discussion
Quite all studies used a chemometric method to explain results, most common of which were PCA and PLS analysis. Until now, the geographic origin of olive oil is verified by documented traceability supported by the stable isotope characterization as an official method to confirm the documented information provided by the producers. However, an important question that still needs an answer is how the geographic origin of an unknown sample is guaranteed using this method. Thus, the need for a method able to reliably verify the geographic origin of both known and unknown samples of olive oil is an urgent requirement. Literature data demonstrate that gas chromatography is a popular method to use when geographic origin should be declared. Olive oil is a complex mixture; however, as demonstrated by the above-mentioned studies, chemical interpretation of this mixture is achievable through gas chromatography. Consequently, a vast amount of data that describe the chromatographic profile of the examined olive oil are available, which, in combination with chemometric methods that point out differences between the studied samples, provide valuable information. In this regard, gas chromatography is a suitable method to use when the geographic origin of olive oil is required. In addition, Infrared and Raman spectroscopy consist of another analytical, quick technique to consider when authenticity must be declared. Both methods deal mostly with the detection of possible adulteration of EVOO and VOO with minor quality or vegetable oils. According to the articles presented in this review, adulteration with sunflower oil is the most studied, followed by soybean oil. The range of the concentration that the adulterant was detected was from 1 to 80%; however, this depends on the type of the adulterant. PLS and PCA were the most common chemometric methods used in combination with various algorithms. Although the number of studies that use IR spectroscopy to estimate authentication of olive oil is greater than those that use only Raman spectroscopy, results obtained from these studies showed that adulteration was successfully detected using both techniques based on spectral differences between olive oil of best and minor quality or olive oil and vegetable ones. Consequently, it cannot with certainty be estimated whether, for example, IR outweighs Raman spectroscopy. In addition, both techniques allow the analysis of a large sample size within a day, while at the same time, no chemical waste is produced. Analysis can be qualitative and/or quantitative, and the information obtained is based on the presence of characteristic chemical groups that belong to specific chemical families. EVOO and VOO authenticity via IR and Raman spectroscopy is mainly based on their predominant constituents, namely, fatty acids and triglycerides. However, the determination of these constituents can also be achieved by gas chromatography. Although GC allows precise chemical characterization of the FA and triglycerides presented, it is a time-consuming method that requires sample pre-treatment with the use of non-environmentally friendly solvents. IR and Raman spectroscopy have overcome both these disadvantages which are of major importance, especially when rapid certification of authenticity is required. Nevertheless, GC may provide data that complete the fatty acid profile of an olive oil. Now, the inverse situation is to be explained regarding the volatile profile. As is mentioned above, volatiles are among those compounds according to which authenticity can be determined. Emphasis was given to the chemical characterization and quantification of these compounds via gas chromatography, which, as indicated by literature data, is a popular method adopted by many researchers in pursuit of olive oil authentication. In the study of [PERSON] et al., 2015 [7], it was demonstrated that volatile compounds could be detected at the band near 5280 cm\({}^{-1}\), attributed to carbonyl groups, using FT-NIR spectroscopy.