## 1 Introduction  
The ability to deliver safe and authentic food is of high priority for manufacturers. However, often due to complex network of suppliers, the potential of unintentional contamination of food exists considerably. The deliberate contamination driven by economic gain has also become harder to detect and track. As a result, the vulnerability of products and the risks inherent to food fraud is of considerable concern. An example is fake or diluted extra virgin olive oil (EVOO). In the worst-case and illegal scenario, EVOO is diluted with cheap vegetable/seed oils or lower-grade, refined olive oils. The authenticity or purity of EVOO has a direct impact on consumers' trust and attitude towards the product and may even pose health risks to the consumer when it is adulterated. EVOO is a fruit oil obtained by cold pressing the pulp of olives. It is one of the main ingredients in the Mediterranean diet, and the consumption of EVOO has spread worldwide due to its nutritional properties and health benefits [1]. The rising global consumer need for EVOO commands premium prices, counterfeitingof EVOO with other cheaper oils including vegetable oils, seed oils, refined olive oils, etc. In a study, researchers from the University of California, Davis and the Australian Oils Research Laboratory concluded that as much as 69% of imported European olive oil sold as extra virgin in the delicatessens and grocery stores on the US west coast was not extra virgin at all [2]. An ongoing global debate is setting quality grades for olive oil (e.g., EVOO is the highest grade of olive oil) and its labelling to avoid producers labelling refined or adulterated olive oil as EVOO. The scientific community is increasingly addressing the issues related to adulteration of EVOO [3; 4; 5; 6; 7; 8; 9] in order to ensure its quality for both economic and health reasons. Food authenticity (or purity of food) is not a new concept [3; 10; 11; 12; 13]. Sensory evaluation carried out by a panel of trained tasters is one of the most important evaluations for determining the quality of EVOO [14]. However, the sensory analysis may not be sensitive enough to detect the purity of EVOO or sophisticated adulterations or mislabeling. Several techniques have been applied to detect the authenticity of EVOO including mass spectrometry, gas chromatography, high-performance liquid chromatography, capillary gas chromatography and nuclear magnetic resonance spectroscopy [4; 5; 11; 12; 15; 16]. Although these methods are very powerful and provide very low detection limits, they are time consuming and expensive, requiring dedicated laboratories and trained professionals. Hence there is an increasing demand to develop a quick, sensitive, portable, easy-to-use and cost-effective online analytical approach for quantifying either the purity or adulteration of EVOO and potentially the additives. EVOO is composed mainly of mixed triglyceride esters of oleic acid and palmitic acid and of other fatty acids, along with traces of squalene and sterols, which all can be identified with vibrational spectroscopy techniques. The general chemical structure of EVOO is shown in Figure 1. Vibrational spectroscopy including infrared, near-infrared and Raman spectroscopy in combination with chemometrics are useful techniques for determining the authenticity of EVOO [6; 8; 13; 17; 18; 19; 20; 21; 22; 23]. Raman spectra occurs as a result of a molecular vibration causing a change in polarizability of the molecule. The molecular vibrations reflect the molecular structure and are therefore used as a spectroscopic fingerprint for detection, identification, and imaging of molecules. The Raman scattered (Stokes) signal can be obtained both via a spontaneous process as well as a stimulated process using two lasers having a frequency difference corresponding to the energy difference in the Raman transition involved [24; 25; 26].