## 1 Introduction  
Olive oil is obtained from the fruit of the olive tree (_Olea europaea_) and extra virgin olive oil (EVOO) has been prized throughout the ages for possessing unique qualities based on its medicinal, cosmetic, nutritional and taste, as well as even ceremonial value. As a result, it is one of the most expensive--and also one of the most adulterated--food products in recorded history [1]. Researchers at the University of California, Davis released the findings of a now famous 2010 study [2], showing that more than two-thirds of the EVOO sold in California is neither extra-virgin nor in some cases even olive oil, as the real oil is often adulterated with cheaper, more readily available oils. In 1981, more than 600 people died because of adulterated olive oil that contained rapeseed oil that was denatured with aniline [3], causing a huge scandal known as the \"toxic oil syndrome\" [4; 5]. Extra virgin olive oil is typically produced via pressing the olive fruits into a paste followed by centrifugation [6]. The process of extracting oil from olives is slow and different from other seed oils and this in part results in EVOO being a high-cost commodity with a price that is typically 3-5 times that of other edible oils [7]. Despite its premium price, olive oil is in very high demand due to its superior quality over other edible oils. Therefore, adulteration of EVOO with low-cost or poor-quality edible oils for economic benefit is of considerable concern to the olive oil industry. Trade organisations such as the International Olive Council (IOC) have laid out clear definitions for the different categories of olive oil and olive-pomace oil, as well as having clear trade standards and approved testing methods for their identification. These include fatty acid composition analysis by gas chromatography, stigmastadiene content by liquid chromatography [8], as well as organoleptic tests. Although these methods are very powerful and provide very low detection limits for the adulterant, they are time consuming and expensive, requiring dedicated laboratories and trained professionals, and as such rely on the sample being taken to centralised laboratories for testing. Hence, there is an increasing demand to develop a quick, sensitive, portable, easy-to-use and cost-effective analytical approach for quantifying either the purity or adulteration of EVOO and potentially any additives that may be present in the oil [9]. In addition to the IOC approved approaches, several analytical techniques have been applied to detect the authenticity of EVOO, and these include mass spectrometry, gas chromatography, high-performance liquid chromatography and nuclear magnetic resonance (NMR) spectroscopy [5; 10; 11; 12; 13; 14]. Vibrational spectroscopic techniques such as infrared in the near (NIR) or mid infrared (MIR) ranges and Raman offer alternative approaches for rapid screening and detection of adulteration in EVOO. The potential of Raman spectroscopy combined with multivariate statistics has been investigated for evaluating the authenticity (or purity) and concentration of EVOO [15], as well as for the detection and quantification of specific adulterants [16]. Recently, spatially offset Raman spectroscopy (SORS) has been developed as a powerful technique that enables measurement at surface positions laterally offset from the excitation laser [17; 18] allowing analysis of materials within container packaging [19]; with more recent advances in SORS having led to the development of a handheld device [20]. Though the main applications of portable SORS relate to pharmaceutical analysis [21], security, and chemical detection [22], there have also been developments in food adulteration analysis with through-container detection of chemical markers associated with counterfeit alcohol [23], detection of adulterated butter [24], sugar [25], and subsurface analysis of salmon through skin [26] or assessment of freshness of intact prawns [27].