**Optical Absorption Spectroscopy for Quality Assessment of Extra Virgin Olive Oil**

Anna Grazia Mignani1, Leonardo Ciaccheri1\*, Andrea Azelio Mencaglia1 and Antonio Cimato2 *1CNR – Istituto di Fisica Applicata "Nello Carrara" 2CNR – Istituto per la Valorizzazione del Legno e delle Specie Arboree Italy* 

#### **1. Introduction**

46 Olive Oil – Constituents, Quality, Health Properties and Bioconversions

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Thermal Extraction and Gas Chromatographic–Mass Spectrometric Determination of Volatile Compounds of extra-Virgin Olive Oils *Journal of Chromatography A,* Vol. Light travels through space in the form of electromagnetic waves of different wavelengths. The entire wavelength range represents the electromagnetic spectrum. Spectroscopy studies the interaction between light and matter, in order to draw information about the chemical composition inside (Lee et al., 2011). Figure 1 shows the various bands of the electromagnetic spectrum. This chapter refers to measurements performed in the 200-2500 nm band, which is usually subdivided into three portions: the ultraviolet (UV), the visible (VIS) – perceivable by human eyes – and the near-infrared (NIR). They correspond to the 200-400 nm, 400-780 nm, and 780-2500 nm ranges, respectively.

Fig. 1. The electromagnetic spectrum

<sup>\*</sup> Corresponding Author

Optical Absorption Spectroscopy for Quality Assessment of Extra Virgin Olive Oil 49

Absorption spectroscopy in the UV-VIS-NIR range is one of the most popular measuring methods of conventional analytic chemistry (Mellon, 1950; Bauman, 1962). The most


The conventional instrument for absorption spectroscopy is the double-beam spectrophotometer, the working principle of which is depicted in Figure 3. Since quartzbased optical fibers are transparent in the UV-VIS-NIR range, they are used to equip conventional spectrophotometers by flexible means. Indeed, optical fibers offer the unique possibility of localized probing, a particularly attractive feature for online measurements, which can be carried out in real time without any sample drawing. Moreover, the recent availability of bright LEDs and miniaturized spectrometers further enhances the intrinsic optical and mechanical characteristics of optical fibers and makes it possible to implement

olive oil sample

The conventional spectrophotometer, implemented by means of optical fiber technology, is depicted in Figure 4. In this case, optical fibers are used for both illumination and detection,

Coupler

UV-VIS

NIR

UV-VIS-NIR Light source

Quartz cuvette with oil sample reference

Si-InGaAs detectors **+** ratioing and processing electronics

**M1 S1 S2 S3**

Transmission spectra to spectrometers

absorption spectrum

I0

I

analysis is simple, fast, and does not require manual intervention; - non-destructive analysis by means of a small quantity of sample;


**2. Instrumentation** 

relevant advantages offered are:

compact and moderate-cost instruments.

monochromator

splitting optics

Fig. 3. Working principle of a double-beam spectrophotometer

**DEUTERIUM HALOGEN LIGHT SOURCE**

Fiber optic bundle

Fig. 4. Fiber optic setup for absorption spectroscopy

**TOP Sensor Systems**

coupling optics

sources Deuterium-Tungsten

A light beam illuminating an olive oil sample gives rise to reflected, transmitted, and scattered intensities. Optical absorption spectroscopy, as shown in Figure 2, makes use of a broadband UV-VIS-NIR source of intensity I0 to illuminate the olive oil sample. Then, the transmitted light intensity I, as a function of the illumination wavelength is measured. The change in light intensity, providing the transmittance T, is determined by the molar absorptivity the concentration of absorbing species C, and the optical path L, via the Lambert-Beer relationship, expressed by Equations 1 and 2. T is frequently expressed logarithmically as in Equation 3, to give the so called optical absorbance A, which results linearly dependent on concentration.

$$I = I\_0 \exp\left(-\varepsilon \mathcal{C} \, L\right) \tag{1}$$

$$T = \frac{I}{I\_0} = \exp\{-\varepsilon \gets L\} \tag{2}$$

$$A = \log \frac{I\_0}{I} = \mathcal{E} \stackrel{\circ}{C} L \tag{3}$$

This chapter focuses on extra virgin olive oil quality evaluation achieved by means of UV-VIS-NIR absorption spectroscopy. The composition of olive oil is about 98% triglycerides and approximately 2% non glycerid constituents.


In practice, the entire UV-VIS-NIR absorption spectrum can be considered an optical signature, a sort of univocal *fingerprint* of the olive oil. The spectroscopic data can be suitably processed for obtaining a correlation to quality indicators, to the geographic origin of production, to product authenticity as well as to adulteration detection.
