4 February / March 2020


Vacuum Assisted Headspace Solid-phase Microextraction: A Powerful Tool for Olive Oil Analysis


by Steven Mascrez1 1


2 , Elefteria Psillakis2 , Giorgia Purcaro1 , *


Gembloux Agro-Bio Tech, University of Liège, Gembloux, 5030, Belgium School of Environmental Engineering, Technical University of Crete, Greece


* Corresponding author. Contact information: Giorgia Purcaro, gpurcaro@uliege.be Gembloux Agro-Bio Tech, University of Liège


Bât. G1 Chimie des agro-biosystèmes, Passage des Déportés 2, 5030 Gembloux, Belgium Office phone: +32 (0)81 62 22 20


The performance of vacuum-assisted headspace solid-phase microextraction (Vac-HS-SPME) was compared to regular headspace solid-phase microextraction (HS-SPME) for the analysis of extra-virgin olive oil. Vac-HS-SPME proved beneficial in particular for semi-volatile compounds, significantly improving the kinetics of extraction. Moreover, for viscous oil samples combining the effects of heating the sample and vacuum was proven beneficial since it reduced the viscosity of the sample, increased the diffusivity of compounds in the liquid phase and improved volatilisation of less volatile compounds.


1. Introduction


Volatile secondary metabolites are an important class of compounds in many fields of applications such as clinical [1- 3], environmental [4-7, and food [4,8,9]. In the latter one, volatiles can provide highly informative hints on botanical and geographical origin and/or on the quality of food in terms of aroma profile, spoilage, or technological impact on the secondary metabolite profile. Among the many high-value food commodities for whom volatile metabolites play an essential role, extra virgin olive oil represents an urgent challenge. The goal is to support the official sensor evaluation with a more objective and robust method.


Headspace (HS) solid-phase microextraction (SPME) is most widely applied for volatile profiling and fingerprinting since it provides easy automation, solvent-free applications, and flexibility due to the different sorbents commercially available [10]. HS-SPME is a technique based on the equilibrium between three-phases, namely sample- headspace-fibre. The equilibrium can be reached in a few minutes or several hours, depending on the physicochemical properties of the analytes and the sample. Therefore, a compromise is needed between sensitivity and throughput.


An exciting possibility to minimise such


a compromise is the use of reduced pressure conditions during sampling, a technique termed vacuum-assisted HS– SPME. The theory for water-based and solid samples was effectively clarified in a tutorial published by Psillakis et al. [11]. More recently, the theory was extended to oily samples [12]. From a thermodynamic viewpoint, the equilibrium concentration is not affected by reduced pressure conditions [11,13], while the kinetics is mostly dependent on medium, temperature, and pressure. This means that an increase in the mass transfer is recorded when increasing the temperature and/or decreasing the pressure [11,13].


The mass transfer in the HS towards the SPME fibre is not considered a limiting process [14,16], while the mass transfer from the liquid to the HS, although highly analyte-matrix dependent, is usually the limiting step [11,16]. This behaviour is explained by considering the concentration gradient located in the stagnant film layers at the liquid/HS interface, assuming that the bulk of the two phases is well mixed. Such a theory proved successful in describing the Vac-HS-SPME process in water-based samples [11,14]. Recently [12], it was clarified that the overall resistence to transfer from the liquid to the HS (1/kO


) is due to two


diffusional resistances in series, namely the sum of the gas-phase resistance (1/(KGL


kG )) and the liquid-phase resistance (1/kL ),


in particular for viscous liquid as olive oil, according to the following equation:


1 k0


= + kL


1 where, kG and kL 1 KGL kG are the mass transfer


coefficients for the gas and olive oil boundary layers and KGL


is the gas phase-


olive oil partition coefficient representing the ratio of the equilibrium concentrations in the gas phase over that in the liquid sample. Moreover, the diffusivity should be taken into account in viscous liquid samples, leading to additional resistance in the liquid-film compared to an aqueous phase [17]. On the other hand, the diffusion coefficient in the gas-phase shows an inverse proportionality to the total pressure in the system, regardless of the model chosen for describing it [14]. Therefore, sampling by Vac-HS-SPME is beneficial for analytes where gas-phase resistance controls their volatilisation rate, improving their extraction kinetics, while for compounds where the limiting process is the liquid diffusion, the temperature will play a beneficial role.


This work aimed to investigate the Vac- HS-SPME sampling on the extra-virgin olive oil profile, comparing the extraction temperature and time profile under reduced and normal pressure conditions.


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