The Determination of Low Levels of Benzene, Toluene, Ethylbenzene, Xylenes, and Styrene in Olive Oil Using HS-GC–MS

Levels of benzene, toluene, ethylbenzene, xylenes, and styrene (BTEXS) are a concern in olive oil. These compounds find their way into olive trees and hence into olives and olive oil mainly as a result of emissions from vehicles, bonfires, and paints into ambient air near orchards. This analysis uses a PerkinElmer® TurboMatrix™ 110 Headspace (HS) sampler with a Clarus® SQ 8 gas chromatograph–mass spectrometer (GC–MS) to achieve detection limits below 5 ng/g and concentrations up to 100 ng/g.

For this method, sample preparation is extremely easy. 10 g of olive oil is weighed into a standard headspace vial, then sealed with a crimped cap. The HS sampler heats the vial at 90 °C for 20 min. During this period the BTEXS components transition from the sample matrix into the vapor phase above. A fixed volume of the HS vapor is extracted from the vial and introduced into a Carbowax-type capillary column for GC separation. A quadrupole mass spectrometer is used to detect and quantify the BTEXS components. By using the MS single-ion monitoring (SIM) mode of operation, the detector sensitivity and selectivity is significantly enhanced and clean chromatography is obtained even at low analyte levels. The Clarifi™ SQ 8 MS detector is able to operate in SIFI mode in which both SIM and full scan data are collected. This enables library searches of MS spectral data to assist with confirmation of the identity of specific components while at the same time benefitting from SIM operation.

Figure 1: Chromatogram of 17 ng/g BTEXS in 10 g "cleaned" olive oil.

The analysis is fully automated and takes just 10.5 min for the chromatography plus 3.5 min for cool-down and equilibration between analyses. Sub-ppb levels are possible using standard headspace sampling of light aromatics in a complex natural oil matrix without the need for vapor pre-concentration (for example with an HS Trap).

The figure below shows typical chromatography obtained from a sample of cleaned olive oil fortified with approximately 17 ng/g of the BTEXS components. Note: the only sample that had low levels of BTEX was in the California sample. It is likely that the olive oil was produced by a solvent extraction method.

Excellent quantitative performance was demonstrated from ten samples of cleaned olive oil fortified with approximately 45 ng/g of the BTEXS components. Each was analyzed using the HS-GC-MS system and an overall quantitative precision of 1.69 to 3.76% relative standard deviation was obtained, which is a very good result for this complex matrix.

Seven different brands of olive oil were purchased from a local supermarket and analyzed using this method. The results are given in the table below. Alarmingly high levels of BTEXS components were found in many of the supermarket samples. The only sample that had low levels was one from California. Solvents were found in the California sample so it is likely that the olive oil was produced by a solvent extraction method rather than the more traditional olive-pressing method.

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