Using tandem solid phase extraction (SPE) in conjunction with gas chromatography tandem mass spectrometry (GC-MS/MS), Xu et al (2015) present a sensitive assay for identifying and quantifying dangerous polycyclic aromatic hydrocarbons (PAHs) in edible oils1.
Since the discovery of their toxic attributes, PAHs have been under close scrutiny by regulatory authorities worldwide. They are a priority for monitoring in food products within the European Union, and the Environmental Protection Agency in the United States considers them a major health concern. Arising from incomplete combustion of carbon-containing organics, PAHs form highly stable contaminants within the environment. Research has shown they can act as carcinogens, teratogens and mutagens, and as a result many countries impose limits on their presence within foodstuffs.
Unfortunately, analysis of PAHs is not simple; they are often present within complex organic matrices and often at low concentration. Sample preparation is therefore time-consuming and complicated. Strict quality controls are therefore in place for approved assays.
Edible oils are one of the many important sources of PAHs that concern food safety regulators, with uptake of these contaminants in humans exceeding amounts taken in from both air and drinking water, the next most common sources. However, preparing the PAHs for assay in oils and fats is difficult since they must be first separated from highly abundant matrix constituents such as triacylglycerides and fatty acids.
Although a number of different extraction techniques have been tried, Xu et al. recognized that the physical characteristics of SPE were ideally suited to PAH assay. In conjunction with highly sensitive GC-MS/MS, the researchers have validated a fast and effective tandem SPE method capable of successfully identifying and quantifying 24 PAHs in edible oils.
First, the researchers used olive oil spiked with commercial PAHs to optimize and validate the tandem SPE/GC-MS/MS workflow. Concentrating on elution characteristics that would maximize PAH yield, the team defined a toluene-ethyl acetate flow through approach using two different SPE cartridges coupled together. After finding the optimal SPE cartridge combination and order, the researchers optimized recovery volume flow to maximize PAH recovery. They then analyzed the eluates using a Trace gas chromatograph coupled with a TSQ Quantum XLS triple mass spectrometer (Thermo Scientific).
During initial validation steps, Xu et al. found that diluting the oil in cyclohexane improved flow through the SPE cartidges by cutting down viscosity. Once this was established, the team obtained good flow through and PAH recovery by optimizing elution flow parameters and SPE cartridge position. Spiking studies gave good recoveries for all 24 PAHs under investigation (64 -115%). Further investigation showed that concentrating the solvent following elution could result in loss of certain PAHs; the team overcame this by evaporating eluates gently to a final volume of 200µL.
With optimization of the sample preparation steps, the team turned to establishing assay parameters through measuring standard concentration curves to achieve linearity. From this they estimated limits of quantification, detection and precision for the PAHs under investigation. Once determined as suitable for assay, the researchers then measured levels of PAHs in samples of commercially obtained edible oils, selecting peanut, olive, rapeseed and soybean as popular consumer products. The assay detected high PAH concentrations in the peanut oil samples compared to the other varieties. Although limits were generally within food safety limits set by China, the team found edible oil PAH levels exceeded those set by EU regulations in a number of samples tested. Xu et al. believe that the SPE-GC-MS/MS workflow described in their paper is a suitable method for rapid and efficient assay of PAHs in edible oils and will contribute to food safety monitoring in the future.
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1. Xu, T. et al. (2015) “Simultaneous Determination of 24 Polycyclic Aromatic Hydrocarbons in Edible Oil by Tandem Solid-Phase Extraction and Gas Chromatography Coupled/Tandem Mass Spectrometry“, Journal of the Association of Analytical Communities International 98(pp.529-37)