Determination of perfluorinated alkyl acids in corn, popcorn and popcorn bags before and after cooking by focused ultrasound solid–liquid extraction, liquid chromatography and quadrupole-time of flight mass spectrometry
Introduction
Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been broadly used since the late 1940s in different industrial and commercial applications due to their effect of the reduction of the surface tension and their hydrophobic and oleophobic properties [1], [2]. Hence, they have been extensively distributed in the environment.
However, perfluorinated compounds show high thermal, biological and chemical inertness owing to carbon–fluorine is the strongest existing covalent bond (450 kJ/mol) [3]. Moreover, it has been also proved that perfluoroalkyl acids (PFAAs) exhibit toxicity in laboratory animals causing developmental diseases, liver cancer, affect the lipid metabolism and disturb the immune system [4]. Additionally, these compounds may come from the degradation of other PFASs, such as polyfluoroalkyl phosphate surfactants (PAPs) and fluorotelomers (FTOHs), which may be atmospherically or metabolically degraded to them, increasing the PFAAs concentration, such as perfluorocarboxylic acids (PFCAs) and perfluorooctane sulfonate (PFOS), in the environment and the human exposure [5], [6], [7].
Due to their hazardous, PFASs have been determined over the last few years in a wide variety of matrices, such as human and wildlife biological ones (urine, milk, plasma, serum, blood, liver, brain and kidney), environmental liquid (river water, seawater and wastewater) and solid matrices (dust, sewage sludge, sediments and soil), consumer products (textile, carpet, cookware and food packaging), food and even in indoor and outdoor air [1], [8].
One of the main applications of the PFASs has been as additives in food-contact packaging due to their ability to make the covering oil, stain and water resistant [9]. In previous studies, PFOA (perfluorooctanoic acid) has been found at levels up to 198 ng/g and 290 ng/g in microwave popcorn packaging [10], [11], but fortunately, the concentration of long chain PFASs in packaging have decreased in recent years [12], [13] because the manufacture of PFOS and other PFASs have been banned in the U.S. and in Europe. However, these compounds can still be present in food contact packaging due to the acquisition of products that can still contain PFASs from other countries outside the U.S. or Europe.
PFASs have been typically extracted quantitatively by classical solid–liquid extraction (SLE) [12], [14], by ultrasound assisted extraction (USE) [10], [15], [16], [17] and by pressurized liquid extraction (PLE) [11], [13], [18], [19] from different kinds of food-contact packaging samples. However, the focused ultrasound solid–liquid extraction (FUSLE) has offered an efficient extraction in only several seconds [20]. FUSLE is a low-cost, fast, simple and safe extraction technique based on the cavitation phenomenon. It is more reproducible and more efficient than traditional ultrasonic bath extraction (USE) due to its 100 times higher ultrasonic power and the immersion of the ultrasound microtip directly in the extracting solution [21], [22].
FUSLE has also been used for the fast extraction (seconds or few minutes) of organic analytes, such as UV filters [23] and bisphenols [24] from packaging, as well as, polychlorinated biphenyls [25], phthalate esters [25], nonylphenols [25], polycyclic aromatic hydrocarbons [21], [22], [25], [26] and brominated diphenyl ethers [27] from environmental matrices. However, longer extraction times were needed for the extraction of metals from sediments [28] using FUSLE.
Regarding to extract PFASs from food, this matrix has been more widely studied than packaging. The most commonly used extraction methods have been based on SLE using an orbital shaker [29], [30], [31], [32] and USE [12], [33], [34]. Ion pair extraction (IPE) [35], [36], alkaline digestion [36], [37], PLE [38] and QuEChERS methods [40] have also been employed. However, any of these techniques are more time-consuming or difficult to implement than FUSLE technique, and this is the first time that this extraction has been used to preparation of food samples.
In this study, a fast and simple method based on FUSLE and UHPLC–(QTOF)MS/MS has been developed, validated and applied for the detection and quantification of ten PFAAs in six different microwave popcorn bags and the popcorn inside them, before and after microwave cooking. Thereby, the absence of migration from packaging to food has been shown and the effect of the microwaving process on PFAAs has also been studied.
Section snippets
Materials and reagents
Individual standards of perfluorooctanesulfonic acid tetraethylammonium salt (PFOS) 98%, perfluorobutanoic acid (PFBA) 98%, perfluoropentanoic acid (PFPeA) 97%, perfluorohexanoic acid (PFHxA) >97%, perfluoroheptanoic acid (PFHpA) 99%, perfluorooctanoic acid (PFOA) 98%, perfluorononanoic acid (PFNA) 97%, perfluorodecanoic acid (PFDA) 98%, perfluoroundecanoic acid (PFUnA) 95% and perfluorododecanoic acid (PFDoA) 95%, were provided by Sigma–Aldrich (Madrid, Spain).
Isotopically labelled internal
Preliminary experiments
In order to select the optimal chromatographic conditions, different mobile phases and flow rates were tested (data not shown).
Formic acid concentrations between 0.1% and 1.0% in the mobile phase were studied. PFOS and PFDA cannot be separated chromatographically employing 0.1% formic acid mobile phase, but when formic acid concentration was increased up to 1.0%, PFOS retention time decreases in such a way that PFNA and PFOS began to overlap. Accordingly, a 0.8% formic acid–ACN mixture and a
Conclusions
A FUSLE–UHPLC–(Q-TOF)MS/MS method has been developed to determine nine PFCAs and PFOS in microwave popcorn packaging, corn and popcorn.
An efficient and simple extraction of PFAAs has been carried out by FUSLE in only one cycle of 10 s. Additionally, the chromatographic separation of the ten PFAAs took place in only 4 min. Therefore, this method allows a fast screening for these emerging pollutants in microwave bags and their content.
The whole method has been validated for the three matrices,
Acknowledgements
The Spanish Ministerio de Educación y Ciencia is thanked for supporting this work through the CTM 2010-16935 project (within the Plan Nacional de Investigación Científica y Desarrollo e Innovación Tecnológica co-financed with FEDER funds). C. Moreta also thanks the Government of La Rioja for the FPI fellowship.
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