Effect of cetyltrimethylammonium bromide on the migration of polyaromatic hydrocarbons in capillary electrokinetic chromatography
Introduction
Polyaromatic hydrocarbons (PAHs) are characterized by their fused-ring structures consisting of between two and seven aromatic rings. The conjugated π electron systems of these aromatic compounds determine their physical and chemical properties. Recently, much attention has been focused on their possible carcinogenic effects on human health. PAHs are generally formed by incomplete combustion and high temperature pyrolysis reactions. Major sources of PAHs can be associated with transportation vehicles, coke ovens, metal smelters, industrial wastes, petroleum cracking, and domestic heating. Due to the widespread distribution of PAHs in the environment and their lipophilic nature, most types of food contain measurable levels of PAHs, generally in the parts per billion (ppb) and parts per million (ppm) range. Certain technological processes and cooking procedures can also cause important levels of PAHs in some foods. Especially, determination of PAHs from the oil samples has been attractive since their legal limits are not completely defined for the vegetable oils and fats. It is indicated that burning and cooking processes can increase the PAH content by destroying about ≥70% of which is originally present in the oils [1].
Due to their unique toxic effects, it is important to develop new analysis methods for the simultaneous identification and determination of PAH mixtures. More recently, CE has become more popular and a complementary technique to the other chromatographic methods. The main advantage is the considerable diminution in the sample preparation and analysis time, as well as in the reagent consumption. Moreover, fused silica capillaries are much less expensive than chromatographic columns, easily washed between runs, and free of irreversible contamination of the matrix, unlike the packed columns. Micellar electrokinetic chromatography (MEKC) is a widely used CE technique based on differential partitioning between aqueous and micellar pseudo-stationary phase [2]. Retention of solutes in MEKC depends mainly on the hydrophobicity of the solute. In order to provide a participation of highly hydrophobic PAHs from the hydrophobic core of micelles to solvent phase, high concentration of organic solvents are needed. However, in organic-rich media, surfactants lack the capability of aggregating to form micelles [3]. Recently, neutral analytes were separated interacting with ionic surfactant monomers added to the buffer in the presence of high concentration of organic solvents. This special CE technique is generally called capillary electrokinetic chromatography (CEKC). In the literature tetraalkylammonium [4], [5], [6], [7], dioctylsulfosuccinate (DOSS) [8], sulfonated Brij-30 [9], dodecylbenzene sulfonate (SDBS) [10] and cetylpyridinium ions [11] have been reported for the electrokinetic separations of PAHs.
In this work, the separation and determination of three aromatic and eight polyaromatic compounds (Fig. 1) were achieved using the selective interaction of cetyltrimethylammonium bromide (CTAB) monomers in 40% ACN containing aqueous solution by CE. The effects of CTAB and acetonitrile (ACN) concentrations, voltage and pH on electroosmotic mobility (EOM), i.e., the mobility of electroosmotic flow and electrophoretic mobilities (EPM) of PAHs were investigated. Linear concentration range, reproducibilities, limit of detection (LOD) and limit of quantification (LOQ) values of investigated analytes were defined and developed electrophoretic conditions were finally applied to the analysis of PAHs in cooked soybean oil sample.
German Society for Fat Science has fixed the legal limits at 25 ppb for total PAHs and 5 ppb for the heavy fraction [12]. For benzo[a]pyrene (BaP), this limit is 1 ppb in smoked foodstuff [13], but there are no legal limits for PAH content in oils and fats, one of the major sources of PAHs in the diet because of their lipophilic nature. Especially BaP has been reported in fumes from refined vegetable, soybean, and vegetable oils [14], [15], [16], [17], [18] but the concentration in the oil was not reported. To our knowledge, there are a few studies in the literature reporting the PAHs amount of cooked oil [19], [20], [21].
Liquid–liquid partition, caffeine complexation and saponification are mostly used extraction methods of PAHs from oil samples. In the liquid–liquid partition method, the oil sample is dissolved in an organic solvent such as hexane, and PAHs are mostly extracted with DMF-water or DMSO [22], [23]. Comparing these three extraction methods for an olive oil sample, it has been noted that the saponification method gave highly enough squalene residue which co-elutes with PAHs. This method needed an additional purification step [24], but the liquid–liquid partition method showed high purification power without an important amount of squalene in the samples. However, the described methods takes more time because of many clean-up steps used in order to remove the co-extracts before the detection step. Moreover, usage of the high amount of toxic organic solvents and their costs are the other disadvantages of these methods. In this work, we used ACN for the extraction of oil sample since it is easier to remove and reaches lower quantification limits than the described methods [25].
The extract obtained with one of these extraction methods contains some impurities other than PAHs and they may interfere with the main analyses. For the purification, different clean-up procedures such as thin layer chromatography (TLC) and column chromatography on different adsorbents are widely applied [26]. For this work, silica gel column (30 cm × 1 cm i.d.) was packed with 20 g of silica gel 60 (70–230 mesh) and deactivated with 5% distilled water for the clean-up of PAHs extract. After loading the sample onto the column, extract was fractionated into aliphatic and aromatic hydrocarbons using n-hexane and n-hexane:dichloromethane (4:1), respectively.
Section snippets
Instrumentation
The electrokinetic measurements were performed on Beckman P/ACE MDQ electrophoretic system (Fullerton, CA, USA) equipped with an on-column UV absorbance detector (230 nm) and system 32 Karat software. An uncoated fused silica capillary (50 μm i.d. × 375 μm o.d., Agilent, Switzerland) with total and effective lengths of 57 and 50 cm, respectively, was used. Temperature was maintained at 25 °C. Centrifugation was performed on Heraeus Biofuge Primo (Osterode, Germany) system. For heating the oil sample,
Influence of additive concentration on the separation
Separation in CEKC is based on the partitioning of the analyte between the running electrolyte and the pseudo-stationary phase. In this work, CTAB was used as pseudo-stationary phase. A 0.0–35.0 mM concentration range of CTAB was prepared in a buffer consisting of 10 mM phosphate and 40% ACN at pH 6.0. Between the 0.0–4.0 mM concentration ranges, with the addition of CTAB, EOM decreases, changes direction, and subsequently increases in the reversed direction. The reason for this is that the
Conclusion
In this study, we performed a new and rapid CE method for PAH analysis with the addition of positively charged CTAB into the separation electrolyte containing phosphate and ACN at pH 6.0. The addition of CTAB both creates an electrically driven counter-flow in the capillary and selective interactions of PAHs with this pseudo-stationary phase cause their retardation and separation. The developed method was applied to a cooked soybean oil sample. Results show that PAH contents of oil increase by
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