Article pubs.acs.org/JAFC
Combination of Running-Buffer-Mediated Extraction and Polyamidoamine-Dendrimer-Assisted Capillary Electrophoresis for Rapid and Sensitive Determination of Free Fatty Acids in Edible Oils Jingqing Wu, Ying Ge, and Weidong Qin* Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China S Supporting Information *
ABSTRACT: A method was developed for determining free fatty acids in edible plant oils by incorporation of running-buffermediated liquid−liquid extraction and polyamidoamine-dendrimer-assisted capillary electrophoresis−capacitively coupled contactless conductivity detection. The recoveries for the extraction were in the range of 90.1% and 110.3%. Addition of dendrimer to the running buffer improved the separation of fatty acids. Under the optimized buffer conditions, i.e., 3 mM pelargonic acid, 39 mM tris(hydroxymethyl)aminomethane, 30 mM polyoxyethylene 23 lauryl ether, 35% acetonitrile, 15% 2propanol, 2.5% 1-octanol, and 300 μM polyamidoamine generation 2 at apparent pH 8.53, the 10 model fatty acids were separated in 18 min with detection limits ranging from 0.46 to 3.28 μM. The successful determination of fatty acids in real samples suggests that the method is simple, cost-effective, and easy to operate and is suitable for scanning free fatty acid in edible plant oils. KEYWORDS: capillary electrophoresis−capacitively coupled contactless conductivity detection, polyamidoamine dendrimer, free fatty acids, edible plant oil
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INTRODUCTION Free fat acids (FFAs) in edible plant oils ordinarily originate from crude plant oils and hydrolysis of triacylglycerols (TAGs) during processing and storage. FFAs affect the flavor quality of the oil. Moreover, they are chemically unstable, and a high concentration of FFAs may lead to the rancidity of the oil and formation of peroxides that are toxic to humans. Therefore, FFA content is one of the most concerned indices of edible oils. Total FFAs in edible plant oil can be determined by nonaqueous titrimetry recommended by the American Oil Chemists’ Society (AOCS).1 The method is simple but uses a large amount of organic solvents and cannot determine the contents of individual FFAs. The individual FFA concentration in edible oil can be obtained by chromatography methods.2−4 In gas chromatography (GC), FFAs are derivatized to methyl or trimethylsilyl esters.5,6 In high-performance liquid chromatography (HPLC), they can be directly detected7 but are often derivatized for higher detection sensitivity.8,9 Owing to its high separation efficiency, capillary electrophoresis (CE) has been considered as an alternative technique for fatty acid analysis.10−13 Capacitively coupled contactless conductivity detection (C4D) is based on conductivity differences between the sample zones and the background electrolyte (BGE); it is an ideal detection approach for analytes lacking chromophores. Moreover, C4D overcomes the problem of short path length that is encountered by the on-column photometric detection. Despite the obvious advantages of CE−C4D, only a few works applied this technique in FFA analysis.14,15 Another aspect should be considered for CE analysis of FFA is separation. Since fatty acids (FAs) possess carboxyl groups, they can be separated straightforwardly under normal polarity © 2014 American Chemical Society
mode in neutral or basic running buffers, in which they migrate to the detector following the cathodic electroosmotic flow (EOF).11 However, it was suggested that reversed EOF combined with a negative separation voltage is favorable, because in this case the solutes comigrate with EOF and better reproducibility of migration time could be achieved.16,17 The commonly used strategy for reversing EOF is dynamic coating due to its versatility and simplicity;18 for this application, cationic surfactants such as tetradecyltrimethylammonium bromide (TTAB)19,20 and cetyltrimethylammonium bromide (CTAB) are often used. Usually cationic surfactants at millimolar levels are needed for EOF reversal in aqueous media, but the concentrations can be higher in nonaqueous solutions.21 This is not favorable if C4D is employed, because the baseline noise would markedly increase upon elevated buffer conductivity. Polyamidoamine (PAMAM) dendrimers are highly branched, symmetrical macromolecules with high-density functional groups.22 They possess three kinds of structural components: cores, interior repeating units, and terminal groups. PAMAMs terminated with primary amine groups are full-generation dendrimers (Gn, where n is an integer), whereas those terminated with methyl ester groups are half-generation ones (Gn.5). Due to protonation of the amino groups, the fullgeneration PAMAMs are positively charged at pH
