Letter pubs.acs.org/ac
From Sample Processing to Quantification: A Full Electrochemical Approach for Neutral Analyte Derivatization, Capillary Electrophoresis Separation, and Contactless Conductivity Detection Mauro Sérgio Ferreira Santos, Fernando Silva Lopes, Denis Tadeu Rajh Vidal, Claudimir Lucio do Lago, and Ivano Gebhardt Rolf Gutz* Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748 05508-000, São Paulo, SP, Brazil ABSTRACT: A thin-layer electrochemical flow cell coupled to capillary electrophoresis with contactless conductivity detection (EC-CE-C4D) was applied for the first time to the derivatization and quantification of neutral species using aliphatic alcohols as model compounds. The simultaneous electrooxidation of four alcohols (ethanol, 1-propanol, 1butanol, and 1-pentanol) to the corresponding carboxylates was carried out on a platinum working electrode in acid medium. The derivatization step required 1 min at 1.6 V vs. Ag/AgCl under stopped flow conditions, which was preceded by a 10 s activation at 0 V. The solution close to the electrode surface was then hydrodynamically injected into the capillary, and a 2.5 min electrophoretic separation was carried out. The fully automated flow system operated at a frequency of 12 analyses per hour. Simultaneous determination of the four alcohols presented detection limits of about 5 × 10−5 mol L−1. As a practical application with a complex matrix, ethanol concentrations were determined in diluted pale lager beer and in nonalcoholic beer. No statistically significant difference was observed between the EC-CE-C4D and gas chromatography with flame ionization detection (GC-FID) results for these samples. The derivatization efficiency remained constant over several hours of continuous operation with lager beer samples (n = 40).
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distance from the working electrode surface.10 The cell can be fed with multiple solutions displaced by solenoid pumps. The sequential steps of potentiostatic conversion inside the thinlayer cell, product injection, CE separation, and detection can be freely set by the software. Such a system has already allowed, for the first time, the combination of both electrochemical preconcentration and stripping of trace metals coupled to capillary electrophoresis.10,11 In addition, it also offers a powerful tool for in situ sampling and the determination of certain reaction products of electrocatalytic processes.12 This study shows that such a thin-layer electrochemical flow cell coupled to capillary electrophoresis with contactless conductivity detection (EC-CE-C4D) system, which is fully based on electrochemical phenomena, also has great potential for electrochemical derivatization of neutral analytes for quantification purposes. This approach will be demonstrated for alcohols, which are electro-oxidized to the corresponding carboxylates, and exemplified by the determination of ethanol in beer.
apillary electrophoresis (CE), which has been a natural choice for the separation of ionic species, may also be applied to nonionic analytes provided that they are either previously or dynamically converted into an ionic form. Precapillary chemical derivatization has long been used for such a task, which allows not only the formation of ionic species but also the attachment of either a chromophore or fluorophore group for detection purposes.1−3 In many cases, the timeconsuming derivatization procedure can be avoided by either complex formation with one component of the background electrolyte 4 or micellar electrokinetic chromatography (MEKC), which allows the differential mobilization of neutral analytes according to their affinity for ionic micelles.5 One advantage of precapillary derivatization over the dynamic approach is the possibility of achieving some additional selectivity by the differential reactivity of the sample components. In this regard, electrochemical derivatization (EC), often applied in association with HPLC,6,7 is a strategy also capable of converting certain neutral analytes into charged species amenable to CE separation, as shown by Matysik et al.8,9 However, the evolution of EC-CE to a quantitative analytical method is reported here for the first time. Most instrumental difficulties with precapillary electrochemical processing of samples have been overcome with a recently described fully automated system comprising a special thin-layer electrochemical flow cell with an inlet for the electrophoresis capillary, which is reproducibly placed at a fixed © 2012 American Chemical Society
Received: July 10, 2012 Accepted: August 30, 2012 Published: August 30, 2012 7599
dx.doi.org/10.1021/ac3019312 | Anal. Chem. 2012, 84, 7599−7602
Analytical Chemistry
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Letter
EXPERIMENTAL SECTION Reagents. A 50.0 mmol L−1 Tris and 10.0 mmol L−1 HCl solution (pH 8.6) was used as the background running electrolyte (BGE). Standard solutions of the alcohols were prepared with analytical grade ethanol, 1-propanol, 1-butanol, and 1-pentanol (Merck). The alcohols were diluted in an aqueous electrolyte consisting of 5.0 mmol L−1 HNO3 and 1.0 mmol L−1 HCl, which favors the electrochemical oxidation of alcohols. Samples. To evaluate the system regarding the analysis of ethanol in real samples, two types of beer were analyzed: a regular pale lager (alcohol content 4.5% v/v) and a nonalcoholic beer (alcohol content
