Analytica Chimica Acta 556 (2006) 195–200 Determination of sulfite in beer samples using an amperometric fill and flow channel biosensor employing sulfite oxidase Min Zhao, D. Brynn Hibbert, J. Justin Gooding ∗ School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 6 April 2005; received in revised form 23 May 2005; accepted 25 May 2005 Available online 14 July 2005 Abstract A simple method is described to determine sulfite in beer samples using a fill and flow channel biosensor. A droplet of sample is placed into the inlet of a rectangular flow cell and begins to flow through the channel by capillarity. The flow is maintained and controlled by a porous outlet plug of defined porosity. In a rectangular flow cell, the sample solution flows through three consecutive zones: over a predictor electrode, an enzyme layer and a detector electrode. Together these three zones enable the differentiation between current due to sulfite and current due to other electroactive species in the sample. The predictor electrode is located upstream, and on the opposite channel wall to the enzyme layer and detector electrode, and is poised at the same potential (+0.65 V versus Ag/AgCl) as the detector electrode. On this electrode, the current contribution from all species in the sample solution that are oxidized at that potential is determined. The enzyme layer contains sulfite oxidase, which, in the process of oxidizing sulfite, produces hydrogen peroxide, which itself is reduced by excess sulfite. The current at the downstream detector electrode is therefore different from that at the predictor electrode as a result of the enzyme reaction and the difference of the currents, corrected for the dimensions of the electrodes, is proportional to the concentration of sulfite. The method enables a straightforward correction of the interfering current at the detector electrode and a determination of the analyte concentration. The effect of interferences from ascorbic acid, ethanol, sorbic acid and tartaric acid in the detection of sulfite is efficiently removed. The concentration of sulfite in a sample of beer measured by the biosensor is equivalent to that measured using a reference method based on the AOAC-recommended Monier–Williams method. © 2005 Elsevier B.V. All rights reserved. Keywords: Sulfite; Beer samples; Fill and flow channel biosensor; Enzymatic analysis 1. Introduction Sulfite is adopted as a common preservative in a great vari- ety of foods to prevent enzyme activity causing browning and to inhibit the growth of microorganisms during storage. How- ever, in recent years, it has been increasingly realized that sulfite has a potential to cause an adverse reaction in sulfite- sensitive asthmatic individuals [1–3]. Since 1986, the United States Food and Drug Administration (FDA) has required labeling of products containing more than 10 ppm of sulfite (156 M) in foods or beverages [4]. Since 2002, the Aus- tralian Food Standards Code (AFSC) has regulated that any ∗ Corresponding author. Tel.: +61 2 9385 5384; fax: +61 2 9385 6141. E-mail address: Justin.gooding@unsw.edu.au (J.J. Gooding). wine containing added sulfites in concentration of 10 ppm or more sold in Australia must be labeled as such. In order to accurately control the quality of manufactured products, a sensitive, easily applied and accurate analytical method for the determination of sulfite is required. A number of methods have been developed for sulfite determination, which may be classified according to the com- pounds measured. One approach is to acidify the sample, then determine the amount of sulfur dioxide gas formed using gas chromatography [5]. Alternatively, sulfur dioxide can be oxidized to sulfuric acid using hydrogen peroxide, followed by an acid–base titration [6]. A typical example of the titration approach is the Monier–Williams method, which is recommended by the Association of Official Ana- lytical Chemists (AOAC) [6]. This method is reliable and 0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.05.083