In-line electrochemical reagent generation coupled to a flow injection biamperometric system for the determination of sulfite in beverage samples

Disposable Microchamber with a Microfluidic Paper-Based Lid for Generation and Membrane Separation of SO2 Gas Employing an In Situ Electrochemical Gas Sensor for Quantifying Sulfite in Wine

This work presents a fully disposable microchamber for gas generation of a sample solution. The microchamber consists of a cylindrical well-reactor and a paper-based microfluidic lid (μFluidic lid), which also serves as the reagent loading and dispensing unit. The base of the reactor consists of a hydrophobic membrane covering an in-house graphene electrochemical gas sensor. Fabrication of the gas sensor and the three-layer μFluidic lid is described. The μFluidic lid is designed to provide a steady addition of the acid reagent into the sample solution instead of liquid drops from a disposable syringe. There are three steps in the procedure: (i) acidification of the sample in the reactor to generate SO₂ gas by the slow dispensing of the acid reagent from the μFluidic lid, (ii) diffusion of the liberated SO₂ gas through the hydrophobic membrane at the base of the reactor, and (iii) in situ detection of SO₂ by cathodic reduction at the graphene electrode. The device was demonstrated for quantitation of the sulfite preservative in wine without heating or stirring. The selectivity of the analysis is ensured by the combination of the gas-diffusion membrane and the selectivity of the electrochemical sensor. The linear working range is 2-60 mg L^-1 SO₂, with a limit of detection (3SD of intercept/slope) of 1.5 mg L^-1 SO₂. This in situ method has the shortest analysis time (8 min per sample) among all voltammetric methods that detect SO_2(g) via membrane gas diffusion.

Sulfite may disrupt estrogen homeostasis in human via inhibition of steroid arylsulfatase

Steroid arylsulfatase is an important enzyme in human, which plays an important role in dynamic equilibrium of natural estrogens. On the other hand, sulfite can be endogenously produced as a consequence of human body's metabolism of sulfur-containing amino acids, while its main sources to human are mainly derived from food as it is a widely used additive. Sulfite-sensitivity is a well-known phenomenon to a small proportion of populations. However, its potential adverse effects on healthy individuals have been hardly reported. It was for the first time reported in this study that sulfite could effectively inhibit arylsulfatase, and its IC50 values for the snail- and human urine-derived arylsulfatase were determined to be 71.9 and 142.8 µM, which were lower than the concentration of sulfite in some healthy population. Consequently, it appears that sulfite might disrupt estrogen homeostasis in human, and this deserves further investigation.

Simple Flow-Based System with an In-Line Membrane Gas-liquid Separation Unit and a Contactless Conductivity Detector for the Direct Determination of Sulfite in Clear and Turbid Food Samples

This study presents a simple flow-based system for the determination of the preservative agent sulfite in food and beverages. The standard method of conversion of sulfite ions into SO2 gas by acidification is employed to separate the sulfite from sample matrices. The sample is aspirated into a donor stream of sulfuric acid. A membrane gas-liquid separation unit, also called a 'gas-diffusion (GD)' unit, incorporating a polytetrafluoroethylene (PTFE) hydrophobic membrane allows the generated gas to diffuse into a stream of deionized water in the acceptor line. The dissolution of the SO2 gas leads to a change in the conductivity of water which is monitored by an in-line capacitively coupled contactless conductivity detector (C4D). The conductivity change is proportional to the concentration of sulfite in the sample. In this work, both clear (wine) and turbid (fruit juice and extracts of dried fruit) were selected to demonstrate the versatility of the developed method. The method can tolerate turbidity up to 60 Nephelometric Turbidity Units (NTUs). The linear range is 5-25 mg L-1 SO32- with precision < 2% RSD. The flow system employs a peristaltic pump for propelling all liquid lines. Quantitative results of sulfite were statistically comparable to those obtained from iodimetric titration for the wine samples.

Novel pararosaniline based optical sensor for the determination of sulfite in food extracts

Sulfite, which is a protective agent in various food industries, also is known as an allergen. Therefore, sulfite content in food must be monitored and controlled. In this context, a novel optical sensor is designed for simple, rapid and sensitive determination of the sulfite content in food samples. Acidified pararosaniline (PRA) hydrochloride reagent in cationic form was immobilized on the surface of the Nafion cation exchanger membrane by electrostatic interactions. In formaldehyde medium, the pale purple PRA-Nafion film was converted to rich purple due to the highly conjugated alkyl amino sulfonic acid formation in the presence of sulfite and the absorbance change at 588 nm was recorded. The proposed optical sensor gave a linear response in a wide concentration range for sulfite. The limit of detection (LOD) and the limit of quantification (LOQ) values obtained for sulfite were 0.084 and 0.280 ppm SO₂ equivalent, respectively. The proposed optical sensor was validated in terms of linearity, additivity, precision and recovery parameters. The sulfite contents obtained for real food extracts were found to be comparable to the conventional iodometric titration results (with the exception of highly colored samples containing reducing agents, where iodometry was shown to exhibit a systematic error while the proposed sensor could measure the true value). The proposed optical sensor is insensitive to positive interferences from turbidity and colored components of the sample. Sulfite determination in a complex food matrix can be performed using the rapid, simple and sensitive PRA-based sensor without a need for pre-treatment.

Comparative sulfite assay by voltammetry using Pt electrodes, photometry and titrimetry: Application to cider, vinegar and sugar analysis

Sulfite is a widely applied preservation agent, against oxidative decay in foodstuffs and beverages. Among the analytical methods applied, electrochemical techniques exploit its facility to undergo oxidation. This paper aims at the comparative investigation of the performances exhibited by three analytical methodologies: cyclic voltammetry at different Pt working electrodes, the volumetric method employing sodium thiosulfate as titrating agent and the photocolorimetric method relying on the reduction of Fe3+-orthophenanthroline complex by sulfite. The cyclic voltammetric assay was performed at Pt strip and Pt ring electrodes, by linearly sweeping the potential between -100 and 1,500 mV. The linear range corresponded to 7.5 mg L-1 – 4.0 g L-1 for Pt strip working electrode and to 15.5 mg L-1 – 4.0 g L-1 for Pt ring working electrode. Relative standard deviations smaller than 3% showed repeatability. RSD values smaller than 3% were also obtained in the photometric assay, but the latter was characterised by a narrower linear range. The Pt strip electrode allowed wider linear range and lower sensitivity, whereas the Pt ring electrode with Ag/AgCl reference in the same unit was characterised by better repeatability. Applications involved sulfite assay in vinegar, brown sugar and cider samples with consistency between cyclic voltammetry and titrimetry.

Flow injection analysis-flame atomic absorption spectrometry system for indirect determination of sulfite after on-line reduction of solid-phase manganese (IV) dioxide reactor

A new and simple flow injection method followed by atomic absorption spectrometry was developed for indirect determination of sulfite. The proposed method is based on the oxidation of sulfite to sulphate ion using solid-phase manganese dioxide (30% W/W suspended on silica gel beads) reactor. MnO₂ will be reduced to Mn(II) by sample injection in to the column under acidic carrier stream of HNO₃ (pH 2) with flow rate of 3.5mLmin^-1 at room temperature. Absorption measurement of Mn(II) which is proportional to the concentration of sulfite in the sample was carried out by atomic absorption spectrometry. The calibration curve was linear up to 25mgL^-1 with a detection limit (DL) of 0.08mgL^-1 for 400µL injection sample volume. The presented method is efficient toward sulfite determination in sugar and water samples with a relative standard deviation (RSD) less than 1.2% and a sampling rate of about 60h^-1.