Sulfite represents a preservation agent widely applied in foods and drinks to hinder the oxidative decay of their components. Sulfite is characterised by means of its reductive properties, as it can be easily oxidized to sulfate . Sulfites are incorporated in: processed meats, wines, beer and cider, soft drinks and fruit juices, jams and jellies, dried and pickled fruits and shellfish . Developments in analytical methodologies are required, namely for chemical compounds present in food, that may constitute a safety risk .
WHO/FAO, through Codex Alimentarius, Codex Stan  lists sulfites among the major allergens, and the European Union as well, through the Regulation (EU) 1169 . The latter stipulates the presence of the allergens on the label of the product, and in the case of sulfur dioxide and sulfites, for concentrations greater than 10 mg kg-1 or 10 mg L-1, as SO2.
The term “sulphites” encompasses collectively sulfur dioxide (INS 220), sodium sulfite (INS 221), sodium bisulfite (INS 222), sodium metabisulfite (INS 223), potassium metabisulfite (INS 224), potassium sulfite (INS 225) .
The first renowned method for sulfite assay is Monier-Williams titrimetry , later modified for the use in food and beverages . Redox titration with iodine solution, after distillation, was another method applied to assess the concentration of sulphites . High performance liquid chromatography was also applied, and employed a HPX-87H column with diode array detection. The sulfuric acid concentration in the eluent solution was comprised between 0.0025 N and 0.06 N .
Spectrophotometric sulfite assay can rely on the reaction of pentacyanidonitrosylferrate(II) and sulfite anion. The nitroprusside-SO32- reaction product was stabilized by zinc ethylenediamine complex and the absorbance was measured at λ = 482 nm. The repeatability was 2.37% and the obtained limit of detection was 0.99 μg mL-1 SO32-. The results of the photocolorimetric assay were compared with those obtained iodometrically, by subjecting excess iodine to titration with thiosulfate in the presence of starch . Photocolorimetric sulfite assay can also be based on SO2 reaction with Schiff’s fuchsin reagent [10,11]. The modified para-rosaniline-formaldehyde method exhibits broad linear range (0.05–5.0 mg L−1 as SO2), whereas the 5,5′-dithiobis(2-nitrobenzoic acid) method, operates within a linear range of 0.10–4.30 mg L−1 SO2 .
Electrochemical methods have the advantages of accuracy, rapidity, and low cost, when compared to other time-consuming instrumental methods . Voltammetric and polarographic methods are applied to determine sulfur-containing preservatives [14,15]. Sulfite electroactivity consists in its propensity to undergo facile oxidation to sulfate, and allows for its quantitation by electrochemical techniques, namely voltammetry, amperometry, potentiometry.
Cyclic voltammetry relies on linearly sweeping the potential, the analytical peak being due to sulfite oxidation to sulfate and the oxidation of sulfite was investigated at platinum electrodes [16, 17, 18]. The irreversibility of the process is proved by studies performed at the surface of a graphite electrode, in alkaline media . With respect to the electro-oxidation steps, it was stated that: either sulfite anion can be oxidized to sulfate in two consecutive steps, each implying one electron transfer [20,21],
or sulfite anion can undergo one step oxidation (one electron transfer) to sulfite radical anion. Subsequently two sulfite anion radicals react, resulting in dithionate anion S2O62-occurrence. The latter disproportionates, giving sulfate and sulfite [20,21].
In cyclic voltammetric studies at a carbon paste electrode modified with carbon nanotubes and p-aminophenol, sulfite oxidation was reported to take place at 680 mV. The analytical peak due to electro-oxidation depends linearly on concentration for 2x10-7- 2.8 x10-4 mol L-1, allowing for accurate determination in residual waters .
Square-wave voltammetry using carbon nanotubes-modified carbon paste electrode allows for quantitation of this preserving agent in beverages. The analytical range of linear response corresponds to 25.0–500.0 μmol L-1, with a detection limit of 1.0 mg L-1 SO2 (at 16 μmol L-1 sulfite) .
Simultaneous voltammetric assessment of ascorbic acid and sulfite in beverages, was performed at a glassy carbon electrode modified with multi walled carbon nanotubes and polyallylamine .
Using cobalt layered perovskite or skeletal nickel as electrode material in fuel cells, allows for sulfite direct oxidation at the electrode surface [25,26]. A comparative study using three glassy carbon electrodes modified by Co-porphyrin, ortho-phenylendiamine, or both, simultaneously, allowed for sulfite electro-oxidation in 0.02 mol L-1 NaOH solution. A concentration of 44.0 μg mL-1 of free sulfite was assessed in a Chilean red wine, by standard addition method .
Modification of a Pt electrode with carbon nanotubes and Prussian Blue led to the amperometric assay of sulfite (at 600 mV) in the presence of iodate and periodate, with a linear range of 16.4 - 142.9 μmol L-1 . A novel system comprising a flow gas diffusion unit and a wall-jet amperometric FIA detector, coated with a supra-molecular porphyrin film, allowed fast, reproducible and accurate assay of free sulfite in fruit juices, with optimization of analytical parameters: linearity range 0.64 - 6.4 μg mL-1 sodium sulfite, detection limit of 0.043 μg mL-1, a RSD of +/- 1.5% (n = 10) and an analytical frequency of 85 analyses per hour .
Biosensors combine the rapidity of electron transfer with the specificity of the biocatalyst [13, 30]. Sulfite oxidase-based biosensors can be developed by enzyme immobilization in a Prussian Blue nanoparticles/polypyrrole composite matrix, electrodeposited on an indium oxide electrode . Recently, sulfite oxidase was also immobilized on a magnetite-gold-folate nanocomposite modified carbon-paste electrode. The linearity range corresponded to 0.1-200 mg L-1, with a detection limit of 10 μg L-1, proving enhanced electro-activity towards H2O2 resulted in the enzymatic substrate oxidation . A flow injection biamperometric assay relying on the redox reaction between the I3- and SO32-ions, enabled a linear response ranging from 1.0 to 12.0 mg L-1 .
This study aims to investigate the performances in sulfite determination, of cyclic voltammetry at Pt strip and Pt ring electrodes, sodium thiosulfate titrimetry, and Fe3+-orthophenanthroline photocolorimetry. This research deals comparatively with the application of three analytical techniques for sulfite assay, which imparts the degree of novelty to the present study. Real sample analysis consisted in sulfite assay in vinegar, brown sugar and cider samples.
2 Experimental procedure
2.1 Reagents and apparatus
To achieve the objective of this study, a KSP potentiostat-galvanostat („built in house” by Professor Slawomir Kalinowski, University Warmia and Mazury, Olsztyn, Poland), as well as the respective software Cyclic Voltammetry for recording the cyclic voltammograms, and a three-electrode voltammetric cell, with a working, a counter and a reference electrode were used.
As working electrodes a Pt strip electrode Radelkis OP-0612P (30 mm2 surface) and an Oxidation Reduction Potential (ORP) Pt ring electrode Mettler-Toledo Pt 4805-DPA were handled. The Pt electrode Mettler-Toledo Pt 4805-DPA had an Ag/AgCl reference incorporated in the same unit.
In addition, a saturated calomel reference electrode (SCE), Radelkis was only used when the working electrode was the Pt strip (Radelkis OP-0612P 30 mm2 surface) and finally the counter electrode was a always Pt strip Radelkis OP-0612P.
Standard solutions of sodium sulfite (Na2SO3 anhydrous p.a., Chimopar Bucharest, Romania) with concentrations ranging between 5 mg L-1 and 5 g L-1, were freshly prepared before every determination. KCl (Chimopar Bucharest, Romania) as 0.1 mol L-1 solution (pH=6.5) was used as supporting electrolyte for all standards preparation.
2.2 Working procedure
Before every determination, both Pt working electrodes were cleaned mechanically on alumina slurry and electrochemically by applying a -1.5 V potential pulse for 3 seconds.
The volume of the analysed sodium sulfite solution was 50 ml and all measurements were performed at 240C, using a 0.10 mol L-1 KCl solution as supporting electrolyte.
For the cyclic voltammetry measurements, the potential was scanned within the range -100 to 1,500 mV, at a 50 mV s-1 scan rate. For the investigation of the influence of the scan rate, this parameter varied between 25 and 250 mV s-1.
The voltammetric determinations were performed at 5 minutes after standard sulfite solution preparations, using 0.1 mol L-1 KCl as electrolyte, in order to hamper the confirmed rapid analyte oxidation .
For titrimetric sulfite assay, to 50 mL sample (vinegar, cider), 5 mL iodine solution 0.1 N were added, followed by titration with a thiosulfate solution 0.1 N, in the presence of starch.
The volume used is equivalent to excess iodine, so sulfur dioxide or sodium sulfite amounts can be calculated as per the reference to their equivalents:
In the case of brown sugar, to 15 mL brown sugar solution (that contained 10 g sugar), 1.5 mL iodine solution 0.1 N were added, followed by thiosulfate titration.
For the photometric assay, to 1 mL complex solution, the appropriate volume of a 200 mg L-1 sulfite solution was added. Then distilled water was addded, to the final volume of 10 mL. The final concentration in the analysed standard solutions was comprised between 10 and 100 mg L-1. Readings were performed at λmax = 515 nm.
Ethical approval: The conducted research is not related to either human or animal use.
3 Results and Disscussion
3.1 Comparative investigation of analytical parameters
A series of cyclic voltammograms, at sulfite concentrations ranging between 5.0 mg L-1 and 5.0 g L-1 were recorded using alternatively the Pt strip electrode and the Pt ring electrode with Ag/AgCl reference electrode. The most representative cyclic voltammograms are presented in Figures 1-3. The observed oxidation potentials between 500 and 1000 mV vs saturated calomel or Ag/AgCl references were comprised within the ranges reported in literature .
The variation of the anodic peak with the increase of the potential scan rate was investigated. The electroactive process is diffusion-controlled, which is proven by the linear dependence of the current intensity on the square-root of the potential-sweep rate (Figure 4), which is in agreement with Randles-Sevcik law. It could be concluded that the value of the measured current intensity is controlled by the analyte diffusion from the bulk solution to the electrode/solution interface.
The influence of the scan rate was studied between 25 and 250 mV s-1. For the standard solutions as well for real sample assay, we chose a 50 mV s-1 scan rate, fast enough to ensure analyte diffusion to the electrode (determined by the concentration gradient between bulk solution and electrode surface) and electron transfer facility. In the case of irreversible or quasi-reversible electron transfers, at high scan rates, the electron transfer becomes slow relative to the potential sweep rate, and the rate of establishing the equilibrium at the electrode surface is lowered. Under these circumstances, the analyte concentrations generated at the surface of the electrode are not consistent to those estimated by Nernst dependence. Moreover, very elevated scan rates generally result in peaks characterised
by high current intensities, but distortions may appear on the voltammogram.
The developed calibration curves (Figures 5, 6) showed a linear range of analytical response corresponding to 15.5 mg L-1 – 4.0 g L-1 for the Pt ring electrode and to 7.5 mg L-1 – 4.0 g L-1 for the Pt strip electrode. The details at concentrations below 1 g L-1 are given in figures 7 (Pt ring electrode) and 8 (Pt strip electrode) respectively.
The equations of the calibration graphs on the whole linear analytical range corresponded to y = 525.07x + 63.74, with a correlation coefficient R12 = 0.9906 for the Pt strip electrode, and y = 454.78x + 163.44, with a correlation coefficient R22 = 0.9921, for the Pt ring electrode respectively, where y represents the value of the current intensity, and x the analyte concentration. Hence, the sensitivity given by the slope of the linear dependence was greater for the Pt strip electrode than for the Pt ring electrode with Ag/AgCl reference incorporated.
The equations of the calibration graphs below 1 g L-1 corresponded to y = 695.99x + 15.228, with a correlation coefficient R12 = 0.9987 for the Pt strip electrode, and y = 599.79x + 124.55, with a correlation coefficient R22 = 0.9830,
for the Pt ring electrode respectively, showing a greater sensitivity for the Pt strip electrode.
The precision of the voltammetric assay, as illustrated by the values of the relative standard deviation, was tested for both electrodes. The RSD values were calculated as:
The value of the relative standard deviation (RSD) was 2.68%, with 0.004022 standard deviation (SD) for the Pt strip electrode. The value of the relative standard deviation (RSD) was 2.55%, with 0.003806 standard deviation (SD) for the Pt ring electrode. To assess these analytical parameters, n=10 determinations were performed, at c = 0.15 g L-1 sodium sulfite, exploiting the dependence at small (below 1 g L-1) concentrations.
Moreover, RSD values were calculated for both electrodes, at c = 1.0 g L-1 sodium sulfite (n=10), exploiting the dependence on the whole linear analytical range: the RSD value was 2.54%, with 0.02565 standard deviation (SD) for the Pt strip electrode. The RSD value was 2.46%, with 0.02505 standard deviation (SD) for the Pt ring electrode.
The detection limits were 1.908 mg L-1 for the Pt strip electrode and 3.168 mg L-1 for the Pt ring electrode, respectively. The limit of detection was calculated as:
where s represents the standard deviation corresponding to the blank (KCl 0.1 M electrolyte solution) signal, and m represents the slope of the calibration graph below 1 g L-1,
considered for each electrode: 695.99 μA g-1 L for Pt strip and 599.79 μA g-1 L for Pt ring.
The limits of quantification were 5.781 mg L-1 for the Pt strip electrode and 9.60 mg L-1 for the Pt ring electrode, respectively. The limit of quantification was calculated as:
where s and m have the same significance as before . The calibration graph developed at the photometric sulfite assay based on the reduction of Fe3+-orthophenanthroline complex is presented in Figure 9, with a linear range of 10-80 mg L-1.
The spectrophotometric assay was characterised by a standard deviation (SD) of 0.7261 and a relative standard deviation (RSD) of 2.89% (at c= 25 mg L-1 sulfite concentration, n=10 determinations), and a limit of detection of 2.915 mg L-1.
3.2 Analytical applications on some real samples
Table 1 presents the results obtained at sulfite analysis in samples of cider, vinegar and brown sugar, that were obtained by applying cyclic voltammetry at Pt strip electrode (with better sensitivity and detection limit), and comparatively, the thiosulfate titrimetry.
The application of the standard addition method in vinegar is illustrated in Figure 10.
The application of the standard addition method in elderflower cider is illustrated in Figure 11.
3.3 Statistical analysis
A Student’s t-test was performed, to statistically analyse the differences between the mean concentrations
obtained by the two methods, with respect to sulfite concentration (Table 2). The t-values are presented, as well as the confidence levels for which there are no statistically significant differences between the mean concentrations given by the two methods (tcalculated < ttabulated). The statistical analysis relied on a two-tailed test. It was performed for eight degrees of freedom (df), as n1=5 (voltammetry) and n2=5 (titrimetry). Hence:
For two of the analysed products (one vinegar and brown sugar), there were no significant statistical differences between the mean values provided by the two methods, if a confidence interval of 90% is to be considered as reference. For four of the analysed products (apple cider Elderflower, brown sugar and the two vinegars), there were no significant statistical differences between the mean values provided by the two methods, if a confidence interval of 98% is to be considered. For apple cider Golden apple and apple cider Redberries, statistical differences were recorded between the means of the two methods, at confidence levels inferior to 99.8%.
3.4 Polyphenol interference study
To provide information about the influence of matrix components, total polyphenol content (TPC) was analysed by the Folin-Ciocalteu method. The results in gallic acid equivalents (GAE) can be classified as follows - apple cider golden apple: 106 mg GAE /L; apple cider elderflower: 123 mg GAE /L, apple cider redberries: 210 mg GAE /L. This leads to a maximum of a 1:1 gravimetric ratio TPC/ sulfite, as per reference to sulfite results furnished by voltammetry (Na2SO3 mg L-1).
Moreover, the voltammetric study performed on standard solutions showed that gallic acid perturbs the analytical voltammogram of sulfite at 1:1 (gallic acid/sulfite) gravimetric ratio, determining a 30.4 % increase of the analytical signal.
Nevertheless, when analyzing ciders, the sulfite analytical signal led to a cyclic voltammetric result consistent with that provided by titrimetry (with less than 6% deviation between the two methods), and is confirmed by standard addition. Here, it should be added that Folin-Ciocalteu assay can be prone to interferences  from a series of compounds such as sulfur dioxide itself, but also sugars, organic acids, all being present in the analysed ciders. The analysed products contain, beside sulfite, malic acid, whose content was reported to increase during
apple juice fermentation  and glucose-fructose syrup. Hence, the discrepancy may be explained by the fact that the values furnished by Folin - Ciocalteu method take account on the sum of the above-mentioned compounds and do not reflect only the TPC, because voltammetrically inactive compounds such as malic acid and sugars contribute to the analytical signal in Folin-Ciocalteu assay.
DuPont et al.  reported for cider a sum of phenolic compounds (cinnamic acid derivatives, benzoic acids, flavanols, flavonol-glycosides) of 48.19 mg L-1, assessed by HPLC. Moreover, Ye et al.  obtained for apple juice, also by HPLC, a sum of phenolic compounds of 64.34 mg L-1, with a decrease after fermentation that led to a 55.63 mg L-1 content in cider. Considering that the analysed beverages contain maximum 25% fermented apple juice, we may conclude that this leads to minimum interferences.
Several comparative and conclusive aspects can be inferred with respect to the applied analytical methodologies.
Both working Pt electrodes, measuring against different references, allow viable assay of sulfite, with confirmed analytical parameters and can be applied to quantifying this preservative from food and beverage samples.
The Pt strip electrode allows a larger linear range and a lower sensitivity (1.908 mg L-1 for Pt strip versus 3.168 mg L-1 for Pt ring), whereas the Pt ring electrode with Ag/AgCl reference incorporated is characterised by better repeatability, proven by a lower value of the relative standard deviation. Voltammetric determinations using both electrodes resulted in better precision than the photocolorimetric assay.
The sensitivity is greater for the Pt strip electrode Radelkis than for the Pt ring electrode Mettler-Toledo: 525.07 μA g-1 L for Pt strip vs 454.78 μA g-1 L for Pt ring, as proven by the slopes of the peak intensity dependences versus concentation. The same trend is observed for the slopes below 1 g L-1: 695.99 μA g-1 L for Pt strip, compared to 599.79 μA g-1 L for Pt ring.
The linear range in photometry (10-80 mg L-1) is less broad than that of voltammetry (15.5 mg L-1 – 4.0 g L-1 for the Pt ring electrode and to 7.5 mg L-1 – 4.0 g L-1 for the Pt strip electrode).
The results obtained by applying the voltammetric method to the assay of real samples, are correlatable to those furnished by the titrimetric method.
The beverages that contain significant amounts of reducing agents (such as phenolic antioxidants) cannot be analysed by the proposed techniques. We chose to analyse beverages that are not prone to interferences from phenolic compunds, such as: vinegars obtained from sugar fermentation (not from grapes or apple juice), sugar and commercial ciders containing maximum 25% apple juice.
We wish to thank Professor Slawomir Kalinowski, University Warmia and Mazury, Olsztyn, Poland, for the support in designing the potentiostat-galvanostat and developing the software Cyclic Voltammetry.
Isaac A., Livingstone C., Wain A.J., Compton R.G., Davis J., Electroanalytical methods for the determination of sulfite in food and beverages. Trends Anal. Chem., 2006, 25, 589-598. CrossrefGoogle Scholar
Lelieveld H., Food safety regulations based on real science. AgroLife Sci. J., 2015, 4, 93-96. Google Scholar
WHO - FAO, Codex Alimentarius, General Standard for Food Additives. CODEX STAN 192-1995, revision 2016. http://www.fao.org/fao-who-codexalimentarius/sh-proxy/en/?lnk=1&url=https%253A%252F%252Fworkspace.fao.org%252Fsites%252Fcodex%252FStandards%252FCODEX%2BSTAN%2B192-1995%252FCXS_192e.pdf accessed 15.05.2017
Regulation (EU) No 1169/2011 of the European Parliament and of the Council of 25 October 2011 on the provision of food information to consumers, amending Regulations (EC) No 1924/2006 and (EC) No 1925/2006 of the European Parliament and of the Council, and repealing Commission Directive 87/250/EEC, Council Directive 90/496/EEC, Commission Directive 1999/10/EC, Directive 2000/13/EC of the European Parliament and of the Council, Commission Directives 2002/67/EC and 2008/5/EC and Commission Regulation (EC) No 608/2004, Official Journal L304, 22.11.2011, p. 18–63.
Hillery B.R., Elkins E.R., Warner C.R., Daniels D., Fazio T. et al., Optimized Monier-Williams method for determination of sulfites in foods: collaborative study. J. Assoc. Off. Anal. Chem., 1989, 72, 470-475. PubMedGoogle Scholar
DeVries J., Ge H., Ebert F.J., Magnuson J.M., Ogawa M.K., Analysis for total sulfite in foods by using rapid distillation followed by redox titration. J. Assoc. Off. Anal. Chem., 1986, 69, 827-830. PubMedGoogle Scholar
McFeeters R.F., Barish A.O., Sulphite analysis of fruits and vegetables by High-Performance Liquid Chromatography (HPLC) with ultraviolet spectrophotometric detection. J. Agric. Food Chem., 2003, 51, 1513-1517. CrossrefPubMedGoogle Scholar
Musagala P., Ssekaalo H., Mbabazi J., Ntale, M., A spectrophotometric method for quantification of sulphite ions in environmental samples. J. Toxicol. Environ. Health. Sci., 2013, 5, 66-72. CrossrefGoogle Scholar
Pisoschi A.M., Danet A.F., Negulescu Gh.P. Glucose determination by cellophane–based and nylon-based enzymic electrodes; application on juices and wine analysis. EJEAFChe., 2006, 5(1), 1185-1194. Google Scholar
Alizadeh A.M., Mohseni M., Zamani A.A., Kamali K., Polarographic determination of sodium hydrosulfite residue (dithionite) in sugar and loaf sugar. Food Anal. Methods, 2015, 8, 483–488. CrossrefWeb of ScienceGoogle Scholar
Enache A., Dan M.L., Vaszilcsin N., Anodic oxidation of sulphite in alkaline media on platinum nanoparticles modified nickel electrode. Chem J Mold., 2017, 12, 102-109. CrossrefWeb of ScienceGoogle Scholar
Gasana E., Westbroek P., Temmerman E., Thun H.P., Kiekens P., A wall-jet disc electrode for simultaneous and continuous on-line measurement of sodium dithionite, sulfite and indigo concentrations by means of multistep chronoamperometry. Anal. Chim. Acta, 2003, 486, 73-83. Google Scholar
Skavas E., Hemmingsen T., Kinetics and mechanism of sulphite oxidation on a rotating platinum disc electrode in an alkaline solution. Electrochim. Acta. 2007, 52, 3510-3517. Web of ScienceCrossrefGoogle Scholar
Senning A., Sulfur in organic and inorganic chemistry, vol. 2, Marcel Dekker Inc, New York, 1972. Google Scholar
Holleman A.F., Wiberg E., Wiberg N., Inorganic Chemistry, Academic Press, San Diego, London, 2001, 503. Google Scholar
Ensafi A.A., Karimi-Maleh H., Keyvanfard M., A new voltametric sensor for the determination of sulfite in water and wastewater using modified-multiwall carbon nanotubes paste electrode. Int. J. Environ. Anal. Chem., 2013, 93, 650-660. CrossrefGoogle Scholar
Sartori E.R., Fatibello-Filho O., Simultaneous voltammetric determination of ascorbic acid and sulfite in beverages employing a glassy carbon electrode modified with carbon nanotubes within a poly(allylamine hydrochloride) film. Electroanalysis, 2012, 24 627–634. Web of ScienceGoogle Scholar
Enache A., Vaszilcsin N., Dan M.L., Anodic oxidation of sulphite in alkaline solutions on cadmium doped cobalt layered perovskite type 114 electrode. Anal. Univ. Oradea (Fasc. Protect. Mediu), 2015, XXV, 185-192.http://protmed.uoradea.ro/facultate/publicatii/protectia_mediului/2015B/im/06.%20Enache%20Andreea.pdf, accessed 10.06.2017 Google Scholar
Enache A., Vaszilcsin N., Dan M.L., Anodic oxidation of sulphite in alkaline solutions on skeletal nickel electrode. I. Voltammetric Studies. Chem. Bull. “Politehnica” Univ. (Timisoara), 2016, 61, 12-16. http://www.chemicalbulletin.ro/admin/articole/86968Articol_AndreeaEnache-UPT.pdf,accessed 22.06.2017
Arce R., Aguirre M.J., Romero J., Sensor for quantitative analytical determination of sulphite in wine using a system of modified electrode and a membrane absorption system. ECS Trans., 2014, 64, 37-42. CrossrefGoogle Scholar
Adenkule A.S., Arotiba O.A., Mamba B.B., Electrochemical studies and sensing of iodate, periodate and sulphite ions at carbon nanotubes/ Prussian Blue films modified platinium electrode. Int. J. Electrochem. Sci., 2012, 7, 8503-8521. Google Scholar
Martins P.R., Popolim W.D., Nagato L.A.F., Takemoto E., Araki K., Toma H.E., Angnes L., Penteado M.D.V.C., Fast and reliable analyses of sulphite in fruit juices using a supramolecular amperometric detector encompassing in flow gas diffusion unit. Food Chem., 2011, 127, 249–255. CrossrefWeb of ScienceGoogle Scholar
Sroysee W., Polnlakhet K., Chairam S., Jaujamrus P., Amatatongchai M., A sensitive and selective on-line amperometric sulfite biosensor using sulfite oxidase immobilized on a magnetite-gold-folate nanocomposite modified carbon-paste electrode. Talanta, 2016, 156-157, 154-162. Web of SciencePubMedGoogle Scholar
de Paula N.T.G., Barbosa E.M.O., da Silva P.A.B., de Souza G.C.S., Nascimento V.B., Lavorante A.F., In-line electrochemical reagent generation coupled to a flow injection biamperometric system for the determination of sulfite in beverage samples. Food Chem., 2016, 203, 183-189. CrossrefWeb of SciencePubMedGoogle Scholar
Jankovskiene G., Padarauskas A., Speciation of free and total sulphite in wines by capillary electrophoresis. Chemija, 2003, 14, 159-163. Google Scholar
European Medicines Agency, Note for guidance on validation of analytical procedures: text and methodology, Part I: Validation of analytical procedures: definitions and methodology, CPMP/ICH/381/95, 1995, http://www.ema.europa.eu/docs/en_GB/document_library/Scientific_guideline/2009/09/WC500002662.pdf accessed 24.04.2018.
Prior R.L., Wu X., Schaich K., Standardized methods for the determination of antioxidant capacity and phenolics in foods and dietary supplements. J. Agric. Food Chem., 2005, 53, 4290-4302. PubMedCrossrefGoogle Scholar
DuPont M.S., Bennett R.N., Mellon F.A., Williamson G., Polyphenols from alcoholic apple cider are absorbed, metabolized and excreted by humans. J. Nutr., 2002, 132, 172–175. CrossrefPubMedGoogle Scholar
About the article
Published Online: 2018-12-21
Conflict of interest: Authors state no conflict of interest. No fundings supported this reseach.
Informed consent: No informed consent was necessary in this study
Citation Information: Open Chemistry, Volume 16, Issue 1, Pages 1248–1256, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2018-0139.
© 2018 Aurelia Magdalena Pisoschi, Aneta Pop, published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0