Nail Altunay* and
Ramazan Gürkan
Cumhuriyet University, Faculty of Sciences, Department of Chemistry, TR-58140, Sivas, Turkey. E-mail: naltunay@cumhuriyet.edu.tr
First published on 11th February 2016
The additives used in foods and beverages may be harmful to human health. Thus, there is an increasing demand for analytical methods that allow the reliable identification and quantification of high-risk substances. In this context, we describe a new ultrasonic assisted-cloud point extraction (UA-CPE) method for the preconcentration of sulfite from foods and beverages prior to analysis by flame atomic absorption spectrometry (FAAS). The method is based on the reduction of Fe(III) to Fe(II) by the sulfite, and the subsequent selective complex formation of Fe(II) ion produced, which is linearly related to sulfite concentration, with 5,6-diphenyl-3-(2-pyridyl)-1,2,4 triazine (DPTZ) in the presence of sodium dodecyl sulfate (SDS) at pH 6.0. The method allows the determination of trace levels of sulfite in the range of 0.04–70 μg L−1 with a detection limit of 0.012 μg L−1. The method was successfully applied to food and beverage samples with good results. The method accuracy was controlled by comparing with those of the standard 5,5′-dithio-bis(2-nitrobenzoic acid) (DTNB) method.
Numerous analytical techniques, which are recently published, have demonstrated the importance of the need for developing fast, accurate and selective techniques for analysis of sulfite species in food and beverages. Different techniques in literature have widely been used for the determination of sulfite species. These techniques include dispersive liquid–liquid microextraction (DLLME) coupled to UV-vis Fiber Optic Linear Array Spectrophotometry (DLLME-UV-vis),8 liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS),9 vapor generation combined with potentiometric detection (VG-PD),10 ion chromatography (IC),11 vapor generation-inductively coupled plasma-optical emission spectrometry (VG-ICP-OES),12 amperometric detection using glassy carbon electrode modified with carbon nanotubes–PDDA–gold,13 headspace single-drop microextraction in combination with UV-vis microspectrophotometry (HS-SDME-UV-vis),14 inductively coupled plasma-optical emission spectrometry (ICP-OES),15 and diffuse reflectance Fourier transform infrared spectroscopic (DR-FTIR) analysis.16 Among all these techniques, flame atomic absorption spectrometry (FAAS) has potential due to its simplicity, low cost, wide availability and low susceptibility to matrix interferences for direct and indirect determination of chemical species. Although the other competitive techniques such as ICP-OES, VG-ICP-OES, and LC-ICP-MS offer lower detection limits, FAAS has survived due to its simplicity and wide availability. The indirect determination of sulfite species in foods and beverages by means of FAAS may be difficult due to the matrix effect. In order to overcome this problem, ultrasonic-assisted cloud point extraction (UA-CPE) is preferably adopted as separation and preconcentration tool. The use of the UA-CPE as an alternative to conventional solvent extraction techniques such as liquid–liquid extraction (LLE) and solid phase extraction (SPE) has the following advantages such as relatively low toxicity, high preconcentration factor, lower cost, higher safety and simplicity.17,18 Also, the UA-CPE was efficiently coupled to FAAS, and successfully used in order to enhance its low detection limit as well as the selectivity of the technique.
The purpose of the present study was to develop an accurate and reliable method for the indirect determination of sulfite in foods and beverages using UA-CPE procedure coupled to FAAS. The UA-CPE was adopted as a preconcentration tool prior to detection of Fe(II), which is linearly related to sulfite concentration, by FAAS. The method is selectively based on ternary complex formation of cationic Fe(PDTZ)22+ complex produced after the reduction of Fe(III) to Fe(II) with sulfite at pH 6.0, with PDTZ (as neutral tridentate chelating agent) in presence of sodium dodecyl sulfate (SDS) as counter ion, and then its extraction from aqueous solution into micelles of nonionic surfactant polyoxyethylene(7.5)nonylphenyl ether (PONPE 7.5) as an extracting agent. The method was applied successfully to its determination after the separation/releasing and preconcentration of sulfite (as free sulfite and total sulfite) from foods and beverage matrices pretreated with acidic (pH 2.0, 0.02 mol L−1 methanesulphonic acid/0.01 mol L−1 D-mannitol) and alkaline (pH 9.0, 0.02 mol L−1 Na2HPO4/0.01 mol L−1 D-mannitol) extraction solutions with UA-CPE.
AAS-6300 atomic absorption spectrometer (Shimadzu, Kyoto, Japan) equipped with D2-background correction, an iron hollow cathode lamp, an air–acetylene flame atomizer was used for the indirect determination of sulfite species in surfactant-rich phases. The wavelength, lamp current, slit width and burner height used, was 248.3 nm, 12 mA, 0.2 nm and 7.0 mm, respectively. The measurements were carried out using an air/acetylene flame at flow rates of 18 and 2.2 L min−1. An ultrasonic cleaner (UCS-10 model, Seoul, Korea) was used to maintain the temperature, and efficiently and fastly to induce ternary complex formation in UA-CPE step. A vortex mixer (VM-96B model, Jeio Tech, Co., Ltd., Seoul, Korea) was used for thorough mixing of solutions. A centrifuge (Hettich Universal) was used to accelerate and facilitate the phase separation process. The pH measurements were carried out using a pH-2005 digital pH meter equipped with a glass–calomel electrode (pH-2005, JP Selecta, Barcelona, Spain). Eppendorf vary-pipettes (10–100 and 200–1000 μL) were used to deliver accurate volumes. A refrigerator was used to keep the selected food and beverages fresh and cool till the analysis.
Also, in terms of applicability to real time samples, the UA-CPE/FAAS method was applied to accurate and reliable determination of sulfite (as free, total and reversibly bound) existing in the foods (onion slices, vinegar, seasoning powder, dried apple, dried grapes, nuts, preserved almond, and starch syrup) and beverages (sparkling white wine, white wine, beer, apple juice and mango juice). The reversibly bound sulfite level was calculated from difference between free sulfite and total sulfite levels after pre-treatment based on two different extraction approaches. The recovery rates of known amounts of sulfite spiked to the samples were analyzed by using the proposed method. The results are summarized in Table 1 in detail. It can be seen that the good recoveries are achieved in the range of 95.8–102.4% for foods and 95.6–102.8% for beverages with RSDs of 1.3–4.1% and 1.2–3.6% respectively.
Samples | Added free sulfite (μg L−1) | By the first preparation process | By the second preparation process | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Found (μg kg−1) | RSDs% | Recovery% | Found (μg kg−1) | RSDs% | Recovery% | ||||||
Free sulfite | Reversibly bound sulfite | Total sulfite | Free sulfite | Reversibly bound sulfite | Total sulfite | ||||||
Food samples | |||||||||||
Onion slices | — | 9.4 ± 0.07 | 23.8 | 33.2 ± 0.2 | 3.3 | — | 9.3 ± 0.08 | 23.9 | 33.2 ± 0.1 | 3.5 | — |
1 | 10.1 ± 0.07 | 24.0 | 34.1 ± 0.2 | 2.5 | 96.9 | 10.0 ± 0.08 | 24.0 | 34.0 ± 0.1 | 2.4 | 97.5 | |
5 | 14.1 ± 0.1 | 23.5 | 37.6 ± 0.3 | 1.9 | 98.2 | 14.2 ± 0.09 | 23.7 | 37.9 ± 0.2 | 1.8 | 98.9 | |
Vinegar | — | 11.2 ± 0.09 | 14.7 | 25.9 ± 0.1 | 4.1 | — | 11.4 ± 0.08 | 14.2 | 25.6 ± 0.2 | 3.9 | — |
1 | 11.8 ± 0.1 | 14.3 | 26.1 ± 0.2 | 3.5 | 97.7 | 12.0 ± 0.1 | 14.8 | 26.8 ± 0.2 | 3.3 | 96.9 | |
5 | 16.1 ± 0.1 | 14.8 | 30.9 ± 0.3 | 2.3 | 99.1 | 16.2 ± 0.2 | 14.9 | 31.1 ± 0.3 | 2.0 | 98.8 | |
Seasoning powder | — | 1.5 ± 0.05 | 18.9 | 20.4 ± 0.1 | 3.1 | — | 1.4 ± 0.04 | 18.1 | 19.8 ± 0.1 | 3.0 | — |
1 | 2.4 ± 0.05 | 19.5 | 21.9 ± 0.2 | 2.5 | 95.8 | 2.5 ± 0.04 | 18.7 | 21.2 ± 0.2 | 2.7 | 101.8 | |
5 | 6.4 ± 0.06 | 19.6 | 26.0 ± 0.2 | 1.8 | 98.1 | 6.3 ± 0.05 | 19.0 | 25.3 ± 0.2 | 1.9 | 99.0 | |
Dried apple | — | 11.9 ± 0.09 | 32.0 | 43.9 ± 0.3 | 3.7 | — | 12.1 ± 0.09 | 32.2 | 44.3 ± 0.3 | 3.9 | — |
1 | 13.2 ± 0.1 | 31.8 | 45.0 ± 0.3 | 2.5 | 102.1 | 13.4 ± 0.1 | 32.9 | 46.3 ± 0.3 | 2.6 | 102.4 | |
5 | 16.7 ± 0.1 | 31.5 | 48.2 ± 0.4 | 1.8 | 98.9 | 17.1 ± 0.2 | 32.8 | 49.9 ± 0.4 | 1.5 | 101.1 | |
Dried grapes | — | 0.8 ± 0.04 | 5.4 | 6.2 ± 0.09 | 3.5 | — | 0.9 ± 0.03 | 5.2 | 6.1 ± 0.08 | 3.4 | — |
1 | 1.7 ± 0.05 | 5.8 | 7.5 ± 0.1 | 2.6 | 97.3 | 1.8 ± 0.04 | 5.7 | 7.5 ± 0.08 | 2.2 | 96.9 | |
5 | 5.9 ± 0.05 | 5.3 | 11.2 ± 0.2 | 1.7 | 100.9 | 5.9 ± 0.04 | 5.5 | 11.4 ± 0.1 | 1.4 | 99.5 | |
Nuts | — | 10.5 ± 0.09 | 13.1 | 23.6 ± 0.2 | 3.2 | — | 10.9 ± 0.09 | 13.8 | 24.7 ± 0.2 | 3.3 | — |
1 | 11.0 ± 0.1 | 13.0 | 23.0 ± 0.2 | 2.1 | 95.9 | 11.5 ± 0.1 | 14.0 | 25.5 ± 0.2 | 2.6 | 96.5 | |
5 | 15.1 ± 0.1 | 12.6 | 27.7 ± 0.3 | 1.3 | 97.4 | 15.7 ± 0.1 | 13.3 | 29.0 ± 0.3 | 1.7 | 98.9 | |
Preserved almond | — | 7.3 ± 0.07 | 8.3 | 15.6 ± 0.1 | 3.5 | — | 7.4 ± 0.08 | 8.3 | 15.7 ± 0.1 | 3.3 | — |
1 | 8.2 ± 0.08 | 8.5 | 16.7 ± 0.2 | 2.6 | 98.1 | 8.1 ± 0.08 | 8.8 | 16.9 ± 0.1 | 2.4 | 96.9 | |
5 | 13.6 ± 0.1 | 8.6 | 22.2 ± 0.2 | 1.8 | 101.2 | 12.3 ± 0.1 | 8.2 | 20.5 ± 0.2 | 1.6 | 99.5 | |
Starch syrup | — | 3.8 ± 0.03 | 9.7 | 13.5 ± 0.1 | 3.3 | — | 4.0 ± 0.03 | 9.9 | 13.9 ± 0.1 | 3.5 | — |
1 | 5.3 ± 0.04 | 10.1 | 15.4 ± 0.1 | 2.2 | 101.9 | 5.1 ± 0.03 | 9.1 | 14.2 ± 0.1 | 2.3 | 102.2 | |
5 | 8.7 ± 0.04 | 10.5 | 19.2 ± 0.2 | 1.7 | 99.4 | 9.1 ± 0.04 | 10.4 | 19.5 ± 0.2 | 1.5 | 101.0 | |
Beverage samples | |||||||||||
Sparkling white wine | — | 11.5 ± 0.1 | 15.6 | 27.1 ± 0.2 | 3.6 | — | 11.7 ± 0.1 | 15.4 | 27.1![]() ![]() |
3.4 | — |
1 | 12.0 ± 0.1 | 16.1 | 28.1 ± 0.3 | 2.7 | 95.6 | 12.4 ± 0.2 | 16.3 | 28.7 ± 0.2 | 2.5 | 97.5 | |
5 | 16.1 ± 0.2 | 15.8 | 31.9 ± 0.3 | 1.5 | 97.3 | 16.5 ± 0.2 | 16.0 | 32.5 ± 0.3 | 1.2 | 98.9 | |
White wine | — | 19.9 ± 0.2 | 21.7 | 41.6 ± 0.3 | 3.3 | — | 19.5 ± 0.2 | 21.1 | 40.6 ± 0.3 | 3.1 | — |
1 | 20.3 ± 0.2 | 21.8 | 42.1 ± 0.3 | 2.9 | 96.9 | 19.9 ± 0.2 | 22.0 | 41.9 ± 0.3 | 2.6 | 96.9 | |
5 | 25.2 ± 0.3 | 21.0 | 46.2 ± 0.4 | 1.6 | 101.0 | 24.3 ± 0.3 | 21.8 | 46.1 ± 0.3 | 1.3 | 99.1 | |
Beer | — | 3.1 ± 0.05 | 15.2 | 18.3 ± 0.1 | 3.7 | — | 3.3 ± 0.06 | 15.3 | 18.8 ± 0.1 | 3.8 | — |
1 | 4.2 ± 0.05 | 14.8 | 19.0 ± 0.1 | 2.5 | 102.3 | 4.2 ± 0.06 | 15.7 | 19.9 ± 0.2 | 2.6 | 97.5 | |
5 | 8.0 ± 0.08 | 15.1 | 23.1 ± 0.3 | 1.9 | 98.7 | 8.2 ± 0.07 | 14.4 | 22.6 ± 0.2 | 1.7 | 98.8 | |
Apple juice | — | 7.8 ± 0.09 | 13.4 | 21.2 ± 0.1 | 3.4 | — | 7.7 ± 0.08 | 13.6 | 21.3 ± 0.2 | 3.3 | — |
1 | 8.9 ± 0.1 | 13.6 | 22.5 ± 0.2 | 2.8 | 101.9 | 8.9 ± 0.1 | 13.3 | 22.2 ± 0.2 | 2.5 | 102.8 | |
5 | 13.0 ± 0.1 | 13.9 | 26.9 ± 0.2 | 1.9 | 100.8 | 12.8 ± 0.1 | 13.7 | 26.5 ± 0.3 | 1.4 | 100.7 | |
Mango juice | — | 3.1 ± 0.03 | 5.4 | 8.5 ± 0.07 | 3.1 | — | 3.2 ± 0.03 | 5.5 | 8.7 ± 0.08 | 3.3 | — |
1 | 4.0 ± 0.05 | 5.9 | 9.9 ± 0.08 | 2.5 | 96.9 | 4.1 ± 0.04 | 5.2 | 9.7 ± 0.09 | 2.6 | 96.6 | |
5 | 8.0 ± 0.06 | 5.7 | 13.7 ± 0.1 | 1.4 | 98.8 | 8.1 ± 0.05 | 5.4 | 13.5 ± 0.1 | 1.2 | 98.7 |
FeL + H2O → FeL(OH)− + H+, anionic citrate complex formation at pH 6.0 | (1) |
FeL(OH)− + HSO3− → (HO)FeL(SO3H)2−, anionic bisulfite complex formation | (2) |
(HO)FeL(SO3H)2− + 2DPTZ as tridentate ligand → Fe(DPTZ)22+ + HL2− + HSO4− | (3) |
Fe(DPTZ)22+ + 2SDS− → Fe(DPTZ)2(SDS)2(aqueous phase) ↔ Fe(DPTZ)2(SDS)2(surfactant rich phase) | (4) |
DPTZ is a selective Fe(II) binding reagent, and its metal complexes are easily soluble in water.22,23 Because of high water solubility, the cationic Fe(DPTZ)22+ complex can't quantitatively be extracted into surfactant rich phase. To determine minimum detectable levels of sulfite in a wide working range, the UA-CPE has been explored using anionic surfactant, SDS as ion-pairing agent with opposite charge. The UA-CPE can be used when the target analytical species are hydrophobic in nature. Though the Fe(DPTZ)22+ complex is water soluble, it has been successfully extracted into surfactant rich phase in presence of SDS as counter ion, and it can be explained through the following mechanism. When the concentration of surfactant is lower than the critical micellar concentration (CMC), only slightly soluble ion-associates can be formed between cationic Fe(DPTZ)22+ complex and mixed surfactant monomers causing turbidity. Electrostatic interaction between the cationic metal–ligand complex, Fe(DPTZ)22+ and the anionic surfactant, SDS takes place through the positively charged the metal–ligand complex and the negatively charged head group of the anionic surfactant molecule, SDS in presence of PONPE 7.5 as extracting agent. The solubilizing effect of the nonionic surfactant begins at CMC and above, hence the neutral hydrophobic ternary complex and/or ion-pairing complex gets trapped into the micelles. Once the ternary complex gets incorporated into the micellar core of nonionic surfactant, PONPE 7.5, it becomes easy to separate it from the aqueous phase. Addition of salts to ionic micellar solution reduces the mutual electrostatic repulsions of charged head groups. This leads to an increased aggregation number and micellar diameter. High concentrations of salt cause anionic surfactant solutions to separate into immiscible surfactant rich and surfactant-poor phases.24–26
The pH is the first evaluated parameters to obtain the best extraction efficiency, since the pH is one of the main parameters for ion-pairing formation and/or ternary complex formation reaction with enough hydrophobicity. Therefore, the effect of pH on indirect EE% of 10 μg L−1 of sulfite was investigated using different buffer solutions. The effect of pH on the analyte EE% is shown in Fig. 1(a), which shows higher EE% at pH 6.0 of citrate buffer for sulfite. Thus, a citrate buffer of pH 6.0 was chosen in terms of method development, resulting in RSD values ranging from 1.2% to 3.7%.
After determining the optimum pH, the effect of citrate buffer amount added on the analytes EE% was examined in range of 0.1–2.5 mL in Fig. 1(b). The EE% was maximum when 0.8 mL of citrate buffer solution was added to a final volume of 50 mL of analytical solution. Above 0.8 mL, there was a decrease in the EE% of ternary complex, which has a linearly related to sulfite concentration. In this stage, it was observed that the solution became more unclouded with the increase in citrate buffer amount. Thus, a 0.8 mL of pH 6.0 of citrate buffer solution was selected for the best EE%, for the further experiments.
DPTZ is a pyridylazo compound, which acts as a tridentate ligand. It binds the metal ions such as Fe(II), Cu(II) and Ni(II) through the pyridine nitrogen atom and the triazine–nitrogen atom, so as to give the stable cationic complexes. The chelating reagent was employed as chromogenic-extraction reagent during spectrophotometric determination of iron in acids and acidic solutions.27 It was also employed as precolumn derivatizing reagent in the HPLC method with UV absorbance detection for the Fe(II) determination.28 The stoichiometry of metal–chelate is 1:
2. The EE% as a function of the chelating agent concentration was examined and the results were shown in Fig. 1(c). As could be understood from the results, the EE% for sulfite increased up to a concentration of 0.5 × 10−5 mol L−1. Above this concentration, there was a decrease in the EE% of sulfite, this decrease in EE% may be due to the concentration dependent transfer of DPTZ as a hydrophobic ligand into the surfactant rich phase as well as ternary complex at higher concentrations, so that it causes an increase in blank signal. Thus, a concentration of 0.5 × 10−5 mol L−1 was selected for the best EE% in all subsequent experiments. Moreover, the precision as RSD% at this concentration range are between 1.1% and 2.9%.
The variation of the EE% as a function of the concentration of the Fe(III) in the presence of 10 μg L−1 sulfite was studied in range of (1–10) × 10−3 mg L−1. The results in Fig. 1(d) show that the EE% of the analyte linearly increased with Fe(III) concentration up to 4.0 × 10−3 mg L−1. The maximum EE% gradually decreased with increasing slope at the higher volumes. The cause of this decrease in EE% may be (a) primary hydrolysis giving rise to low-molecular-weight complexes (monomer- and dimer-), i.e., Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+; (b) formation and aging of polynuclear polymers, i.e., Fen(OH)m(H2O)x(3n−m)+ or FenOm(OH)x(3n−2m−x)+; (c) precipitation of ferric oxides and hydroxides, i.e., Fe(OH)3, FeOOH and Fe2O3, so as to cause increase in blank signal after electrostatic interaction with SDS in absence of sulfite. Thus, 4.0 × 10−3 mg L−1 Fe(III) was selected for the best EE% all subsequent experiments. Moreover, the RSD values at this optimal concentration ranged from 1.8% to 3.3%.
The variation of EE% as a function of ionic surfactants such as CPC, CTAB and SDS concentration is shown in Fig. 1(e). The dependence of UA-CPE to ionic surfactants concentration was studied in the range of (0.1–1.5) × 10−5 mol L−1 in the presence of sulfite. As a result of studies, it was found that EE% of ternary complex, which is linearly related to sulfite concentration, is more efficient in the presence of anionic surfactant, SDS. The cationic Fe(II)L22+ complex forms an ion-pairing complex with counter ion, SDS, and is extracted into non-ionic surfactant, PONPE 7.5. A concentration of 0.75 × 10−5 mol L−1 of SDS is chosen as optimum value for the best EE% of sulfite in all subsequent experiments. Moreover, the RSD values at this concentration were in range of 1.2–3.1%. Generally, the existence of chemically active groups in the nonionic surfactants such as Triton X-45, 100 and 114, PONPE 7.5 and Tween 20 can be evaluated to be advantageous under certain conditions when electrostatic interactions are suitable. In this study, the PONPE 7.5 was chosen as surfactant due to its low cloud point temperature (CPT) and high density of the surfactant-rich phase, which facilitates phase separation by centrifugation. Moreover, the surfactant is commercial availability, high purity grade, stable, non-volatile, relatively non-toxic and eco-friendly reagents when compared with organic solvents. Also, the concentration of the PONPE 7.5 is a critical factor for the UA-CPE. The PONPE 7.5 with small concentration was not enough for the complete extraction. When large concentration of PONPE 7.5 was used, the surfactant-rich phase obtained after UA-CPE was too sticky and more difficult for subsequent handling.29 In this context, the effect of PONPE 7.5 concentration on the EE% of sulfite was studied in range of 0.05–1.0% (v/v). As can be seen from Fig. 1(f), the maximum EE% was obtained using 0.6% (v/v) PONPE 7.5. At concentrations above this value, the EE% can be decreased depending on the increase of the surfactant volume, deteriorating the FAAS signal. At concentrations below this value, the EE% of ternary complex, which is linearly related to analyte concentration, was low because there are few molecules of the surfactant quantitatively to entrap the Fe(DPTZ)2(SDS)2 complex. Thus, 0.6% (v/v) PONPE 7.5 was selected for the best EE% in all subsequent experiments. Moreover, the RSD values at this concentration were in range of 1.5–3.0%. Optimal equilibration temperature and incubation time are necessary to the completion of the complex formation and efficient phase separation. These parameters are very important in UA-CPE of sulfite. The cloud point can be varied depending on the experimental conditions and surfactant type. The CPT of the PONPE 7.5 is about 30 °C in aqueous solution. Some experimental studies have stated that in order to obtain a more favorable preconcentration factor, the CPE should be carried out at the temperatures higher than the CPT.29 In this study, the effect of the equilibration temperature (from room temperature to 65 °C) under ultrasonic power (350 watt, 40 Hz) on the CPT was also examined. As a result of experimental studies, it was found that the EE% reached to maximum at 35 °C for sulfite. Higher temperatures lead reversibly to the disassociation of ternary complex, and thus the reduction of EE%. So, an equilibration temperature of 35 °C was selected. Then, at the fixed temperature of 35 °C, the effect of the incubation time on EE% was studied in the range of 2–20 min. The maximum EE% was observed at 10 min. When incubation time above 10 min is used, a significant decrease in EE% has been observed, probably due to instability of the ternary complex. Thus, the equilibration temperature of 35 °C and time of 10 min were selected for the best EE% in all subsequent experiments. In addition to these experiments, centrifugation time and rate have been studied because they are very necessary to preconcentrate trace amounts of sulfite with high EE% in a short time. The experimental results show that centrifugation for 5 min at 4000 rpm leads to the maximum EE% and sensitivity for sulfite.
For the analyte introduction into the nebulizer of the FAAS, because the surfactant-rich phase obtained after separation with UA-CPE is very viscous, it was necessary to decrease the viscosity of the surfactant-rich phase. The highly viscous phase could be decreased using diluting agents such as several synthetic mixtures of varying compositions with respect to organic solvents and their acid mixtures. As a result of studies, the best results were obtained by dilution of surfactant rich phase to 0.75 mL with methanol. In these conditions, the extraction efficiency was approximately up to 100%.
Sample matrix | Regression equationa [y = b(m ± Sm)x + (b ± Sb)b] | Linear range (μg kg−1) | LOD (μg kg−1) | LOQ (μg kg−1) | Intra-day | Inter-day | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Free sulfitec (μg kg−1) | Found reversibly bound sulfited (μg kg−1) | Found, Total sulfitec (μg kg−1) | RSD% | Free sulfitec (μg kg−1) | Found, reversibly bound sulfited (μg kg−1) | Found, Total sulfitec (μg kg−1) | RSDs% | |||||
a From matrix-matched calibration curves.b The Sm and Sb are their standard deviations of slope and intercept of matrix-matched calibration curves (n: 5) obtained in dried fruit and beverage mixtures in 0.1–150 μg kg−1 respectively.c ![]() |
||||||||||||
A mixture of three different dried fruit (1.5 g, 3![]() ![]() ![]() ![]() |
y = 0.0097 ± 0.0002C(sulfite,μg kg−1) − 0.025 ± 0.001 | 0.1–150 | 0.75 | 2.5 | 11.8 ± 0.1 | 36.7 | 48.5 ± 0.3 | 2.5 | 11.5 ± 0.2 | 37.5 | 49.0 ± 0.4 | 2.2 |
A mixture of three beverages (1.75 mL, 3![]() ![]() ![]() ![]() |
y = 0.0075 ± 0.0001C(sulfite,μg kg−1) − 0.042 ± 0.003 | 0.1–150 | 1.2 | 3.9 | 6.5 ± 0.1 | 27.4 | 33.9 ± 0.2 | 2.7 | 6.7 ± 0.2 | 27.5 | 34.2 ± 0.2 | 2.0 |
Secondly, the sulfite levels of the samples similarly pretreated at pH 2.0 and 9.5 were measured and evaluated by comparing with those of the standard 5,5′-dithio-bis(2-nitrobenzoic acid) (DTNB). The analysis of the samples by standard DTNB method30 was carried out as follows: a known amount of the samples was placed in a volumetric tube of 10 mL and diluted with water approximately to 8.0 mL. Then, 1 mL of DTNB solution (0.060 g of DTNB per 100 mL of 10% ethanol) was added and diluted to the 10 mL with the water. The absorbance was measured at 412 nm against water as analyte blank after 15 min reaction at 20 °C. In order to reduce the absorbance of analyte blank and suppress the interference effect of potential ions present in selected samples such as Cu2+, Co2+, Ni2+, Mn2+, Cr3+, VO2+ and MoO2+, the pH of sample solution was initially adjusted to 6.5 with 0.2 mol L−1 phosphate buffer containing 250 μL of 0.02 mol L−1 oxalic acid. When a regression analysis (n: 6, independently) was conducted for a serial standard sulfite solution in range of 0.2–4.0 mg L−1 in presence of oxalic acid at pH 6.5, according to standard method, a good improvement in regression data was obtained as follows:
Abs = (0.265 ± 0.012) × Csulfite (mg L−1) + (0.0132 ± 0.0011) with a correlation of coefficient of 0.9985 |
Linear range was 0.004–3.5 mg L−1 with limits of detection and quantification of 0.0012 and 0.004 mg L−1 respectively. When necessary, in order to prevent possible nitrite interference in analysis of selected samples, 150 μL of 0.01 mol L−1 sulfamic acid was added to the matrix environment before the UA-CPE. The results were shown in Table 3 in detail.
Selected reference samples | Added, free sulfite (μg kg−1) | By the proposed method | By the modified standard DTNB methoda | The calculated Student t- and F-testsb | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Found Free Sulfite (μg kg−1) | RSD% | Recovery% | Found, reversibly bound sulfite (μg kg−1) | Found, total sulfite (μg kg−1) | Found free sulfite (μg kg−1) | RSD% | Recovery% | Found, reversibly bound sulfite (μg kg−1) | Found, total sulfite (μg kg−1) | |||
a The modified standard DTNB method, which is based on detection of anionic degradation product at 412 nm using pH 6.5 phosphate buffer containing oxalic acid after stabilization of sulfite with mannitol for monitoring of free sulfite and total sulfite at pH 2.0 and 9.5 in order to slow down and control sulfite oxidation.b In order to compare the mean values and their standard deviations for independent two samples t- and F-tests with equal sample size the statistical t- and F-critical values at 95% confidence level and 8 degrees of freedom are 2.31 and 6.39, respectively. | ||||||||||||
Dried apricots | — | 15.7 ± 0.1 | 2.8 | — | 29.2 | 44.9 ± 0.3 | 15.5 ± 0.2 | 3.0 | — | 29.7 | 45.2 ± 0.3 | 0.75, 2.4 |
1 | 16.1 ± 0.2 | 2.4 | 96.3 | 30 | 46.1 ± 0.3 | 15.8 ± 0.3 | 2.5 | 95.8 | 30.2 | 46.0 ± 0.4 | — | |
5 | 20.2 ± 0.2 | 1.8 | 97.8 | 30.2 | 50.4 ± 0.4 | 19.9 ± 0.3 | 1.9 | 97.0 | 30.7 | 50.6 ± 0.5 | — | |
20 | 35.4 ± 0.3 | 1.3 | 99.1 | 30.7 | 66.1 ± 0.5 | 35.9 ± 0.4 | 1.5 | 101.3 | 30.8 | 66.7 ± 0.5 | — | |
Red wine | — | 9.80 ± 0.1 | 3.1 | — | 39.8 | 49.6 ± 0.4 | 10.1 ± 0.1 | 3.3 | — | 39.2 | 49.3 ± 0.3 | 1.10, 2.8 |
1 | 10.4 ± 0.2 | 2.3 | 96.8 | 40.1 | 50.5 ± 0.4 | 10.7 ± 0.3 | 2.8 | 96.4 | 39.7 | 50.4 ± 0.3 | — | |
5 | 14.6 ± 0.3 | 1.9 | 98.5 | 40.4 | 55.0 ± 0.5 | 15.5 ± 0.4 | 2.1 | 102.7 | 40 | 55.5 ± 0.4 | — | |
20 | 29.9 ± 0.3 | 1.4 | 100.6 | 38.2 | 68.1 ± 0.5 | 29.7 ± 0.4 | 1.6 | 98.8 | 38.9 | 68.6 ± 0.5 | — |
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