This paper reports a method for the determination of zinc in Brazilian sugar cane spirit, (
Brazilian sugar cane spirit (
Minas Gerais State is by far the largest producer of high-quality artisanal
However, less than 1% of the volume produced is exported. Efforts have been made to increase the export volume and qualify
Metal elements in distilled beverages come from the raw materials, crop treatment, or manufacturing processes [
Analytical methods for metal determination frequently require sample preconcentration and/or pretreatment for the destruction of the organic matrix, such as wet digestion, dry ashing, and microwave oven dissolution [
However, preconcentration methods based on solid phase extraction are attractive when coupled online with the detection instrument and with the use of a high sorption capacity sorbent, such as
According to Brazilian legislation,
Thus, due to the possible sources of contamination by metal ions, such as zinc, during the production of alcoholic beverages, the objective of this study was to develop a methodology for an online preconcentration system, using activated carbon obtained from
A Varian Model SpectrAA 220 (Victoria, Australia) flame atomic absorption spectrometer equipped with a zinc hollow cathode lamp and a deuterium lamp for background correction were used for the detection of zinc. The instrument was operated under the conditions recommended by the manufacturer: lamp current of 5 mA, wavelength of 213.9 nm, slit width of 1.0 nm, burner height of 17 mm, acetylene flow rate of 2.0 L min−1, and air flow rate of 13.5 L min−1.
The flow preconcentration system was constructed using a Gilson Minipuls 3 peristaltic pump (Villiers Le Bel, France) equipped with eight channels and Tygon and polyethylene tubes were used to pump the solutions through the minicolumn (
A fourier transform infrared (FT-IR) spectrometer (Shimadzu, IRPrestige-21, Tokyo, Japan) was used to characterize the functional groups of activated carbon obtained from the husks of
All working solutions were prepared with deionized water obtained from a Gehaka (São Paulo, Brazil) water purification system.
All reagents were analytical grade. Before use, laboratory glassware was kept overnight in 10% (v/v) nitric acid aqueous solution, followed by ultra-sonication for 1 h and finally rinsed with deionized water. Working solutions of zinc were prepared daily by appropriate dilution of a 1000 mg L−1 standard zinc solution (Carlo Erba, Val de Reuil, France). The nitric acid solution used as the eluent was prepared through dilution in water of concentrated nitric acid obtained from Merck (Darmstadt, Germany).
Brazilian sugar cane spirit samples were purchased at local markets in Ituiutaba and Uberlândia (Minas Gerais State, Brazil) and analyzed without prior treatment.
The
The husks were separated from the seeds, crushed in a blender (Black & Decker, São Paulo, Brazil), and finally passed through 500 to 850
The minicolumns were comprised of polyethylene tubes with an inner diameter of 3 mm and were sealed at both ends with glass wool. The minicolumn (
The flow system consists of a peristaltic pump equipped with Tygon tubes, four three-way solenoid valves and a minicolumn filled with biosorbent. The diagram of the flow preconcentration system is displayed in Figure
Diagram of the online preconcentration system used in this study. (a) Adsorption process and (b) desorption process. V, valve; L, open; D, closed; MC, minicolumn containing adsorbent; R, sample or eluent back stream; hatched circle, valve on; white circle, valve off.
The optimization of the parameters affecting the sorption of Zn by the activated carbon obtained from the husks of the
In order to investigate the selective separation and determination of zinc ions from real samples containing different metal ions, a 10
The FT-IR technique was used to study the main functional groups of the activated carbon and
FT-IR spectra for
The thermal degradation of husks occurs through their dehydration and the formation of CO and CO2 molecules, released in vapor form. The carbonization is characterized by the disappearance of chemical functional groups originally present in the precursor molecules and the formation of compounds with low molecular weight.
The most significant bands in the analysis of the activated carbon obtained were for lignocellulosic materials, present in the spectra at around 3400 cm−1, 2920 cm−1, 1730 cm−1, 1650 cm−1, and in the range of 1300–900 cm−1 [
For the husks, the precursor of the activated carbon, a broad band centered at 3400 cm−1 was assigned to O–H stretching, associated with the water absorbed on the surface of the material and silanol groups (SiOH). In the region of 2920 cm−1, there was a strong signal due to O–H stretching of the methyl groups, these groups being present in the structure of lignin [
Analysis of the differences between the spectrum of the pyrolyzed activated carbon at 200°C and that of the
Preliminary tests were performed to investigate which factors exert significant influence on the adsorption of Zn (II) by the activated carbon. The factors selected were eluent concentration, sample pH, adsorbent mass, and sample flow rate. Eluent type and eluent flow rate were fixed as a nitric acid aqueous solution at 1.0 mol L−1 and 4.0 mL min−1, respectively. These were selected based on preliminary studies [
Table
Conditions for preconcentration of zinc and analytical response for the study of multivariate optimization using SPE with detection by FAAS.
Run | Sorbent mass (mg) | pH | Sample flow rate (mL min−1) | Eluent concentration (mol L−1) | Integrated |
---|---|---|---|---|---|
1 | 30 | 4.0 | 3.0 | 0.5 | 0.4473 |
2 | 60 | 4.0 | 3.0 | 0.5 | 0.4806 |
3 | 30 | 9.0 | 3.0 | 0.5 | 0.3160 |
4 | 60 | 9.0 | 3.0 | 0.5 | 0.3926 |
5 | 30 | 4.0 | 6.0 | 0.5 | 0.4136 |
6 | 60 | 4.0 | 6.0 | 0.5 | 0.4430 |
7 | 30 | 9.0 | 6.0 | 0.5 | 0.4473 |
8 | 60 | 9.0 | 6.0 | 0.5 | 0.4358 |
9 | 30 | 4.0 | 3.0 | 1.5 | 0.3780 |
10 | 60 | 4.0 | 3.0 | 1.5 | 0.1696 |
11 | 30 | 9.0 | 3.0 | 1.5 | 0.2877 |
12 | 60 | 9.0 | 3.0 | 1.5 | 0.2055 |
13 | 30 | 4.0 | 6.0 | 1.5 | 0.4236 |
14 | 60 | 4.0 | 6.0 | 1.5 | 0.4277 |
15 | 30 | 9.0 | 6.0 | 1.5 | 0.4894 |
16 | 60 | 9.0 | 6.0 | 1.5 | 0.4864 |
From the results reported in Table
Pareto chart obtained from the optimization study of the variables, with their significance, for the preconcentration of Zn(II) using activated carbon obtained from
The Pareto chart shows that within the range studied, the mass of the adsorbent and the pH showed no significant influence, and these parameters were thus kept at 30 mg and 4.0, respectively. The literature shows that to ensure interaction between the metal ion and the surface of the adsorbent, the pH must be such that Mn+ is the most abundant metal ion species [
The most significant variables indicated by the factorial design (sample flow rate and eluent concentration) were then optimized using a response surface. The results were used to construct the surface response shown in Figure
Response surface for optimization of eluent concentration and sample flow rate: sample volume, 10.0 mL; sample concentration, 10
The critical points obtained were 1.0 mol L−1 and 6.0 mL min−1 for eluent concentration and sample flow rate, respectively. The application of the Lagrange criterion indicated that the critical point is the maximum point of the response surface.
Thus, as a result of all the optimizations, the following working conditions were selected: sample pH 4.0, 1.0 mol L−1 HNO3 as the eluent, and sample flow rate of 6.0 mL min−1.
The effect of Ca2+, Mg2+, Na+, K+, Cd2+, Fe3+, and Cu2+ on the determination of zinc was studied using a mixed solution method, where the solution contained a fixed concentration of zinc and various concentrations of interfering ions. Solutions were prepared containing 10
Resulting matrix of the factorial design and analytical response*.
Run | Ca2+ | Mg2+ | Na+ | K+ | Cd2+ | Fe2+ | Cu2+ | Integrated absorbance |
---|---|---|---|---|---|---|---|---|
1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.2802 |
2 | 500 | 0 | 0 | 0 | 500 | 0 | 500 | 0.2471 |
3 | 0 | 500 | 0 | 0 | 500 | 500 | 0 | 0.0689 |
4 | 500 | 500 | 0 | 0 | 500 | 500 | 500 | 0.1440 |
5 | 0 | 0 | 500 | 0 | 0 | 500 | 500 | 0.1385 |
6 | 500 | 0 | 500 | 0 | 0 | 500 | 0 | 0.0709 |
7 | 0 | 500 | 500 | 0 | 500 | 0 | 500 | 0.0980 |
8 | 500 | 500 | 500 | 0 | 0 | 0 | 0 | 0.0821 |
9 | 0 | 0 | 0 | 500 | 500 | 500 | 500 | 0.0779 |
10 | 500 | 0 | 0 | 500 | 500 | 500 | 0 | 0.0120 |
11 | 0 | 500 | 0 | 500 | 0 | 0 | 500 | 0.0509 |
12 | 500 | 500 | 0 | 500 | 0 | 0 | 0 | 0.0137 |
13 | 0 | 0 | 500 | 500 | 0 | 0 | 0 | 0.0000 |
14 | 500 | 0 | 500 | 500 | 0 | 0 | 500 | 0.1234 |
15 | 0 | 500 | 500 | 500 | 500 | 500 | 0 | 0.0000 |
16 | 500 | 500 | 500 | 500 | 500 | 500 | 500 | 0.0286 |
17 | 250 | 250 | 250 | 250 | 250 | 250 | 250 | 0.0000 |
*Concentration of each interfering ion is given in
Pareto chart of effects of interfering ions on zinc adsorption in the proposed preconcentration system.
This study suggests that all concomitant ions were significant (
With the optimized system, the method was evaluated through the main analytical features. A good correlation coefficient was obtained (0.9996) between the analytical signal predicted by the linear function and the analytical signal observed experimentally in the linear range of 2–50
The proposed method was applied to five Brazilian
Experimental recovery for determination of zinc in Brazilian
Sample | Zn ( | Recovery (%) | |
Added | Found* | ||
1 | 10.0 | 92.5 | |
40.0 | 99.9 | ||
2 | 10.0 | 108.4 | |
40.0 | 97.9 | ||
3 | 10.0 | 103.1 | |
40.0 | 99.8 | ||
4 | 10.0 | 90.4 | |
40.0 | 93.6 |
*Results are expressed as mean values ± S.D. based on three replicate (
In the present study, the method proposed for determining zinc in beverage samples was described and compared according to the detection technique used. The most important details of the published procedures for zinc determination, in terms of kind of sample, are presented in Table
Comparison of methods for determination of zinc.
Sample | Sorbent | Chelating agent/modifier | Eluent | PF | SV (mL) | LOD ( | Linear range ( | Detection | Ref. |
---|---|---|---|---|---|---|---|---|---|
Brazilian | — | — | — | — | — | 6.0 | 0–4000 | FAAS | [ |
Sugar | — | — | — | — | — | 10 | 25–200 | ASV | [ |
Water, hair, urine and saliva | Alizarin Red S | Alumina | HNO3 | 144 | 25 | 0.2 | 1–100 | FAAS | [ |
Seawater | Silica gel | DPTH | Citric acid/tartaric acid | — | 8.8 | 1.7 | 2–500 | ICP-OES | [ |
Water | Amberlite XAD-2 | oVTSC | HCl | 140 | 1000 | 10 | 20–50 | FAAS | [ |
No data | XAD-7 | 8-BSQ | HCl | 10 | — | 1.6 | 5–200 | Spectrophoto-metry | [ |
Sea water | Dowex 1X8-50 | ARS | HNO3 | 5 | 50–200 | 23 | No data | FAAS | [ |
Saline matrices | Amberlite XAD-7 | ARS | HNO3 | 50 | 50 | 29 | 250–2000 | FAAS | [ |
Water | SiO2/Al2O3/ | PAN | HNO3 | 52.6 | 20 | 2.3 | 30–180 | Spectrophoto-metry | [ |
Brazilian | — | HNO3 | 10.9 | 10 | 1.9 | 2–50 | FAAS | This work |
PF = preconcentration factor; SV = sample volume; LOD = limit of detection; PAN = (1-(2-piridylazo)-2-naphtol); DPTH = 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide; ARS = Alizarin red S; OVTSC = vanillin thiosemicarbazone; 8-BSQ = 8-(benzenesulfonamido)quinoline; FAAS = flame atomic absorption spectrometry; ASV: anodic stripping voltammetry, ICP-OES = inductively coupled plasma optical emission spectrometry.
The proposed method represents an alternative low-cost procedure for the determination of zinc in Brazilian
The authors are grateful for financial support from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).