Article pubs.acs.org/JAFC
Simplification of Iron Speciation in Wine Samples: A Spectrophotometric Approach José A. López-López,*,† Gemma Albendín,‡ María I. Arufe,‡ and Manuel P. Mánuel-Vez† †
Department of Analytical Chemistry and ‡Laboratory of Toxicology, Faculty of Marine and Environmental Sciences, Universidad de Cadiz, Puerto Real, Cádiz 11510, Spain ABSTRACT: A simple direct spectrophotometric method was developed for the analysis of Fe(II) and total Fe in wine samples. This method is based on the formation of an Fe(II) complex with 2,2′-dipyridylketone picolinoylhydrazone (DPKPH), which shows a maximum green-blue absorption (λ = 700 nm) at pH 4.9. Operative conditions for the batch procedure were investigated including reagent concentration, buffer solutions, and wavelength. The tolerance limits of foreign ions and sample matrix have been also evaluated. Limits of detection and quantification were 0.005 and 0.017 mg L−1 of Fe(II), respectively, allowing its determination in real wine samples. Finally, the proposed method was used in the analysis of white, rose, and red wines. Results were compared with a reference method of Commission Regulation (ECC) No. 2676/90 of September 1990 determining European Community methods for the analysis of wines for Fe analysis, showing the reliability of the proposed method in Fe analysis in wine samples. KEYWORDS: iron, speciation, spectrophotometry, wine, DPKPH
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Fe(III).10 Despite the scientific and economic interests in direct speciation of Fe in wine samples, there has been a lack of research about this issue recently.11 With regard to methods for wine sample treatment in Fe speciation analysis, solid phase extraction,12−14 and liquid−liquid extraction have been proposed for separation of iron species prior to spectroscopic determination.15,16 In the case of the direct determination of Fe(II), voltammetry has been used, but it presents some drawbacks for routine analysis due to high technical requirements, plus lengthy analysis. Because of the need for iron speciation in wine quality control laboratories, the development of simplified methods that provide a faster response at lower cost is desirable. With this in mind, molecular absorption spectrophotometry appears as an alternative as it has been shown in the case of Fe(II) analysis by complexation with 2-(5bromo-2-pyridiazo)-5-(diethylamino)phenol (Br-ADAP) in different wine varieties after 10 min reaction time17 and in the case of Fe(III) analysis by complexation with thiocyanate.11 The aim of this work is the use of molecular absorption spectrophotometry to simplify the direct determination of Fe(II) and total Fe in wine samples. The proposed system provides instantaneous complexation of the metal with 2,2′dipyridyl ketone picolinoylhydrazone (DPKPH), reducing operation times and cost of analysis. This method constitutes a fast and simple alternative for speciation of iron in wines free of sample treatment with high sensitivity and tolerance to matrix effects.
INTRODUCTION Wine quality depends on the combination of many factors related to its chemical composition.1 In particular, the presence of inorganic ions in wine is of great interest, because in some cases, they are oligoelements that are also related to aesthetic properties of the final product; however, at higher concentrations many of them become toxic.2 Among others, iron is found in substantial quantities in all grapes and wine varieties and is one of its major metallic constituents. In general, the presence of iron in the must is associated with iron concentration in soil and bloom covering grapes. Nevertheless, other possible sources of iron contamination in wine include grape harvesting and winery materials and transporting, fermenting, and storing in tanks, among others. A consequence of these facts is that iron concentration within commercial wine types varies considerably, with average concentrations in the range from 0.5 to 5 mg L−1.3,4 Determining Fe concentration in wines is of major importance either due to the changes in stability it may cause and due to its effects on oxidation and wine aging.5,6 On the one hand, when protected from air, or due to addition of reducing agent (sulfite or ascorbic acid), iron exists mostly as Fe(II), which is highly soluble. On the other hand, when wine is aerated, Fe(III) predominates due to the oxidizing capability of dissolved oxygen. High concentrations of Fe(III) are responsible for the formation of unpleasant and insoluble suspensions with tannin and phosphates, which are known as hazes.7,8 Therefore, reducing agents as sulfite are widely used to prevent Fe(III) precipitation; additionally, some chelators have been evaluated with this objective.9 Several methods for iron speciation in wine samples can be found, most of them based on the measurement of Fe(II) concentration, followed by the determination of total iron.8 In general, wine samples must be pretreated to avoid matrix effects on instrumental determination and to separate Fe(II) from © XXXX American Chemical Society
Received: December 17, 2014 Revised: April 10, 2015 Accepted: April 23, 2015
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DOI: 10.1021/acs.jafc.5b01571 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Figure 1. (a) Mass spectrum of DPKPH (electronic impact ionization); (b) IR spectrum of DPKPH.
Figure 2. UV−vis spectra of Fe−DPKPH complex in water solution (spike of 1 mg L−1), white wine (spike of 0.5 mg L−1), and red wine (spike of 0.5 mg L−1) samples. [DPKPH] = 1.25 × 10−4 mol L−1.
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Solutions of ascorbic acid dissolved in deionized water were prepared fresh daily. Finally, acetic acid/acetate buffer was used to control the pH in the sample during analysis. Buffer solution was prepared by dissolving equimolar proportions of acetic acid and sodium acetate in distilled water. Wine Samples. Wine samples used in this study were purchased from the manufacturers. White wines with designation of origin (D.O.) Jerez-Xéréz-Sherry produced from Palomino fino grapes, Tio Pepe (Tió Pepe, Spain, 2008), and Romate (Sánchez Romate, Spain, 2009), as well as other white wines such as Casón Histórico, Don Garcı ́a, Don Simón (Garciá Carrión, Spain), and Elegido (Viña Tridado, Spain) were analyzed. Rose wines Mateus Rose (Sogrape, Portugal, 2009)
EXPERIMENTAL PROCEDURES
Reagents and Solutions. Unless otherwise noted, all reagents were of analytical reagent grade. Picolinic acid ethyl ester (99.0%), 2,2′-dipyridylketone (97.0%), hydrazine (98.0%), NaOH (99.0%), ethanol absolute (96.0% v/v), acetic acid (97.0%), sodium acetate (98.5%), Br-ADAP, and ascorbic acid (99.0%) were purchased from Merck (Darmstadt, Germany). Solutions were prepared using highpurity deionized water with resistivity 4% was observed. As can be seen in Table 1, most anionic species are tolerated at relatively D
DOI: 10.1021/acs.jafc.5b01571 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry Table 2. Comparison between Results Obtained by Direct Calibration and the Standard Additions Method slope of calibration curve (L mg−1) wine type white redc
b
Fe (mg L−1) a
direct calibration
standard additions method
ta
2.47 ± 0.19 2.87 ± 0.18
2.63 ± 0.24 2.93 ± 0.31
−1.36 1.50
ethanol (% v/v)
direct calibration
standard additions method
t
15.0 12.5
0.1182 ± 0.014 0.1167 ± 0.008
0.1195 ± 0.012 0.1175 ± 0.011
−0.17 −0.14
t = 2.78, 95% confidence level. Uncertainty measured as RSD (%) for three replicates. bTió Pepe (D.O. Jerez-Xéréz-Sherry, Spain, 2008). cRió Mayor (D.O. Cariñena, Spain, 2008). a
Table 3. Determination of Fe(II) and Total Iron (Fe) in Commercial Wine Samples wine type white
rose
red
designation Tió Pepe (D.O. Jerez-Xéréz-Sherry, Spain, 2008) Sánchez Romate (D.O. Jerez-Xéréz-Sherry, Spain, 2009) Casón Histórico Don Garcı ́a Don Simón Elegido Mateus Rose (D.O. Portugal, Portugal, 2009) Don Simón Carrefour Rincón (D.O. Rioja, Spain, 2008) Beronia Crianza (D.O. Rioja, Spain, 2008) Rı ́o Mayor (D.O. Cariñena, Spain, 2008) Carrefour Elegido
Fe(II)DPKPH (mg L−1)
Fe(II)Br‑ADAP (mg L−1)
ta
2.43 ± 0.18
2.23 ± 0.17
2.25
2.18 1.24 1.24 2.15
± ± ± ±
0.13 0.11 0.11 0.19
2.59 1.99 1.31 2.32
2.55 ± 0.12 2.49 ± 0.23
5.31 ± 0.12 4.07 ± 0.31 2.93 ± 0.21
± ± ± ±
FeDPKPH (mg L−1)
FeFAASb (mg L−1)
ta
1.43 ± 0.15 2.29 ± 0.09
1.46 ± 0.02 2.31 ± 0.08
−0.48 0.36
± ± ± ±
0.08 0.11 0.09 0.18
−2.15 −0.89 −2.25 2.62
5.31 ± 0.08 3.96 ± 0.19 2.67 ± 0.13
0.33 2.75 2.76
± ± ± ±
0.18 0.15 0.07 0.15
2.71 2.74 −1.62 −2.00
3.44 1.99 1.26 2.52
2.73 ± 0.20 2.21 ± 0.18
−2.24 2.73
5.35 ± 0.32 3.68 ± 0.21 2.51 ± 0.11
−1.97 1.69 1.51
2.00 1.58 9.55 5.42 3.99
5.51 ± 0.27 3.86 ± 0.17 2.81 ± 0.09
± ± ± ± ±
0.29 0.19 0.28 0.11
0.14 0.12 0.45 0.25 0.37
3.21 1.92 1.03 2.72
1.97 1.62 9.12 5.89 3.56
± ± ± ± ±
0.11 0.12 0.32 0.28 0.24
1.19 −0.94 −2.22 −2.75 −2.22
a
t(4, 0.05) = 2.78, 95% confidence level. Uncertainty measured as RSD (%) for three replicates. bMethod (CEE) no. 2676/90 of European Commission.
reference method using a t test for three independent replicates of each sample (Table 3). Results confirmed the suitability of DPKPH for selective analysis of Fe(II) in wine samples. In all cases the concentration of Fe(III) was smaller than the Fe(II) concentration. This can be explained by the reductive conditions of wine samples that prevents oxidative damage. Similar distribution of Fe ions has been observed for other wines such as sweet wines from Brazil, which could be explained by reductive characteristics of the wines.17 Additionally, higher levels of Fe(III) were observed in the case of red wines if compared with rose and white wines. For these reasons, spectrophotometric determination of Fe(II) and Fe using DPKPH as a colorimetric reagent appears as a simple and fast alternative to existing analytical methods for iron speciation in wine samples.
measurement of 10 reagent blank samples containing ultrapure water and the optimized concentrations of DPKPH, ascorbic acid, and acetate buffer. In these conditions, the method offered a limit of detection of 0.005 mg L−1, a limit of quantification of 0.017 mg L−1, and a linear response of up to 6 mg L−1; for this reason samples containing higher concentrations were diluted before analysis. These analytical features allowed the use of this method in the analysis of Fe at the concentration level in which it normally appears in wine samples. Application to Commercial Wine Samples. Finally, the capability of this method to selectively determine Fe(II) in wines was evaluated using different wine samples. In particular, five red wines, six white wines, and three rose wine were analyzed. In this section, results for Fe(II) and total Fe are discussed. In the case of total iron concentration results were compared with those obtained by the reference method Commission Regulation (ECC) No. 2676/90 of September 1990 determining Community methods for the analysis of wines for Fe analysis.20 Comparisons were carried out using a t test for three independent replicates of each sample. Results for total concentration of Fe by both methods were in good agreement in the two types of wine used (Table 3). This confirms the applicability of the spectrophotometric analysis of Fe in wine samples using DPKPH as a ligand, free of sample matrix effects for white, rose, and red wines. Although some Fe(III) could be measured as Fe(II), the broadly accepted method of spectrophotometric determination by complexation of Fe(II) with Br-ADAP has been used as a reference method for selective analysis of Fe(II).17 Results obtained by DPKPH complexation were compared with the
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AUTHOR INFORMATION
Corresponding Author
*(J.A.L.-L.) Phone: +34956016165. E-mail: joseantonio.
[email protected]. Notes
The authors declare no competing financial interest. This study was supported by Plan Nacional de I+D (CICYT) Project CTM2004-05718 and in part by Junta de Andalucı ́a (PAI group RNM345).
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DOI: 10.1021/acs.jafc.5b01571 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jafc.5b01571 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
