J. Agric. Food Chem. 2008, 56, 4943–4954
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Searching the Most Appropriate Sample Pretreatment for the Elemental Analysis of Wines by Inductively Coupled Plasma-Based Techniques ´ A. GONZALVEZ , S. ARMENTA, A. PASTOR,
AND
M.
DE LA
GUARDIA*
Department of Analytical Chemistry, Research Building, University of Valencia, 50th Dr. Moliner St., E-46100 Burjassot, Valencia, Spain
Different sample preparation methods were evaluated for the simultaneous multielement analysis of wine samples by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). Microwave-assisted digestion in closed vessel, thermal digestion in open reactor, and direct sample dilution were considered for the determination of Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Y, Mo, Cd, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Tl, Pb, and Bi in 12 samples of red wine from Valencia and Utiel-Requena protected designation of origin. ICP-MS allows the determination of 17 elements in most of the samples, and using ICP-OES, a maximum of 15 elements were determined. On comparing the sample pretreatment methodology, it can be concluded that the three assayed procedures provide comparable results for the concentration of Li, Na, Mg, Al, K, Ca, Mn, Fe, Zn, and Sr by ICP-OES. Furthermore, ICP-MS data found for Cu, Pb, and Ba were comparable. Digestion treatment provides comparable values using both total decomposition in open system and microwave-assisted treatment for Cu by ICP-OES and for Cr, Ni, and Zn by ICP-MS. Open vessel total digestion provides excess values for Cr, Mn, Fe, and Zn by ICP-OES and defect values for Se. However, direct measurement of diluted wine samples provided uncomparable results with the digestion treatment for Mn, Cu, Pb, Zn, Ba, and Bi by ICP-OES and for Mg, Cr, Fe, Ni, and Zn by ICP-MS. Therefore, it can be concluded that microwave-assisted digestion is the pretreatment procedure of choice for elemental analysis of wine by ICP-based techniques. KEYWORDS: Wine; ICP-OES; ICP-MS; multielemental analysis; sample pretreatment
INTRODUCTION
The determination of the elemental composition of wines is of great interest for the characterization of samples and also for the identification of wine origin (1–4). To do it in an appropriate way, there is no single element to be determined, but it is necessary to accurately determine, as much as possible minor, trace and ultra trace elements. Different techniques such as flame atomic absorption spectroscopy (FAAS) (5), graphite furnace atomic absorption spectroscopy (GFAAS) (6), and voltammetry (7) have been employed for the determination of metals in wines. However, the most extensively employed techniques to do the elemental characterization of wines are inductively coupled plasma mass spectrometry (ICP-MS) (8–10) and inductively coupled plasma optical emission spectrometry (ICP-OES) (11–13), which are suitable for the simultaneous or fast sequential determination of trace and ultratrace elements. Those techniques provide a multielement detection capability and a wide linear dynamic * Corresponding author. Tel. +34-963544838. Fax: + 34-963544838. E-mail:
[email protected].
range (4-6 orders of magnitude), which permits the determination of several elements in a same sample. Wine, from an analytical point of view, is a complex matrix with water and ethanol as the main components containing Fe, K, Mg, Ca, Na, and Al at the ppm level and around 15 elements that are commonly present at the ppb level. Additionally, there are hundreds of organic compounds present in wines, which can cause spectral and nonspectral interferences. Therefore, it is clear that sample pretreatment as well as the method of determination is critical for the accuracy of the determination of trace and ultratrace elements, as has been probed previously (14). The selection of a sample preparation procedure implies that one must obtain the best results in the shortest time, with minimum loses or contamination risks, using the smallest quantities of reagents and samples, and generating little amounts of residues and wastes. Thus, different sample pretreatments have been described in the literature for the determination of metal content in wines by ICP-MS and ICP-OES, such as microwave-assisted acid digestion (8, 11, 24), thermal digestion in open reactor (15, 16), sample dilution (17–20), or dry ashing (21).
10.1021/jf800286y CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
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Table 1. Operating Conditions for Multielemental Analysis of Wine Using ICP-OES and ICP-MS
Be, Bi, B, Cd, Ca, Cr, Co, Cu, Fe, Pb, Li, Mg, Mn, Mo, Ni, K, Se, Na, Sr, Tl, Ti, V, and Zn) dissolved in 5% HNO3 from Scharlau (Barcelona, Spain) and a multielemental (100 µg mL-1) solution containing 16 lanthanides dissolved in 5% HNO3 from Alfa Aesar (Karlsruhe, Germany) were employed for calibration. HNO3 69% (w/v) from J.T. Baker (Deventer, The Netherlands), H2O2 35% (v/v) reagent grade from Scharlau, perchloric acid 60% (w/v) from Panreac (Barcelona, Spain), absolute ethanol from Scharlau, and nanopure water with a maximum resistivity of 18.2 MΩ cm-1 obtained from a Milli-Q Millipore system (Bedford, MA, USA) were used for sample treatment and sample dilution. A microwave laboratory system Ethos SEL from Millestone (Sorisole, Italy), equipped with an optical fiber sensor for automatic temperature control, an automatic gas detector and operating at a maximum exit power of 1000 W, was employed for microwave-assisted digestion of wine samples introduced inside 10 high pressure vessels of 100 mL inner volume. An ultrasound water bath from Selecta (Barcelona, Spain) of 350 mL volume, with 50 W power and 50 Hz frequency, was employed for sample sonication. Measurements by ICP-OES were carried out using a PerkinElmer Model Optima 5300 DV spectrometer (Norwalk, CT, USA) equipped with an autosampler AS 93-plus and a Meinhard nebulizer. Argon C-45 (purity higher than 99.995%), supplied by Carburos Meta´licos (Barcelona, Spain), was employed as plasmogen and carrier gas. ICP-MS measurements were carried out with a Perkin-Elmer SCIEX Elan 6100 DRCII apparatus (Norwalk, CT, USA), equipped with a Meinhard nebulizer. An autosampler Perkin-Elmer AS-91 was used for sample introduction in both measurement systems. Samples. Twelve red Spanish wine samples were employed in this study. Samples were categorized into three classes as a function of their origin (P.D.O.), three samples from Utiel Requena P.D.O., three from Valencia P.D.O. and six from Fontanars dels Alforins, the last ones also corresponding to the Valencia P.D.O. Wine bottles were purchased in liquor retail stores and markets. The alcoholic content ranged from 12 to 14.2% v/v ethanol, and their vintages in all cases were between 2001 and 2005. The plastic containers used for storing and diluting the samples were cleaned to avoid contamination by trace metals. Containers were treated with nitric acid and washed with milli-Q water before washing with the samples. Once opened, the wine samples were treated according to the three evaluated pretreatment procedures.
ICP-OES parameters
value
radio frequency power gas plasma gas auxiliary gas nebulizer gas sample aspiration rate view background correction number of replicates nebulizer
1400 W argon 15 L min-1 0.2 L min-1 0.8 L min-1 1.1 mL min-1 axial 2-point 1 Meinhard ICP-MS
radio frequency power gas plasma gas auxiliary gas nebulizer gas lens voltage detector dwell time sweeps/readings replicate/readings replicates nebulizer
1125 W argon 15 L min-1 1.2 L min-1 0.95 L min-1 6.5 V dual 50 ms 6 1 2 Meinhard
The aim of this study has been to do a critical evaluation of sample pretreatments commonly employed for the determination of trace elements in wines by inductively coupled plasma-based techniques. Accuracy has been the main criterion for selecting the best sample treatment. In our study, accuracy has been evaluated by the comparison of results found by ICP-OES and ICP-MS for 12 different samples after being treated by two digestion procedures and just dilution. EXPERIMENTAL PROCEDURES Reagents and Apparatus. Stock solutions of a multielemental standard solution (100 µg mL-1) containing 26 elements (Al, As, Ba,
Table 2. Analytical Features of ICP-OES and Semi-Quantitative ICP-MS for the Determination of Trace Elements in Wines after Dilution or Digestion of Samplesa ICP-MS
ICP-OES
dilution b
-1
LOD (µg L ) Li Na Mg Al K Ca Sc Ti V Cr Mn Fe Ni Cu Zn Se Sr Ba Pb Bi
0.016 20 0.07 0.16 0.12 15 0.7 3 0.8 2 0.03 0.6 0.12 0.06
digestion % RSD 7 3 10 4 10 5 8 4 9 7 11 8 11 2 7 5 3
LOD (µg L )
% RSD
LOD (µg L-1)
% RSD
7.5 × 10 2.9 × 10-6 8.9 × 10-5 2.9 × 10-5 0.2 0.5 0.5 0.5 0.25 0.5 3.7 × 10-6 4.1 × 10-4 1.5 × 10-6 0.05 7.9 × 10-6
5 6 4 17 3
-6
2 3 4 2
0.11 0.3 0.06 0.8 0.2 0.16 0.02 0.06 0.08 0.4 0.03 0.03 2 0.19 0.2 1 0.05 0.09 2 1.9
0.4 0.3 0.6 7 0.9 1.6 1 2 0.7 0.9 0.7 20.0 8 1.6 3 3 0.9 0.4 25 3
0.07 0.12 0.10 0.08 0.12 0.13 0.05 0.09 0.05 0.11 0.03 0.07 0.1 0.4 0.07 6 0.03 0.02 11 4
0.1 0.5 1.9 3 1.2 1.9 0.2 0.9 0.3 0.5 0.3 2 2 0.5 0.5 20 0.9 0.5 20 5.7
LOD (µg L ) -6
1.9 × 10 5.1 × 10-7 0.05 1.7 × 10-6
8 5 6
5
-1
digestion
% RSD
b
-1
dilution c
c
a Se was not determined by ICP-MS, and for mayor elements, the LOD values in ICP-MS have no sense because we did not found any sample undetectable contents. LOD values were calculated as the concentrations corresponding to signals equal to three-times the standard deviation of a 0.5 µg L-1 solution signal (three replicates). c LOD values were calculated as the concentrations corresponding to signals equal to three-times the standard deviation of a blank solution signal (three replicates). b
Elemental Analysis of Wines
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Figure 1. Contour plots of the effect of radio frequency power (RF), sample aspiration flow rate, and sample dilution factor on the sensitivity of La, Al, Fe, and Zn determination by ICP-OES. Sample Pretreatment. The most common preparation methods used for wine analysis are wet acid digestion and dilution. Wet digestion is made for partial or total decomposition of the organic matter and can be performed in closed or open vessels, using convective thermal energy or microwave radiation. Alternatively, direct analysis of wine samples diluted with water seems to be a fast and precise way for sample preparation. MicrowaVe-Assisted Digestion. The microwave-assisted treatment was adapted from that of Galani-Nikolakaki et al. (22) according to the microwave laboratory system manufacturer’s recommendations. Teflon digestion vessels were previously cleaned in nitric solution to avoid cross-contamination. Two milliliters of red wine were weighted inside the digestion vessels, and 8 mL of concentrated nitric acid was added. Once the vessel was capped, it was placed in the microwave oven, and the following program was run: 3 min to reach 85 °C; 12 min to reach 145 °C; 6 min to reach 180 °C; 15 min at 180 °C; and a final cool down. After cooling to ambient temperature, reactors were
opened and sonicated to eliminate nitrous vapors. The resultant solution was transferred inside a plastic flask and diluted to a final volume of 25 mL. Open Vessel Total Digestion. Acid digestion of wine samples in open vessels was based on the method reported by Lara et al. (23). One milliliter of concentrated perchloric acid and 2 mL of concentrated nitric acid were added to 5 mL of wine, and the sample was digested by heating the mixture until dryness in a pyrex glass of 50 mL volume. To avoid external and cross-contamination of samples, single-use plastic Pasteur pipettes were used to transfer samples and reagents. The white ashes obtained by this procedure were dissolved in 0.5 mL of concentrated nitric acid, and the solution was diluted with milli-Q water to a final volume of 25 mL. Dilution. A 10-times dilution of wine samples, without any further sample preparation, was also checked as sample pretreatment. In this sense, 1 mL of sample was introduced in a plastic flask, and 75 µL of concentrated
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Table 3. Data of the Elemental Composition of Wines Obtained by ICP-OES after Different Sample Pretreatmentsa D.O. Valencia 1
D.O. Valencia 2
acid digestion Li Na Mg Al K Ca Sc Ti V Cr Mn Fe Ni Cu Zn Se Sr Ba Pb Bi
acid digestion
closed vessel
dilution
open vessel
closed vessel
dilution
open vessel
closed vessel
dilution
0.1014 ( 0.0001 42.4 ( 0.2 81 ( 1 1.01 ( 0.03 1003 ( 12 55 ( 1
