Aim: The control of geographical origin is one of the most challenging topics regarding wine authenticity. The aim of the present study was to assess the 87Sr/86Sr ratio of vineyard soils from Portuguese Denominations of Origin (DO) and evaluate its suitability as a tool for origin authentication.
Methods and results: An analytical protocol was optimized (chromatographic separation of Sr and Rb, followed by inductively coupled plasma-mass spectrometry (ICP-MS) analysis) for 87Sr/86Sr isotopic ratio determination in soil-wine system. The 87Sr/86Sr ratios of soils from four vineyards located in three Portuguese DO (Dão, Óbidos and Palmela), established on distinct soil types, were determined. Significant differences were found between soils of different DO regions. The soil in the Dão DO, developed on granites, showed a statistically higher 87Sr/86Sr ratio than the other soils, which were developed on sedimentary formations.
Conclusion: The results show clearly that 87Sr/86Sr ratio may represent a suitable fingerprint for these Portuguese DO.
Significance and impact of the study: This study highlights the relevance of setting up an international databank of 87Sr/86Sr values for use for geographical identification and authentication.
The globalization of food markets has raised consumer concerns for product origin and quality. The place of origin of foodstuff is regarded as value-added information and as a guarantee of quality and authenticity. For wine in particular, geographical origin has a direct effect on its quality and commercial value, being one of the most studied products in terms of food authentication (Barbaste et al., 2002; Almeida and Vasconcelos, 2003).
The control of the geographical origin of wine based on its chemical composition is one of the most challenging issues in relation to wine authenticity. In the last decade, many efforts have been made to identify potential markers and develop reliable analytical methods to determine the wine’s authenticity. Among these “fingerprints”, isotopic ratios play an increasingly important role (Almeida and Vasconcelos, 2001; Barbaste, 2001; Ferreira, 2008; Rosner, 2010). The application of methods using stable isotopes of light elements (H, C, N, O, S) started two decades ago, providing information on climate, distance from the sea, altitude, latitude, and technological practices (Ferreira, 2008).
More recently, the study of isotopic ratios of heavy elements such as Pb and Sr came into use in this field of application, providing additional information on the geographical origin, since plants inherit the isotopic signature of these elements from the geological and pedological environment (Horn et al., 1993; Barbaste, 2001; Rummel et al., 2010).
The use of Sr isotope ratio is a well established tool in earth sciences for dating and tracing the origin of rocks and minerals. Furthermore, some studies indicate that Sr isotope analysis could be very useful for determination and verification of the geographical origin of food.
Sr has four naturally occurring stable isotopes with ranges of natural abundance as follows: 84Sr, 0.55-0.58%; 86Sr, 9.75-9.99%; 87Sr, 6.94-7.14%; and 88Sr, 82.29-82.77% (Berglund and Wieser, 2011). The 84Sr, 86Sr and 88Sr isotopes occur in constant relative proportions, while 87Sr gradually increases in minerals due to the radioactive β-decay of the 87Rb isotope. The relative abundance of 87Sr varies with geological ages and consequently with geographical locations, providing a fingerprint for different rock types (Capo et al., 1998; Vanhaecke et al., 1999; Almeida and Vasconcelos, 2001).
The proportion of 87Sr to total Sr increases at a rate dependent on available Rb. Geological environments rich in Rb relative to Sr will have a high 87Sr/86Sr ratio, while regions with low Rb relative to Sr will retain low 87Sr/86Sr ratios for long periods of geological time. Therefore, the Sr isotopic composition of a geological sample depends on the Rb/Sr concentration ratio and age of the material.
Weathering of the underlying rock and/or sediments is a significant source of strontium for the soil. Pre-Cambrian granitic bedrock and alluvial sands derived from felsic rocks show high 87Sr/86Sr ratios (0.710-0.716) reflecting the age of the continental crust and high Rb/Sr ratios from which these materials originated. Limestones have intermediate 87Sr/86Sr ratio values (0.706-0.709) and young oceanic basalts and their sediments show the lowest values (0.702-0.705) (Faure, 1986; Capo et al., 1998).
Biological processes involved in plant metabolism do not significantly fractionate Sr isotopes (Capo et al., 1998). Elements are absorbed by the plants in the same isotopic proportions in which they occur in soil. Thus, plants reflect the growth environment, such as bedrock, soil and soil water content (Horn et al., 1993; Capo et al., 1998; Stewart et al., 1998), including all sources of Sr: natural (bedrock weathering, precipitation) and anthropogenic (e.g., fertilizers). To establish correlations between the 87Sr/86Sr ratio of wines and soils, a deep knowledge of the region’s geological and pedological features is a prerequisite.
Studies by Horn et al. (1997) demonstrated that the 87Sr/86Sr values of several wines were within the respective ranges for rocks and soils. In the cases in which the isotopic ratio values were inconsistent with their origin, double-blind studies were performed and the wines were found to be falsely labeled. Barbaste (2001) reported values of 87Sr/86Sr ratio in wine in agreement with the literature data for the corresponding soils. In addition, Almeida and Vasconcelos (2004) studied the Sr isotopic composition in two wines from the Douro Portuguese Denomination of Origin (DO) by means of quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) and compared it with the isotopic composition of the respective soil and grape juice. It was shown that the 87Sr/86Sr values in soil, grape and wine were statistically identical.
Precise and accurate Sr isotope data is required for origin discrimination. Faraday multi collector-equipped thermal ionization MS (TIMS) or ICP-MS are the techniques of choice to determine the 87Sr/86Sr ratio of food, with precision values of 0.002% (RSD) (Barbaste et al., 2002; Rosner, 2010). The lower precision of Q-ICP-MS, typically below 0.1% (RSD), can be a limiting factor, especially in studies involving samples with very close 87Sr/86Sr values. Nevertheless, Q-ICP-MS is robust and less time consuming, and proved to be suitable to distinguish the Sr isotopic composition of wines (Vanhaecke et al., 1999; Almeida and Vasconcelos, 2001).
Due to the isobaric overlap of 87Sr and 87Rb, an effective Rb/Sr separation is a pre-requisite for the accurate determination of Sr isotope ratios by Q-ICP-MS. In order to perform this separation, ion-exchange chromatography is employed and some elution procedures are described in the literature. The most widely used procedure involves the elution of Rb with a weak HCl solution, followed by the elution of Sr using a stronger HCl solution (Vanhaecke et al., 1999). An alternative approach to the conventional HCl method is the use of a two-step chromatographic separation, described by Almeida and Vasconcelos (2001). Alternatively, Vorster et al. (2008) proposed a highly specific separation method using the complexing agent ethylenediaminetetraacetic acid (EDTA), which forms strong chelates with Sr without interacting with Rb.
Data on the isotopic composition of Portuguese vineyards are scarce and no information is available concerning different DO. Therefore, within the “Multi-elemental and isotopic composition as fingerprints of wine geographical origin” research program on wine fingerprinting strategies, a study was developed to investigate whether between-region variation in Sr isotopic composition could be used as a tool for the traceability of Portuguese DO (Dão, Óbidos and Palmela), where soils are developed on different geological formations. For that, we first optimized an analytical protocol suitable for the determination of 87Sr/86Sr by Q-ICP-MS in vineyard soil samples, grape must and wine, Sr being separated from Rb by cation-exchange chromatography using EDTA. Then, 87Sr/86Sr ratios of soils from selected vineyards in Portuguese DO, established on distinct soil types, were determined and compared.
Materials and methods
Four vineyards from three Portuguese DO (Dão, Óbidos and Palmela) were studied (Figure 1). In the Dão DO (Central Portugal), one vineyard was considered: Quinta dos Carvalhais (40º 33´ N, 7º 47´ W), Sogrape Vinhos. It is established on Dystric Cambisols and Dystric Regosols (IUSS Working Group WRB, 2006) soils which were developed on monzonitic granites (Hercynian granites), the most representative geological formation of Dão DO (Teixeira et al., 1961). In the Óbidos DO (Centre of Portugal), characterized by a larger variety of lithological formations, two vineyards were considered: Quinta do Sanguinhal (39º 15´ N, 9º 09´ W) and Quinta de S. Francisco (39º 11´ N, 9º 10´ W), both of Companhia Agrícola do Sanguinhal. Quinta do Sanguinhal is established on soils (Dystric Regosols) developed on Jurassic sandstones; Quinta de S. Francisco is established on soils (Eutric Regosols) developed on Cretacic sandstones (Zbyszewski and da Veiga Ferreira, 1966). In the Palmela DO (Southern Portugal), one vineyard was considered: Vinha de Algeruz (38º 34´ N, 8º 49´ W), José Maria da Fonseca Vinhos. This vineyard is established on Eutric Regosols developed on Pliocene sedimentary formations (clays and sands), the most representative geological formation of Palmela DO (Manuppella et al., 1999). The area of each vineyard plot, year of planting, rootstock, vine spacing, row direction and training system are indicated in Table 1. All the vineyards have the same red variety in production (Vitis vinifera L., cv Aragonez).
Figure 1. Location of the four experimental vineyards in three Portuguese DO (Dão, Óbidos and Palmela).
Table 1. Characteristics of the vineyards.
|Vineyard / Portuguese DO||Area of the vineyard plot (ha)||Year of planting||Rootstock||Vine spacing (m)||Row direction||Training system|
|Quinta dos Carvalhais / Dão||2.5||1995||1103P||2.0x1.2||NE-SW||bi-lateral cordon|
|Quinta do Sanguinhal / Óbidos||2.6||2000||R110||2.7x1.0||N-S||bi-lateral cordon|
|Quinta de S. Francisco / Óbidos||5.0||2001||R110||2.7x1.0||N-S and E-W||bi-lateral cordon|
|Vinha de Algeruz / Palmela||3.0||1990||1103P||2.8x1.2||N-S||bi-lateral cordon|
The study area has a temperate, Mediterranean-type climate characterized by dry summer and wet autumn/winter/early spring. The Quinta dos Carvalhais, Quinta do Sanguinhal and Quinta de S. Francisco vineyards have a humid temperate climate with dry summer and cold nights (Csb, Köppen classification); the Vinha de Algeruz vineyard has a humid temperate climate with dry hot summer and temperate nights (Csa, Köppen classification) (Instituto de Meteorologia; IM, 2008).
Soil sampling took place in December 2007 (Vinha de Algeruz, Quinta do Sanguinhal and Quinta de S. Francisco) and May 2009 (Quinta dos Carvalhais). In each vineyard, soil samples were collected with a probe, from nine sampling sites distributed along three non-contiguous vine rows (representative of the entire vineyard area), from four depth layers (0-20, 20-40, 40-60 and 60-80 cm), and sealed in plastic bags. The samples used in this study were those collected at the 40-60 cm layer, where root density was higher. Soil samples were dried, ground and then forced to pass through a 2-mm sieve.
Nitric acid (HNO3) 65% EMSURE® and hydrofluoric acid (HF) 48% EMSURE® (Merck) were used for soil digestion. Ultrapure concentrated HNO3 (J.T. Baker) and Trace Select® 30% (v/v) hydrogen peroxide (H2O2) (Fluka, Sigma-Aldrich) were used for grape must and wine digestion. Ammonia (NH3) 25% (v/v) and EDTA Titriplex III of analytical grade (Merck) were used for ion-exchange chromatography.
Monoelement standard solutions of Be, Co, In (1000 mg/L; Merck) and a multielement solution with Mg, Cu, Rh, Cd, In, Ba, Ce, Pb and U (10 mg/L; Perkin-Elmer) were used for ICP-MS optimization. Ultrapure concentrated HNO3 (J.T. Baker) was used for washing, blank, and standard solutions. ICP-MS semi-quantitative calibration (for Rb and Sr determinations) was established with a multielement standard solution with 30 elements: Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Ag, Cd, In, Cs, Ba, Hg, Tl, Pb, Bi and U (10 mg/L; Perkin-Elmer). ICP-MS internal standardization was performed with standard solutions of Rh and Re (1000 mg/L; Merck).
The standard reference material (SRM) 987 (SrCO3) from the National Institute of Standards and Technology (NIST; Gaithersburg, USA) was used as isotopic standard (external correction of mass bias) and for ICP-MS instrumental parameter optimization. A stock solution of Sr (1000 mg/L) was prepared by dissolving 1 g of powder in 1% (v/v) HNO3. Monoelement standard solutions of Sr (1000 mg/L; Reagecon) and Rb (1000 mg/L; Perkin-Elmer) were used for the study of the influence of Rb concentration on Sr isotope ratio measurements.
De-ionized water (conductivity < 0.05 µS/cm) prepared by using a Seralpur Pro 90CN system (Seral, Ransbach-Baumbach, Germany) was used for all solutions. To avoid contamination of the samples, all the material (polypropylene and Teflon PFA) was soaked in 20% HNO3 (v/v) for at least 24 hours and then rinsed thoroughly with de-ionized water before use. For decontamination solution preparation, HNO3 reagent grade was double-distilled using an infra-red sub-boiling distillatory system (model BSB-939-IR, Berghof, Germany).
Prior to ion-exchange chromatography, the samples were acid digested using a microwave system (CEM MDS 2000). Four mL of HNO3 (65% v/v) and 4 mL of HF (50% v/v) were added to each subsample of 0.20 g of dried soil (fraction < 270 mesh). The digestion program consisted of one 30-minute step at 630 W (100 psi). The dissolved material was dried, dissolved with HNO3 (2 M) to 10 mL, and finally diluted to 25 mL with de-ionized water. The efficiency of the digestion procedure was confirmed with the certified reference material Geo PT 25, Basalt HTB-1.
Strontium and rubidium separation by ion-exchange chromatography
Ion-exchange chromatography was carried out in a column (internal diameter of 1.1 cm and bed length of 13 cm) filled with Dowex 50W-X8/400 mesh resin (Sigma Aldrich). The procedure described by Vorster et al. (2008) employing EDTA as eluent was used as a starting point. After pretreatment and resin conditioning, samples were loaded into the column, and sequential elution of Ca, Sr and Rb was performed with EDTA solutions. Experimental conditions, namely pretreatment and resin conditioning, volume and concentration of EDTA solution at each elution step, and resin washing, were optimized in order to obtain Sr and Rb separation in a single separation step. Experimental tests were conducted using soil, grape must and wine samples after high power microwave (HPMW) digestion and dilution with 0.1% HNO3 to a specific volume depending on the required concentration: soil samples were treated as previously described; grape must and wine samples were mineralized by an adaptation of the HPMW procedure previously optimized by Catarino et al. (2010).
To prevent the precipitation of residual EDTA in the ICP-MS instrument, samples were filtered using a Millipore PVDF syringe filter (0.45-μm pore size, 3.0-cm diameter).
Analytical measurements were carried out with a Perkin-Elmer SCIEX Elan 9000 ICP-MS (Norwalk, CT, USA) equipped with a cross-flow nebulizer, a Ryton Scott-type spray chamber, and nickel cones. A four-channel peristaltic sample delivery pump (Gilson model) and a Perkin-Elmer AS-93 Plus autosampler protected by a laminar-flow-chamber clean room class 100 (Max Petek Reinraumtechnik) were used. The ICP-MS instrument was controlled by Elan 6100 Windows NT software (version 2.4).
The operating conditions of the ICP-MS equipment, optimized daily, were as follows: RF power of 1200 W; Ar gas flow rates of 15 L/min for cooling, between 0.94 and 0.98 L/min for nebulizer and 1.5 L/min for auxiliary; and solution uptake rate of 1.0 mL/min.
Rb and Sr concentrations were determined in mineralized soil samples (after 100-fold dilution) by using an ICP-MS semi-quantitative approach, as described by Catarino et al. (2006). The contents of Rb and Sr in Sr chromatographic fractions were also determined before isotopic measurements. Between samples or standards, the sampling system was rinsed with 2% HNO3 (v/v) for 90 s.
The ICP-MS instrumental parameters were optimized with the Sr isotopic standard (50 µg/L) to obtain the best precision for 87Sr/86Sr measurement and, therefore, to allow the detection of small variations in 87Sr/86Sr ratios and between samples of different origin. The influence of Sr concentration (between 10 and 50 µg/L) and instrumental parameters on precision was studied: number of replicates (between 3 and 8), dwell time (between 10 and 60 ms) and number of sweeps per reading (between 100 and 1000).
The measurement of the 87Sr isotope suffers from interference by the 87Rb isotope. As residual Rb can still be present in the pretreated samples (HPMW digestion and chromatographic separation), mathematical correction for both 87Rb interference on 87Sr (-0.385617×85Rb) and 86Kr interference on 86Sr (1.505657×83Kr) was systematically carried out using the software described above. Additionally, the influence of Rb was assessed by adding defined amounts of Rb to a 25 µg/L Sr solution and determining 87Sr/86Sr ratios.
With the purpose of determining 87Sr/86Sr accurately, the SRM 987 (SrCO3) from the NIST (Gaithersburg, USA) was used as isotopic standard for the instrumental mass bias correction (certified value: 0.71034 ± 0.00026). The concentration of the isotopic standard was similar to that estimated for the soil samples (50 µg/L). Between samples or standard, the sampling system was rinsed with 2% HNO3 for 120 s.
Under optimized conditions, each soil sample was treated (HPMW digestion followed by chromatographic separation) and analyzed in duplicate (each analysis consisted of three replicates). Periodical calibration between samples was established in order to underscore a possible shift over time.
The statistical treatment of soil 87Sr/86Sr ratios was performed by one-way analysis of variance and comparison of means (Fisher LSD, 95% level) using Statistica 7.0 software (StatSoft Inc., Tulsa, USA). In an early stage, normal distribution and homogeneity of variance were verified by Normal p-p (distribution of within-cell residuals) and Cochran C tests (p < 0.05), respectively.
Results and discussion
Strontium and rubidium separation by ion-exchange chromatography
Figure 2 presents the optimized procedure scheme for Sr and Rb ion-exchange separation. In order to overcome the lack of specificity of the resin for Sr, a pretreatment step with 30 mL of 10% NH3 (v/v) for resin particles conversion to NH4+ form was performed, followed by excess NH3 removal with 20 mL of de-ionized water. A volume of 20 mL of 0.02 M EDTA (pH 5.5) was used to adjust the column pH (conditioning step). By applying the correct EDTA concentration, at a suitable pH for complex formation, cations are eluted from the column (Vorster et al., 2008). Soil samples, grape must and wine contain high concentrations of cations, which saturate the resin causing a loss of Sr. To avoid this problem, samples were diluted with 0.1 M EDTA (pH 7.0) to 15 mL. As cations preferentially bind to EDTA rather than to the resin, they are selectively removed from the column. The sample solution (pH 5.0) was loaded into the column, with the cations being retained by the resin. During the first elution step with 40 mL of 0.02 M pH 5.5 EDTA, Ca was removed. During the second elution step with 50 mL of 0.05 M pH 7.0 EDTA, Sr was removed. At this stage, the column was washed with 40 mL of de-ionized water to remove excess EDTA. Since EDTA does not form chelates with alkali metals, Rb remains in the column. The elution of this element was carried out with 40 mL of 3 M HNO3, following the removal of EDTA by de-ionized water. The Sr-containing fraction was evaporated to dryness and dissolved with HNO3 (0.5% v/v) to 20 mL.
Figure 2. Procedure scheme of strontium and rubidium ion-exchange separation. HPMW, high power microwave; Q-ICP-MS, quadrupole - inductively coupled plasma - mass spectrometry.
In order to verify the chromatographic separation efficiency, Rb and Sr concentrations in Sr-containing fractions were determined. The results indicated that the optimized ion-exchange separation method using the complexing properties of EDTA could be used reliably to separate Sr and Rb.
Accuracy and precision of 87Sr/86Sr measurement by ICP-MS
The ICP-MS acquisition parameters for 87Sr/86Sr measurement, under optimized conditions, are given in Table 2. As mentioned above, mass bias correction was carried out with a Sr isotopic standard solution. The correction factor (automatically calculated) generated through several studies ranged between 1.020197 and 1.025118. The instrumental stability during measurements was investigated by repeating the analysis of a 50 µg/L Sr solution and no drift effect was observed.
Table 2. Optimized ICP-MS instrumental parameters for the measurement of 87Sr/86Sr in wine, grape must and soil samples.
|Scanning mode||peak hopping|
|Dwell time||30 ms (86Sr)|
|35 ms (87Sr)|
|20 ms (88Sr)|
|Sweeps per reading||500|
|Readings per replicate||1|
|Time per run||247 s|
It was noted that 1 µg/L of Rb in a 25 µg/L Sr solution could influence the 87Sr/86Sr ratio, resulting in values higher than in reality.
Since the application of this method to sample discrimination is based on relative differences between samples and not on absolute ratios, its success is more dependent on precision than on accuracy. The precision of 87Sr/86Sr measurement ranged between 0.1 and 0.2% (RSD) and proved not to be related to Sr concentration (10-50 µg/L). As expected, this precision is poor as compared to that provided by TIMS, but similar or slightly better than that obtained by several authors (Almeida and Vasconcelos, 2001; Vorster et al., 2008) using a similar mass separator device (Q-ICP-MS).
87Sr/86Sr in soils from Portuguese Denominations of Origin
87Sr/86Sr determination in soil samples from different Portuguese DO was performed using the optimized analytical protocol. Table 3 presents the total Rb and Sr concentration, Rb and Sr concentration ratio, Rb and Sr levels in Sr chromatographic fraction, and 87Sr/86Sr ratio for each vineyard. In addition, Table 4 shows the LSD test P-values for the 87Sr/86Sr ratio of the different vineyards and DO.
Table 3. Soils from Portuguese DO vineyards: identification, classification, total Rb and Sr concentration, Rb/Sr ratio, Rb and Sr levels in Sr chromatographic fraction, and 87Sr/86Sr ratio.
|Vineyard / Portuguese DO||Soil classification||Soil||
Sr fraction after
|Quinta dos Carvalhais / Dão||
Dystric Cambisols /
Regosols from monzonitic granites
|Quinta do Sanguinhal / Óbidos||
Dystric Regosols from
|Quinta de S. Francisco / Óbidos||
Eutric Regosols from
|Vinha de Algeruz / Palmela||
Eutric Regosols from
Pliocene clayey sands
|NIST SMR 987||Measured value:||0.710±0.00054|
Means followed by the same letter are not significantly different at the 0.05 level of significance.
Table 4. LSD test P-values for the 87Sr/86Sr ratio of the different vineyards and DO regions.
|Quinta dos Carvalhais / Dão vs Quinta do Sanguinhal / Óbidos||0.000000|
|Quinta dos Carvalhais / Dão vs Quinta de S. Francisco / Óbidos||0.000000|
|Quinta dos Carvalhais / Dão vs Vinha de Algeruz / Palmela||0.000000|
|Quinta do Sanguinhal / Óbidos vs Quinta de S. Francisco / Óbidos||0.579584|
|Quinta do Sanguinhal / Óbidos vs Vinha de Algeruz / Palmela||0.003171|
|Quinta de S. Francisco / Óbidos vs Vinha de Algeruz / Palmela||0.013087|
Rb was successfully removed from Sr-containing fraction, and its residual content in the samples (less than 1.5% of the Sr content) had a minimal contribution to the 87Sr/86Sr ratio.
The precision of Sr isotope ratio measurements, expressed as RSD (%), ranged between 0.04 and 0.23, which allowed to distinguish variations in isotopic abundance between soil samples.
Regarding the 87Sr/86Sr ratio, significant differences were found between soils from different regions. The two soils from the Óbidos region displayed the same 87Sr/86Sr ratio (0.714) and were significantly different from the soils of the Dão and Palmela regions (0.737 and 0.711, respectively). Despite being developed on different geological materials, the soils from the Óbidos DO showed identical ratios, demonstrating the suitability of 87Sr/86Sr as fingerprint for this DO.
The lowest values of 87Sr/86Sr ratio and Rb content were observed in the soil of the Palmela region, developed on the youngest parent material considered in this study. The Dão region 87Sr/86Sr ratio was distinctively higher than in the other regions and is in agreement with values reported for granitic rocks of the same region (Costa, 2006). This value is also in accordance with the results of Almeida and Vasconcelos (2004) for one soil (0.732) and wines (0.729) from the Douro DO, a mainly schistous region located in the northeastern Portugal. According to the literature, high 87Sr/86Sr values are characteristic of granitic and, therefore, older rocks (Costa, 2006).
The results show that Rb/Sr ratio is a good indicator of the 87Sr/86Sr values and level of similarity between soils of these vineyards. Considering the evolution of 87Sr/86Sr in geological systems, in general older rocks, such as granites in continental crust, with higher Rb/Sr ratios will develop higher 87Sr/86Sr ratios than younger ones with lower Rb/Sr ratios (Capo et al., 1998). Dão has the highest Rb/Sr, the two vineyards from Óbidos have similar intermediate Rb/Sr, and Palmela shows the lowest Rb/Sr values. These values are in agreement with the ones obtained for 87Sr/86Sr.
With the exception of the Douro region (Almeida and Vasconcelos, 2004), no reference to the Sr isotopic composition of vineyard soils from Portugal was found in the literature. In this study, 87Sr/86Sr values for three major DO regions (Dão, Óbidos and Palmela) were assigned, with the specific purpose of discriminating between vineyard soils from different regions. The 87Sr/86Sr ratio may represent a suitable fingerprint for the studied DO and may be used in conjunction with other tracers. In a previous study within the referred research program, we showed the potentialities of rare earth element (REE) patterns as fingerprint for origin authentication (Catarino et al., 2011). Regarding the vineyards/DO of the present study, it was shown that REE patterns can represent a suitable fingerprint for wine origin authentication.
The developed analytical protocol for 87Sr/86Sr determination by Q-ICP-MS showed sufficient precision and accuracy to allow the detection of variations in isotopic abundance between soil samples.
The most important result of this study was the confirmation that 87Sr/86Sr ratio could be a viable tool for origin identification for these three Portuguese DO regions. In addition, the scarce knowledge of 87Sr/86Sr ratios in Portuguese vineyard soils was extended.
In this work, only the most representative lithological formations of each DO were considered. Studies using 87Sr/86Sr should be performed on the other lithological formations of these DO, in order to characterize each region, and on other Portuguese DO. Besides providing geological and pedological background, this information may be integrated in an international wine databank, together with other parameters, for use in geographical identification and authentication.
Acknowledgments: The authors would like to acknowledge the wine companies “Companhia Agrícola do Sanguinhal Lda”, “José Maria da Fonseca Vinhos” and “Sogrape Vinhos” for providing their facilities regarding the project development; Paulo Marques and José Correia for help in field sampling; the staff of the Soil Laboratory (ISA, Lisbon) for soil analysis; and Otília Cerveira for help in Mineral Analysis Laboratory activities (INIAV, I.P., Dois Portos). The present study was developed within the “Multi-elemental and isotopic composition as fingerprints of wine geographical origin” (PTDC/AGR-ALI/64655/2006) project funded by the Fundação para a Ciência e a Tecnologia.
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