Luis Gras

Atomic spectrometry methods for wine analysis: A critical evaluation and discussion of recent applications

The analysis of wine is of great importance since wine components strongly determine its stability, organoleptic or nutrition characteristics. In addition, wine analysis is also important to prevent fraud and to assess toxicological issues. Among the different analytical techniques described in the literature, atomic spectrometry has been traditionally employed for elemental wine analysis due to its simplicity and good analytical figures of merit. The scope of this review is to summarize the main advantages and drawbacks of various atomic spectrometry techniques for elemental wine analysis. Special attention is paid to interferences (i.e. matrix effects) affecting the analysis as well as the strategies available to mitigate them. Finally, latest studies about wine speciation are briefly discussed.

Ultratrace determination of Pb, Se and As in wine samples by electrothermal vaporization inductively coupled plasma mass spectrometry

The determination of Pb, Se and As in wine has a great interest due to health risks and legal requirements. To perform the analysis of wine, two considerations must be taken into account: (i) the low concentration level of the analytes; and (ii) the risk of interferences due to wine matrix components. The goal of this work is to evaluate electrothermal vaporization (ETV) sample introduction for ultratrace determination of Pb, Se and As in wine samples by inductively coupled plasma mass spectrometry (ICP-MS). The results obtained with ETV-ICP-MS were compared to those obtained with conventional liquid sample introduction in ICP-MS and electrothermal atomic absorption spectrometry (ETAAS). Analytical figures of merit of ETV sample introduction strongly depend on the amount of wine sample, on the modifier nature (i.e. Pd, ascorbic acid or citric acid) and concentration and on the temperature program. Wine matrix components exert a great influence on analyte transport efficiency. Due to this fact, the analysis of wine cannot be performed by means of external calibration but the standard addition methodology should be used. The determination of Pb and Se in wine by ETV-ICP-MS provides similar results as conventional liquid sample introduction ICP-MS. For As, the concentration values obtained with ETV sample introduction were between two and four times lower than with the conventional system. These differences are related to the lower intensity of polyatomic interferences (i.e. (40)Ar(35)Cl(+) vs. (75)As(+)) obtained for ETV sample introduction when compared to the conventional system. Finally, no differences for Pb determination were observed between ETV sample introduction and ETAAS. Unfortunately, the limits of detection for As and Se in ETAAS were not low enough to quantify these elements in the wine samples tested.

Application of a microwave-based desolvation system for multi-elemental analysis of wine by inductively coupled plasma based techniques.

Elemental wine analysis is often required from a nutritional, toxicological, origin and authenticity point of view. Inductively coupled plasma based techniques are usually employed for this analysis because of their multi-elemental capabilities and good limits of detection. However, the accurate analysis of wine samples strongly depends on their matrix composition (i.e. salts, ethanol, organic acids) since they lead to both spectral and non-spectral interferences. To mitigate ethanol (up to 10% w/w) related matrix effects in inductively coupled plasma atomic emission spectrometry (ICP-AES), a microwave-based desolvation system (MWDS) can be successfully employed. This finding suggests that the MWDS could be employed for elemental wine analysis. The goal of this work is to evaluate the applicability of the MWDS for elemental wine analysis in ICP-AES and inductively coupled plasma mass spectrometry (ICP-MS). For the sake of comparison a conventional sample introduction system (i.e. pneumatic nebulizer attached to a spray chamber) was employed. Matrix effects, precision, accuracy and analysis throughput have been selected as comparison criteria. For ICP-AES measurements, wine samples can be directly analyzed without any sample treatment (i.e. sample dilution or digestion) using pure aqueous standards although internal standardization (IS) (i.e. Sc) is required. The behaviour of the MWDS operating with organic solutions in ICP-MS has been characterized for the first time. In this technique the MWDS has shown its efficiency to mitigate ethanol related matrix effects up to concentrations of 1% (w/w). Therefore, wine samples must be diluted to reduce the ethanol concentration up to this value. The results obtained have shown that the MWDS is a powerful device for the elemental analysis of wine samples in both ICP-AES and ICP-MS. In general, the MWDS has some attractive advantages for elemental wine analysis when compared to a conventional sample introduction system such as: (i) higher detection capabilities; (ii) lower ethanol matrix effects; and (iii) lower spectral interferences (i.e. ArC(+)) in ICP-MS.

Introduction of organic solvent solutions into inductively coupled plasma-atomic emission spectrometry using a microwave assisted sample introduction system

A microwave assisted sample introduction system based on the use of a TM010 cavity (MWDS2) has been employed for the introduction of 10% w/w organic solvent solutions in inductively coupled plasma atomic emission spectrometry (ICP-AES). Ethanol, propan-2-ol, formic and acetic acids have been used. Firstly, the effect of the incident microwave power and the sample uptake rate on the emission signal was evaluated. For all matrices tested, the higher emission signals were obtained when operating at the highest microwave power (i.e., 290 W) and sample uptake rate (400 μL min−1). Results with the MWDS2 were compared with those afforded by a desolvation system based on the use of a domestic microwave oven (MWDS) and by a conventional sample introduction system (CS). The MWDS2 provides the highest emission signals (up to 7 and 17 times higher than those with the MWDS and the CS, respectively). As regards the matrix effects originated by the organic solutions, results demonstrate that the use of a microwave-based sample introduction system, mainly the MWDS2, affords a noticeable reduction in the matrix effects originated by the use of organic solvent solutions in ICP-AES with a conventional sample introduction system. This behaviour can be explained by taking into account the solution transport rates afforded by the different sample introduction systems. For all the analytical lines and matrices tested and operating at 400 μL min−1, the MWDS2 gives rise to signal values that are, on average, 1.2 ± 0.2 times the signal obtained with water. For the MWDS and the CS, this factor takes values of 1.6 ± 0.2 and 2.0 ± 0.5, respectively. Transient matrix effects have been observed operating with the MWDS2 when switching between water and an organic matrix solution. These transient effects result in a drift time about 4.5 times higher than those with a CS.

Behaviour of a desolvation system based on microwave radiation heating for use in inductively coupled plasma atomic emission spectrometry

Abstract The present paper describes the preliminary results obtained with a desolvation system for inductively coupled plasma atomic emission spectrometry that incorporates a heating unit based on microwave (MW) radiation. This system has been called Microwave Desolvation System (MWDS). The results have proved that MW radiation can be considered as a good choice for aerosol heating in a sample introduction system. MW radiation seems to be a more uniform way of aerosol desolvation than conductive/convective heating (i.e. lower radial temperature gradients), the degree of vaporization of the droplets is less dependent on the liquid flow rate ( Q l ), and also the background noise associated with the vaporization of droplets is reduced. As regards the results obtained with MWDS, in comparison with a conventional desolvation system (CDS), they are very dependent on Q l . When heating is applied, the amount of analyte that leaves the heating step increases by 30–60% with the MWDS, irrespective of Q l , whereas for the CDS this increase is very high (up to 300%) at low Q l values (0.4 ml min −1 ), but almost negligible at high Q l values (2.4 ml min −1 ). In agreement with this, the analytical figures of merit are favourable to the CDS at low flow rates, and to the MWDS at high liquid flows. Under all the conditions studied, the amount of solvent that leaves the condensation unit are lower for MWDS than for CDS.

New ultrasound assisted chemical oxygen demand determination.

A new method for the chemical oxygen demand (COD) determination assisted by ultrasonic radiation has been evaluated for the first time. The suggested method uses an instrumentation simpler and cheaper than the previous ones used for the same purpose. With this preliminary version of the method the optimized experimental conditions are: high ultrasonic power (143 W, 95% of maximum nominal power; 126.5 W/cm2 of power density); high sulphuric acid concentration (> 60%); and a sonication time of 2 min. Under these conditions this first prototype shows the same limitations as the official COD method as regards the type of organic compounds. It works successfully with easily oxidative organic matter (sodium oxalate, glucose and salicylic acid) but the COD values obtained with more difficult organic matter are poor. When the ultrasonic assisted method is applied to real waste waters the precision is statistically not different to that of the conventional semi-micro method but the COD values obtained lie between 50 and 60% of the values obtained with the conventional semi-micro method. Hence, the use of the ultrasonic radiation for COD determination seems to be an interesting and promising alternative to the conventional ones but still much more research efforts must be done in order to improve the instrumental set-up. The suggested direction of these developments must be a more efficient interaction between the ultrasonic radiation and the sample.

Electrothermal vaporization of mineral acid solutions in inductively coupled plasma mass spectrometry: comparison with sample nebulization

Abstract The analytical behaviour of an electrothermal vaporization (ETV) device for the introduction of mineral acid solutions in inductively coupled plasma mass spectrometry (ICP-MS) was evaluated. Water, nitric acid, hydrochloric acid, perchloric acid and sulphuric acid in concentrations within the 0.05–1.0 mol l −1 range were studied. For all the acids tested, increasing the acid concentration increases the ion signal and deteriorates the precision. The magnitude of the signal enhancement depends on the analyte and on the acid considered. Acid solutions give rise to ion signals that are between 2 and 10 times higher than those with water. Among the acids tested, sulphuric acid provides the highest signals. The addition of palladium reduces matrix effects due to the acids and increases the signal in ETV ICP-MS. In comparison with conventional sample nebulization (CS), the ETV sample introduction system provides higher sensitivities (between 2 and 20 times higher) at the same acid concentration. The magnitude of this improvement is similar to that obtained with a microwave desolvation system (MWDS). The ETV sample introduction system gives rise to the lowest background signals from matrix-induced species. Due to this fact, the limits of detection (LODs) obtained for the isotopes affected by any interference are lower for ETV sample introduction than those obtained with the CS and the MWDS. For the isotopes that do not suffer from matrix-induced spectral interferences, the ETV gives rise to LODs higher than those obtained with the CS. For these isotopes the lowest LODs are obtained with MWDS.

Microwave desolvation for acid sample introduction in inductively coupled plasma atomic emission spectrometry

Abstract This study deals with the behaviour of a microwave desolvation system (MWDS) with acid solutions in inductively coupled plasma atomic emission spectrometry. Hydrochloric, nitric, sulphuric and perchloric acids at different concentrations (up to 0.6 mol l −1 ) have been tested. Sample uptake rate ( Q l ) was also varied. The parameters evaluated for each variable were analyte and solvent transport rates and emission intensity. The combination of low acid concentrations (0.05–0.1 mol l −1 ) and low liquid flows (0.4 ml min −1 ) leads to the highest analyte transport rate and emission signal and to the lowest solvent transport rate. For Q l higher than 1.9 ml min −1 , the use of an impact bead is advisable. Among the acids tested, sulphuric and perchloric acids give rise to higher emission intensities than hydrochloric acid and nitric acid. Nonetheless, the limits of detection (LODs) obtained with the MWDS are about the same magnitude irrespective of the solution employed. The LODs reached when using the MWDS are similar to those obtained with a desolvation system based on infrared heating of the aerosol.

Insight into the interaction of the microwave radiation with droplets of interest in analytical chemistry

Abstract The idea of heating aerosols by applying a microwave field has recently been taken up again for liquid sample introduction in atomic spectrometry. Heating of the droplets can take place directly, because of microwave energy absorption by the droplets themselves, or indirectly, as a result of heat transmission from the radiated parts of the system to the droplets by conduction, convection or thermal radiation. There is some degree of discrepancy among the specialists on this subject, i.e., about the contribution of each of these mechanisms to the temperature of the aerosol. Some of them state that there is no direct coupling between the microwave radiation and the droplets, only indirect heating. However, we claim that both ways of heating occur. In this work we support our point of view on the basis of the existing bibliographical background, a theoretical analysis and our own experimental results.

Desolvation of acid solutions in inductively coupled plasma atomic emission spectrometry by infrared radiation. Comparison with a system based on microwave radiation

Abstract The behaviour of an infrared desolvation system with acid solutions in inductively coupled plasma atomic emission spectrometry (ICP-AES) is evaluated, and the influence of the liquid uptake rate and of the nature and concentration of the acid on the solvent and analyte transport rates and on the analytical figures of merit is studied. The results are compared with those obtained with a desolvation system based on the absorption of microwave radiation. The infrared desolvation system performs best at low sample uptake rates (0.4 ml min −1 ) and its behaviour strongly depends on the nature and concentration of the solution used. With nitric and hydrochloric solutions, there is almost no effect of the acid concentration on the emission intensity, while for sulfuric and perchloric acids the signal decreases as the acid concentration is increased. These effects seem to be related with the different capability of the acid aerosols to be heated in an IR field. The microwave desolvation system seems to be more prone to matrix (acid) effects, specially when using sulfuric and perchloric acids, resulting in emission intensities which are usually lower than those obtained with the infrared desolvation system, though their limits of detection are quite similar.