Spectroscopy Focus Analysis of Wine by direct Aspiration and Radial ICP-OES


The ability to determine trace elements in wine utilising direct aspiration without prior sample digestion was demonstrated using the Spectro ARCOS with radial plasma observation. Other alcoholic beverages can be analysed using the same methodology. It could be shown that the required sensitivity to fulfil the regulations can be met. Excellent results were obtained for spike recovery measurements performed for a variety of different wines.


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Without the usually required digestion procedure, preparation related errors are eliminated; the total analysis time is drastically reduced.


Today, wine analysis helps the wine maker to determine the maturity of the grapes...


Author Details:


Olaf Schulz, Product Manager, Spectro Analytical Instruments GmbH, Germany


1. Introduction


Over production and a growing consumer demand for quality wine has lead to optimised production methods. Scientific, enological methods are standard practice - the times when wine makers relied on inherited experience belong to the past.


Today, wine analysis helps the wine maker to determine the maturity of the grapes, to control of the fermentation process, the clarity of the wine, the taste and the shelf life and thus to guarantee a high, equal quality of the final product.


Wine is currently among the most popular alcoholic beverages. In many cultures it is part of the daily diet. Its good effect to prevent cardio vascular deceases, if consumed moderately, is described in many publications. As a natural food, wine is influenced by the surrounding environmental conditions, however. In order to avoid that elements or substances are contained at toxic, harmful levels, maximum concentrations are set for pesticides, insecticides, toxic elements and substances allowed for the production of wine in national and international regulations.


In Europe, Germany, Australia and the US the following norms apply:


• European Commission Regulations EC466/2001 [1] and EC1622/2000 [2]


• German Wine Regulation [3] • Australian Wine Production Standard [4] • US 27 CFR Part 24 Segment C [5]


The elemental analysis of wine includes the toxic heavy metals, As, Cd and Pb, but also elements influencing the quality of the wine like Fe, Zn, Cu and Mn. For instance, as per the German wine regulation, the maximum limits allowed are:


Table 1. Limiting values as per the German Wine Regulation


Limiting Values [mg/l]


Al


As Pb


B (H3BO3) Br (total) Cd Cu Zn Sn


8 0.1


0.25 80 1


0.01 2 5 1


While the determination at such levels principally does not present any difficulty for a modern ICP-OES, this is different when the samples need to be digested prior to the analysis, which typically means a 100 fold dilution.


Digestion is applied, since the analysis utilising direct aspiration is difficult and includes a number of challenges, namely:


• Wine is a partly volatile matrix;


• Alcohol and sugars cause molecular interferences in relevant spectral ranges;


• The strongly structured spectra makes the application of conventional background correction methods impractical;


• Conventional interference correction methods can neither be applied;


• The concentration of alcohol and sugar ‘cannot’ be quantified; and


• The level of interference strongly changes with the alcohol and sugar content of the wine.


For these reasons, in order to achieve the required sensitivity, GFAA and ICP-MS are currently common analysis methods.


The present report investigates the analysis of wine by direct aspiration utilising a novel pixel-intensity dilution based correction approach. It considers all matrix relevant components and, even though the composition is not precisely known, the relevant spectral effects are corrected. Alcoholic beverages (wine) represent a good example, since the described varying concentrations of alcohol, sugar, and flavouring substances, without correction, strongly limit calibration and sample analyses.


2. Experimental


2.1 Instrumentation All measurements were performed with the Spectro ARCOS optical emission spectrometer (Spectro Analytical Instruments, Kleve, Germany) with radial plasma observation. The Spectro ARCOS features a Paschen- Runge spectrometer mount, employing the proprietary Optimised Rowland Circle Alignment (ORCA) technique. Consisting of two hollow section cast shells, optimised small volume and 32 linear CCD detectors, the wavelength range between 130 and 770 nm can be simultaneously analysed, allowing complete spectrum capture within 2s. Due to the unique reprocessing capabilities of the system, a new measurement is not required even if additional elements or lines are to be determined at a later date.


The optic is hermetically sealed and filled with argon, continuously circulated through a filter, which absorbs oxygen, water vapour and other species. High optical transmission in the VUV is achieved, allowing the determination of non-metals as well as the use of prominent and interference free lines in this region.


Utilising Spectro’s patented ‘Intelligent Calibration Logic (ICAL)’, which normalises the wavelength scale, the state of the optical system is automatically monitored.


An air-cooled ICP-generator, based on a free-running 27.12 MHz system, is installed, which ensures excellent stability of the forward power even in the case of rapidly changing sample loads. All relevant ICP operating parameters are software controlled, allowing easy selection of the optimum operating conditions. For sample introduction, a jacketed cyclonic spray chamber, cooled to 0°C and a Burgener T2002 nebuliser were used. The ICP operating parameters are given in Table 2.


Table 2. ICP Operating Conditions. Power


Coolant flow Auxiliary flow Nebuliser flow


Sample aspiration rate Plasma Torch


Spray Chamber chamber


Replicate Read time


1500 W 20 L/min 1.0 L/min 0.85 L/min 1.0 mL/min Quartz, fixed


1.8 mm Injector tube Tracy jacketed cyclonic spray 26 sec/replicate


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