Chromatographic analysis of volatile sulphur compounds in wines using the static headspace technique with flame photometric detection

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Abstract

This study describes the development of a method for determining eleven sulphur compounds in wine, which takes into account that thiols are easily oxidizable. The equilibria of the analytes between air and aqueous ethanol were studied and optimised using static headspace gas chromatography in order to obtain the best sensitivities. The influences of parameters such as temperature, time, ionic strength, headspace volume and the volume of headspace injected were determined. A cryogenic trap was used to concentrate the headspace analytes and they were chromatographically analysed using GC temperature programming on a poly(ethylene glycol) capillary column with FPD detection at 394 nm. The power relationship was observed between the chromatographic response and a concentration of sulphur compounds in the range 2–150 μg l−1 in the sample. Recoveries were determined by the standard addition technique and were higher than 90% for sulphides and disulphides and close to 80% for thiols. The overall method was succesfully used to determine the sulphur compounds in white and red wines.

Introduction

For many years it has been known that sulphur compounds (S-compounds) are present in a variety of foods, including cheese [1], fish [1], poultry [1], meat [2]and mushrooms [3], as well as in wine 4, 5, whisky 6, 7, beer 8, 9and some other alcoholic beverages [10].

These compounds can be classified according to their molecular mass. Compounds which contain sulphur and which have a low molecular mass, such as thiols (R-SH), sulphides (R-S-R) and disulphides (R-S-S-R), have a considerable influence on the aroma of foods and beverages, even at trace amounts. This is because of their high volatility and low sensory threshold. This last property is an aspect that should be taken into account for thiols, because they are very oxidizable and quickly convert into disulphide forms in the presence of oxygen. As a result, making standard solutions and working with samples which contain thiol is cumbersome and requires special care.

In the case of wine, some of these compounds, such as dimethyl sulphide, may have a beneficial effect 11, 12, but generally the influence of those compounds is considered to be negative and they contribute to strong, objectionable flavours. Sulphurous off-flavours, which cause some of the major defects in the quality of wine aroma, [13], lead to unpleasant tastes and odours which can be described as rubber, onion, garlic, cabbage, and so on 9, 14, 15.

Different S-compounds have been identified in wine 5, 11, 16. Their concentration depends on the conditions of wine production, because they have different origins: natural causes [17], the use of some S-containing phytosanitary products 13, 18, thermal treatments (Maillard and Strecker reactions) 2, 14, production during alcoholic fermentation 19, 20and photochemical reactions [15].

Amperometric, colorimetric, fluorimetric, gas chromatographic, potentiometric and titrimetric techniques have all been used to determine these compounds 8, 21, 22. However from the point of view of sensitivity, specificity and reliability, the usual method is gas chromatography with sulphur-specific detection: flame photometric detection 4, 5or chemiluminescence detection 23, 24.

Because of the low boiling point of some of the S-compounds studied, liquid–liquid or solid-phase extraction could not be used as the concentration technique. So, the most widely used techniques are headspace, either static 5, 23or dynamic [25], and purge and trap 26, 27.

For the procedure proposed here, the static headspace technique with cryogenic trap was tested to improve the recoveries for the determination of volatile S-compounds. The method was then used to determine these compounds in red and white wines.

Section snippets

Chemicals and reagents

The S-compounds studied were: hydrogen sulphide [7783-06-4], methanethiol [74-93-1], ethanethiol [75-08-1], dimethyl sulphide [75-18-3], diethyl sulphide [352-93-2], methyl-n-propyl sulphide [3877-15-4], methyl thiolacetate (S-methyl acetate) [1534-08-3], ethyl thiolacetate (S-ethyl acetate) [625-60-5], carbon disulphide [75-15-0], dimethyl disulphide [624-92-0] and diethyl disulphide [110-81-6]. Ethyl-methyl sulphide [624-89-5] and thiophene [110-02-1] were chosen as internal standards (istd),

Results and discussion

Volatile S-compounds were chromatographically analysed using the headspace technique. Therefore, different parameters were studied in order to obtain the best sensitivities. The parameters optimised were: temperature, time, ionic strength, volume of headspace and volume of headspace injected. EDTA (50 mg/10 ml wine) was added to form complexes with the metals, so avoiding their catalytic effect in the oxidation of thiols [29].

The first parameter examined was the temperature of the sample vials.

Conclusions

The method developed appears to be suitable for determining a relatively large number of usual volatile S-compounds in wines. The interference of other compounds in a matrix as complex as the wine was eliminated by using a specific detector and S-compound low concentrations could be detected by concentrating the headspace with a cryogenic trap.

Furthermore this new procedure for preparation and validating standard thiol solutions enables accurate concentration standards of these volatile and

Acknowledgements

The authors wish to thank CICYT (project ALI 94-0505) for the financial support given.

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