Headspace solid-phase microextraction of sulphides and disulphides using Carboxen–polydimethylsiloxane fibers in the analysis of wine aroma

doi:10.1016/S0021-9673(98)01050-4

Journal of Chromatography A

Volume 835, Issues 1–2, 12 March 1999, Pages 137-144

https://doi.org/10.1016/S0021-9673(98)01050-4 Get rights and content

Abstract

Headspace solid-phase microextraction was applied to gas chromatography coupled to flame photometric detection to develop a method for analysing volatile sulphides and disulphides in wine. The Carboxen–polydimethylsiloxane-coated silica fiber was tested and different parameters such as presampling time, ionic strength, stirring, headspace volume, ethanol concentration, time and temperature of extraction were optimized to make extraction as efficient as possible. The optimized conditions enabled limits of detection to be obtained at the ng/l levels. The fiber tested has a strong affinity for the sulphur compounds studied and enables these analytes to be quantitatively determined in wines. The Carboxen–polydimethylsiloxane-coated fiber is more efficient at extracting than fibers such as those which are polydimethylsiloxane-coated and polyacrylate-coated, but its repeatability is worse. The overall process was successfully applied to identify and quantify sulphur compounds in white, red, rosé and vintage wines.

Introduction

Sulphur is a constituent of the amino acids cysteine, cystine, methionine and glutathione. Via Maillard or Strecker reactions, these substances may be degraded into different kinds of sulphur compounds that are present in a great deal of foods and beverages. Many of these analytes, particularly those of high volatility, are powerful odorants with very low sensory thresholds [1]. In wines, the presence of these compounds is usually considered as an off-flavour and, according to the literature, the concentration of these compounds may be affected by the cloudiness of the grape juices [2], the sulphur containing pesticides used 3, 4, thermal and photochemical reactions 4, 5and ageing [6].

Since they are commonly found at trace levels, sulphur compounds need to be identified and quantified by such sensitive techniques as gas chromatography (GC) coupled to flame photometric (FPD) or sulfur chemiluminescent detection (SCD). However, before the chromatographic analysis, the analytes have to be preconcentrated. The techniques most widely used for the determination of sulphur compounds, in wines and other alcoholic beverages, are liquid–liquid extraction 7, 8, direct static headspace (HS) 8, 9, static HS with cryogenic trapping 10, 11, and dynamic HS (purge and trap) 12, 13.

Solid-phase microextraction (SPME) is a recently developed technique 14, 15which uses a polymer-coated silica fiber to extract analytes from a variety of matrices and, directly, transfer them into the injector of a GC system for thermal desorption and analysis.

SPME was originally developed for sampling organic contaminants in water by direct immersion of the fiber into the sample [16], but nowadays it is also applied to the headspace above solid or liquid samples (HS-SPME) in food analysis 17, 18, 19, 20. Polydimethylsiloxane (PDMS) and polyacrylate (PA) fibers have already been used to analyse the volatile sulphur compounds present in wines 21, 22. This study tests the suitability of the Carboxen–polydimethylsiloxane (CAR–PDMS) fiber. This is a new recently marketed fiber which seems to have a strong affinity for the sulphur compounds [23].

Section snippets

Chemicals and reagents

The volatile sulphur compounds studied were: dimethyl sulphide (Me₂S) [75-18-3], diethyl sulphide (Et₂S) [352-93-2], methyl-n-propyl sulphide (MeSPr) [3877-15-4], methyl thioacetate (MeSAc) [1534-08-3], ethyl thioacetate (EtSAc) [625-60-5], carbon disulphide (CS₂) [75-15-0], dimethyl disulphide (Me₂S₂) [624-92-0], diethyl disulphide (Et₂S₂) [110-81-6]. Ethylmethyl sulphide (MeSEt) [624-89-5] and thiophene [110-02-1] were used as internal standards (I.S.s).

MeSEt and Et₂S₂ were supplied by

Results and discussion

The CAR–PDMS (75 μm) fiber is coated with porous carbon which makes it possible to use SPME to analyse volatile analytes at trace levels. This fiber has been used to extract volatile organic compounds from water and air 23, 25, and sulphur gases [23]. In this study, this new fiber was used to analyse eight volatile sulphur compounds commonly found in wines.

In the analytical method developed, several variables before the sample injection were optimized. The experiments were carried out with five

Conclusions

HS-SPME using the CAR–PDMS fiber is a good technique for determining the sulphur compound content in wines at ng/l levels. This simple, solventless and fast technique enables volatile and less volatile sulphur compounds to be determined simultaneously without analyte loss. CAR–PDMS makes the technique more sensitive than when other coated fibers are used, but matrix interferences have to be taken into account, and a specific synthetic wine must be used to validate the method.

Acknowledgements

The authors wish to thank CICYT (project ALI97-0765) for the financial support given.

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Headspace solid-phase microextraction of sulphides and disulphides using Carboxen–polydimethylsiloxane fibers in the analysis of wine aroma

Abstract

Introduction

Section snippets

Chemicals and reagents

Results and discussion

Conclusions

Acknowledgements

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr.

J. Chromatogr.

J. Chromatogr. A

J. Int. Sci. Vigne

Sci. Aliment.

Wine Ind. J.

Ital. J. Food Sci.

Analusis

Am. Soc. Brewing Chem.