Effect of Ethanol on the Adsorption of Volatile Sulfur Compounds on Solid Phase Micro-Extraction Fiber Coatings and the Implication for Analysis in Wine

Michael C. Qian,Peter M. Davis

doi:10.3390/molecules24183392

Abstract

Complications in the analysis of volatile sulfur compounds (VSC) in wine using solid-phase microextraction (SPME) arise from sample variability. Constituents of the wine matrix, including ethanol, affect the volatility and adsorption of sulfur volatiles on SPME fiber coatings (Carboxen- polydimethylsiloxane(PDMS); DVB-Carboxen-PDMS and DVB-PDMS), which can impact sensitivity and accuracy. Here, several common wine sulfur volatiles, including hydrogen sulfide (H2S), methanethiol (MeSH), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), diethyl disulfide (DEDS), methyl thioacetate (MeSOAc), and ethyl thioacetate (EtSOAc) are analyzed, using SPME followed by gas chromatography (GC), using a system equipped with a pulsed-flame photometric detection (PFPD) system, at various ethanol concentrations in a synthetic wine matrix. Ethyl methyl sulfide (EMS), diethyl sulfide (DES), methyl isopropyl sulfide (MIS), ethyl isopropyl sulfide (EIS), and diisopropyl disulfide (DIDS) are evaluated as internal standards. The absorption of volatile compounds on the SPME fiber is greatly affected by ethanol. All compounds exhibit a stark decrease in detectability with the addition of ethanol, especially between 0.0 and 0.5% v/v. However, the ratio of interested sulfur compounds to the internal standard becomes more stable when the total alcohol concentration exceeds 2%. EMS was found to best resemble DMS. EIS and DES were found to best resemble DMDS, MeSOAc, and EtSOAc. DIDS was found to best resemble DEDS, DMTS, H2S, and MeSH.

Highlights

Volatile sulfur compounds (VSC), often having very low odor thresholds, are responsible for many common off-flavors in wine [1]
A coated fiber is extended into the headspace of a sample vial, allowing a finite number of volatiles to adsorb until later thermal desorption
It was postulated that the ethanol competed with adsorption of small VSCs on the active Carboxen site, such that the addition of ethanol could dramatically decrease the absorption of small VSCs on the fiber

Summary

Introduction

Volatile sulfur compounds (VSC), often having very low odor thresholds, are responsible for many common off-flavors in wine [1] Such compounds often have offensive odors, imparting notes of cabbage, onion, garlic, or rubber to wines [1,2,3,4,5,6]. The relatively minute scale of the fiber0 s capacity compared to the over sample ensures that the removal of a small fraction of the volatile content will not disturb the sample equilibrium. This allows for very fast equilibration between the air-fiber interface as compared to static headspace analyses [9]. Due to the limitation of space for volatiles to adhere to the fiber, the presence of other volatiles from the matrix can interfere with analysis [10]

Methods

Results

Conclusion