Ultratrace Determination of Aroma-Active Thiols by Automated Headspace Solid-Phase Microextracion With In-Fiber Derivatisation and GC-ECD and GC-MS-NCI

By: Laura Mateo-Vivaracho Vicente Ferreira and Juan Cacho, Reporter EU Volume 20

Laura Mateo-Vivaracho, Vicente Ferreira and Juan Cacho, Departamento de Química Analítica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, España

Thiols very often have strong odors. Low molecular weight members are extremely unpleasant, but some higher molecular weight members at low concentrations have powerful and penetrating aromas responsible for the sensory characteristics of numerous products, such as mango, passion fruit, grapefruit, coffee and some wines. The analysis of these compounds is extremely difficult because of the low concentrations at which they must be determined (ng L-1 range), their reactivity and instability and their poor chromatographic and spectrometric properties. Present methods are long and tedious and require many steps.

In the present work, the applicability of a fast and automated headspace solid phase microextraction (HS-SPME) with in fiber derivatization with 2,3,4,5,6-pentafluorobenzyl bromide (PFBBr) has been studied and applied to wine. Studied analytes were 2- methyl-3-furanthiol, 2-furanmethanethiol, 4-mercapto-4-methyl- 2-pentanone, 3-mercaptohexanol and 3-mercaptohexyl acetate. For this work poly(dimethylsiloxane)-divinylbenzene (PDMS-DVB) SPME fibers at 65 μm thickness were used. Method optimization has been carried out by using both GC-FID and GC-electron capture detection (ECD) and the final analytical determination is based on GC-MS in the Negative Chemical Ionization (NCI) mode.

One of the most critical steps of the method setup is to transfer an adequate and reproducible amount of derivatization reagent to the fiber. Different alternatives have been developed, making possible to transfer variable amounts ranging from 0.6 to 200 nmol of reagent to the fiber. An excess of reagent can cause serious contamination problems, while a deficit makes the response unstable. A value of 3 nmol has been selected as compromise. This is achieved by exposing the fiber to the vapors of 10 mL of a 200 ppm solution (10 % acetone) of the derivatization reagent at 55oC for 5 minutes.

Different parameters that impact the process were investigated to optimize the analytical signal. Apart from the mass of reactive loaded, the stirrer speed, the time and temperature of the extraction-derivatization, the effect of salt, and the number of injection cycles that can be performed from one vial of the PFBBr reagent have been investigated.

In general, it has been found that the critical parameter and the formation of derivatives, and therefore, high temperatures, times increase the analytical signal. Conditions of 55oC, 10 min and 250 rpm have been selected as the best compromise for getting good signals and not ruining the chromatographic performance. The method proposed is extremely sensitive, and it is possible to get clear signals for three analytes at concentrations below 0.1 ng L-1 when using GC-MS in NCI mode. The method is also repeatable (rsd 12%), and the response is not matrix-sensitive because it does not differ between white and red wines. Other aspects considered in the method setup were the oxidation of analytes during the process.

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Acknowledgments

This work has been funded by project AGL2004-06060/ALI. Lab. Análisis del Aroma y Enología. Dep. Analytical Chemistry, University of Zaragoza. Pza. San Francisco, s/n, 50009 Zaragoza, Spain.

* Contact: Laura Mateo
E-mail: laura.mateo@unizar.es Tel.: 976761000 Ext. 3509

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