Gerstel: Intelligent Automation for GC/MS and LC.MS

Analysis of Aroma Compounds in Edible Oils by Direct Thermal Desorption GC/MS using Slitted Microvials

INTRODUCTION
The determination of aroma compounds in edible oils is important for manufacturers and vendors of these products. Off-flavors derived from unsaturated fatty acid degradation are of interest. These compounds can compromise the taste and quality of a product even in the ng/g concentration range. Sensitive and fast analysis methods, ideally combined with simple sample preparation, are needed.

In 2008, GERSTEL developed a sensitive analysis method employing dynamic headspace sampling. At that time direct thermal desorption from standard microvials placed in TDU thermal desorption tubes was also tested. Although the technique performed well for volatile analytes, the resulting sensitivity for high boiling compounds was not satisfactory. In this project we returned to the microvial approach and evaluated different microvial designs to improve the transfer of high boiling compounds while maintaining the excellent performance for the volatile
fraction.

EXPERIMENTAL
Instrumentation. Thermal desorption of oil samples placed in microvials was conducted in the GERSTEL Thermal Desorption Unit (TDU). Analytes were refocused in a GERSTEL Cooled Injection System (CIS 4), PTV-type inlet, before being transferred to the GC column. A 7890/5977 GC/MS system from Agilent Technologies was used for separation and detection of the analytes of interest. Thermal desorption tubes containing the samples were delivered to the TDU automatically using a GERSTEL MultiPurpose Sampler (MPS).

Materials. Standard empty TDU tubes with single notch were used for thermal desorption. Oil samples were placed in either standard microvials or newly designed microvials with a slit located 1 cm from the bottom. Both styles of microvials are commercially available from GERSTEL.

RESULTS AND DISCUSSION
Thermal extraction of aroma compounds from edible oils employing microvials was shown to work well. The microvial prevents contamination of the analysis system by high boiling matrix compounds while allowing effective transfer of analytes onto the analytical column. After sample processing the microvial can be disposed of and the desorption tube is ready to take up the next sample.

In this study the performance of different microvial designs was investigated. A relatively short desorption time of 15 min was chosen to clearly reveal differences. It turned out that microvials with the slit placed at a height of 1 cm from the bottom were the most effective for analyte transfer – especially for high boiling compounds.

Relative standard deviations for 10 repeat measurements with the 1 cm slitted microvial were between 7.2 and 16.8% with a median of 9.7%. This is highly acceptable considering the complex matrix, the low concentrations and the straightforward sample preparation. A longer desorption time would likely improve the relative standard deviations further.