Abstract
Sequential-Stir Bar Sorptive Extraction (Seq-SBSE) was investigated as a technique for the extraction and concentration of volatiles in beer prior to gas chromatography–time-of-flight mass spectrometry (GC–TOFMS) analysis for standard solutions prepared in an imitation beer solvent. Twenty-seven beer analytes across a range of polarities (Ko/w’s) were investigated. The technique provided acceptable quantitation for most of the 27 analytes investigated, but calibration curves for five of the most polar analytes were nonlinear. Thin Film SPME (TF-SPME) sample preparation, which has recently been shown to provide quantitative extraction of polar analytes, was compared to Seq-SBSE results for the standard solutions. TF-SPME provided linear calibration curves for all 27 analytes. To compare test precision, a commercial sample of beer was analyzed in triplicate by both techniques. Eighty-five analytes were detected by Seq-SBSE with an average percentage of relative standard deviation (%RSD) of 9.7% and 95 analytes were detected by TF-SPME with an average %RSD of 8.9%. Triplicate samples from three brands of commercial beers at the end of shelf life were analyzed. Ten potential off-flavor chemicals analyzed by TF-SPME were subjected to principal component analysis. Examination of principal component analysis (PCA) loadings results showed the most significant concentration differences in off-flavor chemicals in the different brands occurred with E-2-nonenal, 3-methyl-2-butene-1-thiol, and vinyl guaiacol. Studies showed that some analytes showed better detection sensitivity by Seq-SBSE, while other analytes were detected with higher sensitivity by TF-SPME. Linear least squares correlation coefficients for calibration curves were higher for Seq-SBSE except for highly polar volatiles, while TF-SPME was able to quantitatively detect more chemicals including highly polar ones. TF-SPME appears to be an excellent supplemental technique for the analysis of beer volatiles.
Acknowledgments
The authors are grateful to Jack Stuff, Ph.D., and Jackie Whitecavage at GERSTEL Inc. for their support and suggestions with implementing TF-SPME technology as well as to Gary Spedding, Ph.D., BDAS, LLC, for his helpful discussions.