Evaluation of Sequential-SBSE and TF-SPME Extraction Techniques Prior to GC-TOFMS for the Analysis of Flavor Volatiles in Beer

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.