Multi-stir bar sorptive extraction for analysis of odor compounds in aqueous samples

As reproducible coating of stir bars with more polar phases was found to be very difficult, a supporting grid was used in the development of an ethyleneglycol-modified Silicone (EG Silicone) coated stir bar. This new polar coating showed good performance for the extraction of polar solutes, but long term use also showed degradation of the coating due to friction while stirring. In order to address the lower robustness of the EG Silicone stir bar which has a much softer coating compared to a conventional polydimethylsiloxane (PDMS) stir bar, a novel SBSE procedure termed multi-SBSE (mSBSE) was developed. mSBSE consists of the robust PDMS stir bar stirring at the bottom of the vial and the EG Silicone stir bar attached on the inner side wall of the vial (a magnetic clip is used for the set-up). After extraction, the two stir bars are placed in a single glass desorption liner and are simultaneously thermally desorbed. The desorbed compounds were analyzed by thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS). Compared to conventional SBSE, mSBSE provides more uniform enrichment of a wide range of odor compounds in aqueous sample since both stir bars can complement each other, while eliminating the damage of the EG Silicone phase during the extraction. The robustness of the EG Silicone stir bar was dramatically increased and more than 30 extraction and desorption cycles were possible without loss in performance. The recoveries for polar solutes such as 2-acetyl pyrrole (log Kow: 0.55), benzyl alcohol (log Kow: 1.08), guaiacol (log Kow: 1.34), and indole (log Kow: 2.05) were increased by a factor of about 2–7. The mSBSE–TD–GC–MS method showed good linearity (r2 > 0.9913) and high sensitivity (limit of detection: 0.011–0.071 ng mL−1) for the test compounds spiked in water. The feasibility and benefit of the method was demonstrated with analysis of odor compounds in roasted green tea. The normalized areas obtained from mSBSE showed the best enrichment for most of the selected compounds compared to conventional SBSE using the PDMS stir bar or the EG Silicone stir bar. Fifteen compounds were determined in the range of 0.15–210 ng mL−1 (RSD < 14%, n = 6).