Stir Bar Sorptive Extraction: Capacity and Competition Effects
The determination of volatile and semi-volatile analytes in aqueous solutions using stir bar sorptive extraction (SBSE) as the extraction step is gaining acceptance in a wide variety of application areas including water and waste water analysis, beverages and other consumer products. In this study physical capacity of the GERSTEL Twister stir bar was determined gravimetrically by concentrating hexadecane from aqueous isopropanol solutions. Under optimal conditions, the stir bar absorbed milligram quantities of hexadecane. From a practical perspective, the highest analyte concentration that still provides a linear calibration response could be defined as the working capacity of the stir bar. For typical analytes, this upper limit was found to be in the ppm range. To assess competition and displacement effects, model compounds including a pesticide mix and methyl esters were extracted from water with Twister or SPME in the presence of up to 5 ppm limonene. Under these conditions, the most polar analytes showed the greatest reduction in peak area when extracted with SPME. Detection limits determined for model compounds showed a 10 to 25-fold advantage of the increased capacity of the Twister phase compared to SPME.