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Min J. Yang, Maureen L. Orton, and Janusz Pawliszyn*
Caffeine occurs naturally in tea, coffee, and cola nuts. Caffeine analysis is performed for quality control purposes, for example to insure proper caffeine levels in decaffeinated coffee to meet regulatory standards. Current methods for determining caffeine in beverages require pH adjustments and often involve the use of toxic organic solvents. These methods are labor-intensive, generate large quantities of organic waste, and have relatively poor precision. Alternatively, solid-phase microextraction (SPME) can be used. The SPME technique has many advantages including simplicity, portability, time efficiency, sensitivity, and compatibility with a GC system. Initial work on SPME caffeine analysis has been reported (1). The SPME principle is based on an equilibrium process in which the analyte partitions between the fiber and the aqueous phase (2, 3). Many applications of SPME have been investigated (4-8), and a recent review provides a good overview (9). The main objective of this paper is to describe a simple undergraduate experiment for determination of caffeine in common beverages using the combined SPME-GC/MS methods.
Experimental Procedure Materials and Equipment
The SPME device was purchased from Supelco (Supelco Canada, Mississauga, ON). For caffeine analysis, an uncoated fused silica fiber was used for extraction. Commercial fibers are often supplied with a coating such as poly(dimethylsiloxane) (PDMS) and polyacrylate. Uncoated fiber can be prepared by dissolving the fiber coating in hot sulfuric acid.
Two 5-mg/mL stock solutions of regular ^sup 12^C caffeine (Aldrich Chemical Co, Inc., Milwaukee, WI) and isotopically labeled (trimethyl ^sup 13^C) caffeine (Cambridge Isotope Laboratories, Woburn, MA) were separately prepared in methanol. Regular coffee, decaffeinated coffee, and teas were brewed as for normal consumption. Soft drinks were taken from beverage containers.
A Varian 3400 gas chromatograph equipped with a Saturn II ion trap MS (Varian, Mississauga, ON) detector was used. The column was a 30 m x 0.25 mm SPB-5 with a stationary phase thickness of 0.25 (mu)m (Supelco Canada, Mississauga, ON).
Extraction Procedures
Many different versions of SPME procedures are described; which one to use depends on the application (4-8). For this experiment, the adsorption period was set for 5 minutes. The sample was magnetically stirred during extraction (Fig. 1). After the adsorption step, the fiber was directly transferred into the GC injector...