SPME Analysis. The procedure developed by Hora´k et al. (25) was
used with minor modifications. A manual SPME device and carbowax/
divinylbenzene (CW/DVB) 65 μm fibers were obtained from Supelco Co.
(Bellefonte, PA).
Before use, the fiber was preconditioned in the GC injection port at
250 C for 0.5 h. Beer sample (4 mL) with addition of heptanoic acid as
internal standard (final concentration=1.3 mg/L) and 2 g of ammonium
sulfate was placed in a 20 mL vial closed by a polytetrafluoroethylene
(PTFE)/silicone septumpurchased fromChromacol (Herts, U.K.). Before
extraction, the vial was shaken manually for 10 s. Headspace SPME was
carried out at 25 C (ambient temperature).Asampling time of 30min was
used for the analysis. Each analysis was carried out three times.
After exposure, the fiber was thermally desorbed into a GC and left in
the injection port at 250 C for 5 min. The injector was equipped with a
0.75 mm i.d. inlet liner. The same chromatographic parameters were used
as in the SPE method.