by the use of a TriPlus autosampler (Thermo Electron Corporation). Helium (purity 99.999%) was used as a carrier gas at a constant flow of 1 mL/min. A split–spitless injector set at 220 °C was always used, and injections of 2 μL were performed in the splitless mode.A surge pressure of 92 kPa was applied after the injection for 0.5 min.