A Comparison of Cryofocusing Injectors for Gas Sampling and Analysis in Fast GC

The performance of two cryofocusing injectors for fast gas chromatography has been studied. The first system traps analytes onto bare metal tubes and rapidly vaporizes them upon ballistically heating the tube using a capacitance discharge. The second is a microloop injector in which analytes are cryotrapped onto short lengths of narrow-bore fused silica tubing with various coatings. The ballistically heated injector is capable of sampling and injecting compounds from air faster than the microloop system, because the metal tube can be heated and cooled more rapidly. Both systems are capable of cryotrapping compounds as volatile as butane at -90 °C, and the microloop system can trap ethane when a section of a porous layer open-tubular (PLOT) column is used as the sample loop. In addition, the microloop injector can be used without cryointegration to analyze compounds regardless of their volatility, as long as they are present in the samples at detectable concentrations. Because the ballistically heated injector is flushed prior to injection, it can introduce only compounds that are adsorbed onto its metal trap. Comparison of chromatograms obtained using the two injectors show similar chromatographic resolution. Both traps are susceptible to freezing during the cryotrapping step, but the use of an inline Nafion dryer allows air saturated with water vapor to be sampled using both systems for 3 min without plugging the trap. Thermal decomposition during the injection step can occur for labile species in the ballistically heated trap, but even the highly unstable compound ethyl diazoacetate may be injected without breakdown in the microloop system.