Fast micro thermal desorption combined with cryofocusing for ultra fast gas chromatography

Abstract

In safety research, gas chromatography is a commonly used technique for the detection of hazardous substances in air samples for example. When public safety is at risk and decisions about protective measures must be made, the speed of sampling and analysis is a decisive factor. This demand for rapid results in combination with a high sample throughput is also important in industrial applications. In this context, time-saving methods also contribute to cost efficiency. The three investigations presented here focus on the step-by-step development of a rapid system designed to accelerated sample application. <br/> In the initial study, a liquid CO₂ cryofocusing unit was developed to enable rapid refocusing of volatile analytes at the head of the separation column with low CO₂ consumption. A test mixture comprising four gases and 16 solvents was investigated, with the solvent analysis being particularly important for pharmaceutical quality control. The effect of cryofocusing on retention time, full width at half maximum (FWHM), and resolution was evaluated in comparison to measurements conducted without cryofocusing. The entire method, including system cool down, has a run time of less than 90 s. <br/> In the second study, a thermal desorption unit (TDU) and corresponding micro thermal desorption tubes (<em>μ</em>TD-tubes) were developed as an alternative to the conventional split/splitless injector for the flow field thermal gradient GC (FF-TG-GC) system. The low thermal mass of this setup enables for exceptionally rapid heating and cooling cycles and thus a faster analyte desorption compared to existing systems. The study investigates the impact of the reduced <em>μ</em>TDtube dimensions on parameters such as desorption time, FWHM, breakthrough volumes, tube flow rates (and thus linear velocities), porosity and back pressure. The system's functionality is demonstrated by measurements of an <em>n</em>-alkane standard ranging from <em>n</em>-pentane to <em>n</em>-eicosane. <br/> In the next step, a rapid method was developed for analyzing a complex mixture containing six chemical warfare agent (CWA) simulants and ten explosives using the TDU in combination with cryofocusing. Due to the substantial differences in physicochemical properties among the target compounds, the analytical parameters had to be adjusted accordingly. The effects of desorption conditions, cryofocusing and the thermal gradient along the separation column were systematically examined and a cycle time of only 164 s was achieved.