Institute of Agricultural Engineering, University of Bonn , Nussallee 5, D-53115 Bonn, North Rhine-Westphalia, Germany.
Anal Chem. 2013 Oct 1;85(19):9021-30. doi: 10.1021/ac401419j. Epub 2013 Sep 20.
The Deans' switch is an effluent switching device based on controlling flows of carrier gas instead of mechanical valves in the analytical flow path. This technique offers high inertness and a wear-free operation. Recently new monolithic microfluidic devices have become available. In these devices the whole flow system is integrated into a small metal device with low thermal mass and leak-tight connections. In contrast to a mechanical valve-based system, a flow-controlled system is more difficult to calculate. Usually the Deans' switch is used to switch one inlet to one of two outlets, by means of two auxiliary flows. However, the Deans' switch can also be used to deliver the GC effluent with a specific split ratio to both outlets. The calculation of the split ratio of the inlet flow to the two outlets is challenging because of the asymmetries of the flow resistances. This is especially the case, if one of the outlets is a vacuum device, such as a mass spectrometer, and the other an atmospheric detector, e.g. a flame ionization detector (FID) or an olfactory (sniffing) port. The capillary flows in gas chromatography are calculated with the Hagen-Poiseuille equation of the laminar, isothermal and compressible flow in circular tubes. The flow resistances in the new microfluidic devices have to be calculated with the corresponding equation for rectangular cross-section microchannels. The Hagen-Poiseuille equation underestimates the flow to a vacuum outlet. A corrected equation originating from the theory of rarefied flows is presented. The calculation of pressures and flows of a Deans' switch based chromatographic system is done by the solution of mass balances. A specific challenge is the consideration of the antidiffusion resistor between the two auxiliary gas lines of the Deans' switch. A full solution for the calculation of the Deans' switch including this restrictor is presented. Results from validation measurements are in good accordance with the developed theories. A spreadsheet-based flow calculator is part of the Supporting Information.
Dean 切换阀是一种基于控制载气流量而不是分析流路中机械阀的排放切换装置。这种技术具有高惰性和无磨损操作的特点。最近，新型整体式微流控器件已经问世。在这些设备中，整个流系统集成到一个具有低热质量和密封连接的小型金属设备中。与基于机械阀的系统相比，流量控制的系统更难计算。通常，Dean 切换阀用于通过两个辅助流将一个入口切换到两个出口之一。然而，Dean 切换阀也可用于以特定的分流比将 GC 流出物输送到两个出口。由于流动阻力的不对称性，计算入口流量到两个出口的分流比具有挑战性。如果一个出口是质谱仪等真空装置，而另一个出口是大气检测器，例如火焰离子化检测器（FID）或嗅觉（嗅探）端口，则情况尤其如此。气相色谱中的毛细流动通过圆管层流、等温、可压缩流动的 Hagen-Poiseuille 方程进行计算。新微流控器件中的流动阻力必须通过相应的矩形截面微通道方程进行计算。Hagen-Poiseuille 方程低估了通向真空出口的流量。提出了一种源自稀薄流理论的修正方程。Dean 切换阀色谱系统的压力和流量计算通过质量平衡的解来完成。一个特定的挑战是考虑 Dean 切换阀两个辅助气体线之间的反扩散电阻器。提出了一种完整的计算包括这个限制器的 Dean 切换阀的解决方案。验证测量的结果与开发的理论非常吻合。基于电子表格的流量计算器是支持信息的一部分。
