Practical and theoretical aspects of designing a flame-ionization detectormass spectrometer Deans' switch - Pressure-flow relations in gas chromatograpy detector interfaces using vacuum-outlet conditions

In gas chromatographic (GC) practice, straightforward splitting of the colu mn effluent over a mass spectrometry (MS) system and a flame-ionization det ection (FID) system leads to unpredictable split ratios and, thus, to poor quantification. We therefore decided to implement a Deans' switch, which sh ould allow for quantitative transfer of the column effluent to either FID o r MS. Since FID works under atmospheric pressure and an MS under vacuum con ditions, it is difficult to establish suitable dimensions (lengths and diam eters) of the capillaries needed for a "balanced" switch. Generally, the Po iseuille equation, which describes the how of fluids through tubes of circu lar cross-section, is used to this end. However, the motion of gases in sma ll capillaries at low pressures, as is the case in GC-MS interfaces, is fun damentally different from that at near atmospheric pressures. This becomes manifest from a substantial drop of the dynamic viscosity of the gas, eta, to an effective viscosity, eta/F, or, in other words, the Poiseuille equati on is no longer valid. Adapting the Poiseuille equation by the introduction of the correction factor, F, has been reported. In this paper F and the us e of an equation for F expressed in terms of readily accessible parameters will be discussed. In addition, a successful design of a balanced FID/MS De ans' switch will be demonstrated. (C) 1999 Elsevier Science B.V. All rights reserved.