Practical and theoretical aspects of designing a flame-ionization detector/mass spectrometer Deans' switch: Pressure–flow relations in gas chromatograph–detector interfaces using vacuum-outlet conditions

In gas chromatographic (GC) practice, straightforward splitting of the column effluent over a mass spectrometry (MS) system and a flame-ionization detection (FID) system leads to unpredictable split ratios and, thus, to poor quantification. We therefore decided to implement a Deans' switch, whi...

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Bibliographic Details
Published in:Journal of Chromatography A 1999-01, Vol.831 (2), p.257-265
Main Authors: Blomberg, Jan, Brinkman, Udo A.Th
Format: Article
Language:English
Subjects:
Analytical chemistry
Chemistry
Chromatographic methods and physical methods associated with chromatography
Exact sciences and technology
Gas chromatographic methods
Online Access:Get full text
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Summary:In gas chromatographic (GC) practice, straightforward splitting of the column effluent over a mass spectrometry (MS) system and a flame-ionization detection (FID) system leads to unpredictable split ratios and, thus, to poor quantification. We therefore decided to implement a Deans' switch, which should allow for quantitative transfer of the column effluent to either FID or MS. Since FID works under atmospheric pressure and an MS under vacuum conditions, it is difficult to establish suitable dimensions (lengths and diameters) of the capillaries needed for a “balanced” switch. Generally, the Poiseuille equation, which describes the flow of fluids through tubes of circular cross-section, is used to this end. However, the motion of gases in small capillaries at low pressures, as is the case in GC–MS interfaces, is fundamentally different from that at near atmospheric pressures. This becomes manifest from a substantial drop of the dynamic viscosity of the gas, η, to an effective viscosity, η/ F, or, in other words, the Poiseuille equation is no longer valid.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 use 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 Deans' switch will be demonstrated.
ISSN:0021-9673
DOI:10.1016/S0021-9673(98)00955-8