Pumping Speed, Conductance and Gas Flow

Because analytes plus residual mobile phase (and often also collision gas in the case of tandem mass spectrometry) are being injected continuously into a mass spectrometer, the vacuum pumps must be capable of continuously evacuating significant input gas flows in order to maintain a sufficiently low pressure within the analyzer and detector. Before describing the types of pump commonly employed, it is necessary to discuss the concepts of gas flow and its measurement. In this regard two helpful websites are recommended for both introductory and more detailed discussions www.newequation.net and www.vacuumlab.com. 

In the context of gas flow there are three different flow regimes to consider, namely, viscous, molecular and intermediate. The transitions among these states depend primarily on the pressure, which in turn determines X, the mean free path of the gas molecules, i.e., the average distance traveled by a molecule before it collides with another. The kinetic theory of gases (see any textbook of physical chemistry) gives  

Thus at a pressure of 10 torr and for r, o = 1 nm, the mean free path X 7 m, considerably larger than any reasonable dimension within an analytical mass spectrometer. Of course X decreases in inverse proportion to P. 

In contrast with these molecular considerations, from the purely phenomenological point of view the gas flow from orifice 1 to orifice 2 occurs because the pressure Pj 2, so it is convenient to define the conductance of the pipe (www.newequation.net) as  

The molecular and phenomenological descriptions must be equivalent. (number of molecules.time ) and Qg (pressure.volume.time ) are easily related via the ideal gas law in the form  

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