Flow through the sampling orifice

In the continuum flow regime the mean free path X is much smaller than the orifice diameter, and the exit pressure from the reactor is orders of magnitude greater than the backing pressure in the ion source vacuum chamber. The Mach number is unity that is, the flow speed, t , through the orifice is sonic  

In the continuum regime the flux through the orifice, Fc, can be expressed as 

FLASH PHOTOLYSIS WITH TIME-RESOLVED MASS SPECTROMETRY 

In the free molecular flow regime, where the molecular mean free path X is much larger than the orifice diameter, the speed of molecules exiting the reactor can be taken as the mean molecular speed, (v) = SRT/nMW, and the flux, FM, is given by 

The interpolation between these limiting flow regimes is in terms of a Knudsen number-related weighting factor  

---

Gas flow through the sampling orifice is of interest since its volume rate of flow determines the size of vacuum pumps necessary for the mass spectrometer. In addition, orifice flow behaves as a first-order removal process that can lead to erroneous kinetics if it is too large. The perturbation of flow in the reactor has been discussed in Section IV.B. 

---