Mass spectrometric sampling of ions from atmospheric pressure flames-III: Boundary layer and other cooling of the sample

It is shown that when a flame is sampled supersonically, cooling of the sample can occur in two regions: the boundary layer on the high pressure side of the sampling orifice and also in the subsequent supersonic and near-adiabatic expansion into the vacuum chamber. It is possible to divide such a sample into two parts: that which is affected by the boundary layer and a central core which is not. Boundary layer thickness is found to decrease as orifice diameter is increased, with the result that for a tiny sampling hole nearly all the sample passes through the boundary layer. Conversely, the sample is nearly all core and unaffected by the boundary layer when a relatively large orifice is used. Estimates of the residence times of flame gas in these regions are made, as well as the likelihood of the sample composition undergoing change. For this, computations are made of the fall in temperature of the sample, as it passes through the boundary layer. The cooling there is found to depend, amongst other things, on the orifice diameter and is greatest for small holes. The fractional temperature drop in the boundary layer is found to be appreciable under some conditions, being occasionally as large as 0.4.