A combined field ionisation-electron impact ion source for high molecular weight samples of low volatility.

1964 B E C K E Y reported about a combined field ionisation/electron impact source1 the commercial version of which is shown in Fig. 1 2. Vhe vapour to be analyzed enters the source via an inlet line. However such inlet is suitable only for sampes of not too low volatility. For the qualitative analysis of organic compounds of very low volatility evaporation out of an oven (direct inet) in the immediate neighborhood of the ionization region is required. On principle, there are two possibilities to solve this problem for a combined field ionisation/electron impact ion source: a) One uses the arrangement with two separate ionization regions as shown in Fig. 1. Then one must have two separate direct inlet systems, or one must make the direct inlet probe with the vacuum lode or the ion source movable. b) The second possibility is to make the ionization region for field ionization to coincide with the one for electron impact ionization. Then one direct inlet only will suffice. This second possibility is the more elegant and more efficient. But some ion optical problems have to be solved. We have realized this second possibility. The respective ion source is shown in Fig. 2. Electron beam and wire emitter are nearly at the same spot so that the vapour beam out of the oven can be ionized in both operating modes.