Why ionisation outside the detection cell?
Ions can be generated in the detection cell as well, and in fact, the earliest experiments were done that way. However, there are a number of very good reasons why ionisation is now generally done outside the detection cell, i.e. externally. First, some ionisation techniques such as electrospray are performed at high pressures (1 bar), which are incompatible with the ultra-high vacuum in the detection cell (~10^-9 mbar). In fact, multiple pumping stages are required to overcome this pressure difference. Furthermore, the fact that ionisation is done outside the detection cell makes it easier to change over to different ionisation sources and also sample introduction is much easier.The ion transfer optics used to get the ions from the ionisation source to the detection cell are very effective at their task if the necessary parameters are optimised. Hence internal ionisation is very rarely done nowadays, except in the case of MS/MS. By the way, the terms internal and external ionisation a
