Negative repeller voltage when using EI ?

[goran123]

Hi,
I have just started with GC-MS.
If I have understood it right, usually a positive repeller voltage is used with EI and PCI, while a negative voltage is used with NCI.
However, now I have learnt that a negative voltage is used in EI, in combination with some sector MS instruments. This is strange - I think that a negatively charged repeller should attract the positively charged ions, not repell them.

Is it common to use a negative voltage at EI?
Please, could someone explain this?

Kind regards,

Goran Karlsson
Stockholm
Sweden

[willnatalie]

A postitive charge is used; the reason is that the repeller, directs the positively charged fragments into the analyzer (i.e. quads).
You are right about the about the attraction order, but the idea is to send the positive ions away (repell them) into the insturment and neutralize the negative ions.

Hope this helps.