Low mass cut of in CID experiment

[Ricardinio]

Can any one explain why the low mass cut off in CID experiment is about 1/3 of the precursor ion? Thanking you before hand.

[MG]

Well, whether you are doing scan or MS/MS on an ion trap, the low mass cutoff can usually be set by the user. I do not know the mathematical reason why 1/3 is a good value. However, I can tell you that the low mass cutoff is not a sharp boundary between stable and unstable masses. For example, m/z 100 will be transmitted more efficiently with the cutoff set to 50 rather than 60 (at least with the ion traps I've used).