Adduct Help
Posted: Fri Apr 09, 2010 12:06 am
by Stryder08
Running LC-ESI-MS in positive ionization mode with 0.05% formic acid in DI Water and Methanol as mobile phases.
Analyte of interest is detected as a doubly charged ion (M+2). Very little to none singly charged ion (M+1) is detected. But I do detect (M+1)+45.
Anyone have any thoughts on this?
Posted: Fri Apr 09, 2010 6:42 am
by bhuvfe
Is there a chance that you have a quaternary ammonium group in your analyte? That might explain the + 45 (exchange of anion?).
Maybe something like NR2-(CH2)-NR3??
Posted: Fri Apr 09, 2010 10:49 am
by lmh
Sorry to ask the obvious, but are you sure of the [M+1+45]? If this is big (and I assume it's big because in my limited experience only the larger things with several obviously ionisable groups tend to produce predominantly doubly-charged ions) then its mass will have strayed away from the integer value ("mass defect"). If, additionally, you're using something like an elderly ion-trap, the mass accuracy may be poor, in which case it's just possible you're rounding the mass the wrong way and it's actually [M+1+46]. This would be easier to explain as a formic acid cluster. But sorry if this is just one of those obvious suggestions you've already thought of and rejected!