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Why is best to run acids in negative mode in LC/MS/MS ?
Posted: Tue Apr 25, 2006 1:02 pm
by saioa
Hello,
I understand that it could be best to use a negative mode as it is easy to ionize acidic pharmaceuticals. However in LC they should be neutral so they can be separated and that is usually done by having an acidic mobile phase, which I could think it would decrease the signal in the MS.
What would be best mobile phase option in this case?
Thanks
Posted: Tue Apr 25, 2006 2:30 pm
by Noser222
My take on it is that there is always some ionization of your analyte in an acidic mobile phase, and "some" is enough to be detected. If you get too acidic, like if you have TFA in there, then you start killing your signal.
Posted: Tue Apr 25, 2006 5:26 pm
by Mark Tracy
You can also use post-column addition of ammonium hydroxide.
Posted: Tue May 02, 2006 3:13 pm
by H-KP
Hello,
You should test the two modes using a neutral mobile phase.
If ammonium acetate is added in the mobile phase, you can monitore in the positive mode on the +NH4 adduct.
Posted: Tue May 02, 2006 4:38 pm
by Uwe Neue
A buffer instead of a straight acid additive might help as well. Instead of using formic acid, use an ammonium formate buffer at pH 3.75.
Posted: Thu May 04, 2006 4:06 am
by james little
You might find this article useful..
"A Study of Ion Suppression Effects in Electrospray Ionization from Mobile Phase Additives and Solid-Phase Extracts," Claude R. Mallet, Ziling Lu, and Jeff R. Mazzeo, Rapid Communications in Mass Spectrometry, 2004; 18: 49-58
Good article, thanks
Posted: Tue May 09, 2006 7:48 am
by saioa
Thank you for the answers, the article has a lot good information, Thanks.
Saioa
Posted: Fri May 19, 2006 10:53 am
by Magda
If someone should use TFA what's the best concentration for him to avoid ion supression?