Mobile phase ions

[meillid]

Recently I am trying to use LC/MS to do impurity identification of nicotine solution. According to the method that our client provided, I got noting. The first thing that concerns me is the mobile phase ion. I use 0.01%TFA as mobile phase A and 100% acetonitrile as mobile phase B according to this gradient method. I always detect peaks at 142, 163, 204 even when NO column is installed and only mobile phase is running. Then I change the mobile phases to that I used for other experiment: 0.5% FA, 2mM ammonium acetate in H20 for mobile phase A and 0.5% FA, 2mM ammonium acetate in Methanol for mobile phase B to run mobile phase only (same gradient method without column) to see the mobile phase ion. This time I still see ions at 163, and other ions are 146, 361, 361, 375 etc. I just wonder why I always see ion at 163, even for completely different mobile phase? What this ion is? Do you usually see so many ions from mobile phases at this mass range (150-400Da)? Or is there anything wrong with my instrument? (I have completely cleared both HPLC and MS source to prevent contamination). Also what are the chance for the LC/MS (ESI) to be useful for the identification of unknown despite the interfere ions from the mobile phase? What is the normal level of baseline from the mobile phase in CPS? (Say API LC/MS ESI+ mode). Thanks for any suggestions.

[MG]

I always expect some noise ions at the lower masses. To see whether the amount of background you are seeing is normal, it would be good if you could look at old data and see if historically those ions have always been there at those levels (even when the instrument was installed), or if they are new contaminants. I checked my lists of common background ions, and the only one I see from your listing is 146, acetonitrile trimer with sodium, (3ACN+Na)+.

To answer your question of how LC/MS can be useful for identifying compounds when background ions are present: every data system I have used has a background-subtraction feature, that can be used to remove the mobile phase background ions whether you are doing flow-injection or LC.

[meillid]

MG, thank you very much. Since I take over this instrument, most of the time I run MRM for quantification. I really don't have old data to compare with. Would you tell me, what ions do you usually observe? and in what intensity? Thanks again.