Chromatography Forum

I have a polar hydrophilic compound that appears to be a base. I am assaying from plasma via LC-MS. Under acidic conditions, assuming I have the ionized form, there is an expected short retention with a hydrophilic endcapped C18 column. However, there is also a varying endogenous peak when I inject a blank. If I move to basic conditions (i.e. - ammonium carbonate, pH 8.) should I expect a much longer retention or will the hydrophilic nature still overpower the limited endcapping on the column and elute too quickly continuing my interference problem? Unfortunately I only have a single quad, so please, no suggestions of MRM to daughter ions. Any help is much appreciated.

Tough to tell. Depends on the pK, which you do not know. Most bases have a pK above 8, in the 9 to 10 range. If this is the case, you won't get a lot of difference at pH 8, maybe only due to increased silanol activity.

I suggest either to try HILIC on a silica column (Atlantis HILIC silica), or try a column which has been maximized for polar retention (Atlantis T3), or a column that lets you go to a more alkaline pH (XBridge C18).

thanks for the response. I may try another column, but I found we were not monitoring close enough to the void. I don't like to turn on the spray chamber until it's necessary so I can try to keep it cleaner. The compound eluted even sooner with the base, at 2 min, but with much greater signal! I think I can move back the retention using 100 aqueous, hopefully, there is no endogenous peaks behind it.