hydrophilic compound in varying pHs

[neuger]

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.

[Uwe Neue]

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).

[neuger]

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.