phosphate in LCMS

[harringg]

I'm aware that using phosphate buffer in LC mobile phase and ESI MS is not recommended. Are there any known issues with injecting a sample that contains phospate buffer into a 40:60 H2O:ACN mobile phase on a RP C18 column? 10 ul injection, 200 ul/min flow rate, 2.1x100 column. My concern is more the phosphate buildup on the ESI source cone from the sample. Or is that not much of an issue based on the small volume? Ion suppression will probably be a factor too, but that's a minor point for our interests.

Thanks,
Grant

[tom jupille]

Actually, ion suppression should not be an issue. Phosphate is unretained in reversed phase; so long as your analytes are reasonably away from t0, the major factor should be simply the buildup of crud on the source. If you're going to be running a lot of samples, you might consider a diverter valve between the column and the source to cut the area around t0 to waste. Of course, that raises other issues with respect to band spreading, baseline stability, etc.

[harringg]

Actually, ion suppression should not be an issue. Phosphate is unretained in reversed phase; so long as your analytes are reasonably away from t0, the major factor should be simply the buildup of crud on the source. If you're going to be running a lot of samples, you might consider a diverter valve between the column and the source to cut the area around t0 to waste. Of course, that raises other issues with respect to band spreading, baseline stability, etc.

Thanks for the quick reply, Tom. Our current method uses a diverter to send the LC flow to waste both before and after our compound of interest has eluted, which has a RT of ~5 minutes. So it sounds like we should be alright.

I appreciate the help,
Grant