We have been experienced a strange phenomena: We analyze a drug substance released from a device in the release media (pH 7.4, 10 mM PBS and 1.5% SDS)on Shimadzu LC with HP eclipse XDB C18 column, MeOH/Buffer=1/1(buffer: pH 6.6, 40 mM phosphous). The main peak was at 9 min with an impurity at 5 min. After continuous injection, the impurity peak remained stable but the main peak was getting shorter to 6 min. The bracketing standard (in water)shifted too. After the column washed with MeOH/water 1/1, the peak was back to 9 min. The drug is water soluble ionic sodium salt, stable when the pH is above 5. SDS was used to increase the solubility of other drugs in device for release. Without SDS in release media, the drug peak didn't shift. It looks like SDS made the column more saturated with salts so that the peak came earlier. Even we decreased the buffer concentration to 20 mM it didn't help "stop" the peak.

Has anyone experienced something similar? What could we do to "stop" the peak?

* SDS: sodium monododecyl sulfate

I presume you mean "phosphate" for "phosphous". I don't recognize "PBS". I take it the mobile phase does not contain SDS.

Looks like a classic case of accumulating a detergent (SDS) on a C18 column, altering the properties of the column. Washing the column fixes the problem.

Try washing the SDS off the column each cycle using a "step gradient" to 1/1 MeOH/water (or anything else that works).

Alternatively you could try a mobile phase containing SDS. The properties of the column would then not change with the injection of a little more, but, of course, you'd have to adjust your chromatography to regain the separation you have now.

Bruce,
Thanks for your message. Yes, there is no SDS in the m.p. (phosphate). I guess the only way to remove SDS is washing it out. Washing makes the run time longer.

PBS stands for Phosphate Buffered Saline.

Kathy