Serious "double peak" in Amoxycillin assay

[Medkemist]

Hello all.. I'm running a bioequivalency assay on Amox in human plasma. I use a published method that I personally have used in the past to assay a similar amox product. Now I'm experiencing a serious, consistent double peak phenomenon whenever I run my unknowns, interestingly enough, niether calibration standards nor QC's show this splitting, and the splitting is only affecting the Amox peak but not that of the IS (cefadroxil). the unknown samples have been stored @ -70 for ~ 6 months prior to the start of the analysis. Is this a known stability issue with Amox?? all my investigations in literature indicated that it's a pretty stable analyte. Any takes on this issue?? thanks.. I'm using new column, guard, vial inserts, caps, etc.. I'd appreciate any help I can get..

[Kostas Petritis]

Was the published method that you used for Amoxycillin or the amox product? If not, does Amoxycillin retained enough in the column? What do you use to dissolve Amoxycillin (i.e. injection solvent + pH) and what is your mobile phases?

[bartjoosen]

Maybe you can see if it's a degradation product of amoxicillin?
Don't know what detector you are running at, but you can stress your standard with pH 8 as shown in the EP monograph to find the dimers.