Interfering peaks in LC-MS/MS

[Edde]

I am developing an LC-MS/MS method for bisacodyl and metabolite, bisacodyl diphenol in urine. In 3 colleagues' urine, I see bisacodyl and/or bisacodyl diphenol peaks. The response is as high as e6 or e7 with matched RT. I measure 2 mass transitions for each. They are: bisacodyl -362_184, 362_167; bisacodyl diphenol - 278_184, 278_167. Since bisacodyl is a synethetic laxative and all three healthy colleagues (one of whom is myself) claim they have not taken any.

Is there any possibility that it is contaminant?

I have changed the LC gradient and tested one extract and the analytes are still there with matched RT. Seems not a carryover issue as samples before and after are clean.

Anyone please kindly advise? Thanks.

[Uwe Neue]

what is the content and the solvent composition of the "samples before and after" and the urine samples? I am thinking of some carry-over, released by a different sample solvent...

[Edde]

The samples before and after are in the same solvent, i.e. 50% ACN with 1 mM ammonium formate and 0.1% formic acid, as the positive sample.