I am trying to analyse samples of Rosuvastatin in rat plasma using LC-MS/MS and ran into the problem that I get two peaks for Rosuvastatin, one after a retention time of around 7 mins and one around 10. Both peaks are existent if only the plasma that does not contain any Rosuvastatin is measured (blankplasma; see the first Figure) and further regardless of concentration, however the higher the concentration, the smaller the relative size of the second peak (example in the second Figure).
I redid the analysis with newly created samples but the second peak persists.
Since the samples containing rosuvastatin solved in only water ran fine after the plasma based samples, I assume that the LC-MS is clean, and that every component works as intended.
My thoughts were that there is a compound inside of rat plasma that has the same m/z value as Rosuvastatin, but my search has not yet yielded any results.
Does anyone have an idea what the reasons could be or how to interpret this result?
Also, I am a newbie in this forum, if this is more fit for the BiologyExchange feel free to tell me.
Thanks in advance.
Edit below: Clarification of experimental details.
I'm showing multiple reaction monitoring on a Waters instrument (Micromass Quattro Ultima PT & a Waters 2695 HPLC).
In the shown MRM method the sample gets analyzed for the mass transition of rosuvastatin (482 > 258) and the internal standard carbamazepin (237>194).
The chromatograms shown are results of the processing with QuanLynx for quantification. In the QuanLynx method the retention time of rosuvastatin is set to ca. 7 minutes, such that the shaded peak is recognized as rosuvastatin and therefore integrated by QuanLynx.
The non shaded-peak is the unknown peak which appears when measuring with rosuvastatin spiked plasma for the calibration curve, rat plasma samples after rosuvastatin injection in the rat, and in the blank plasma that did not contain any Rosuvastatin.
When I inject only acetontrile however, I only see a small peak at 7.18 min rosuvastatin trace without the unknown second peak.