Hello

I am trying to detect flavonoids - quercetin and its corresponding O-glucuronide (metabolite) form on a UPLC-TQD mass spec instrument using MRM (multiple reaction monitoring) mode. The instrument is very sensitive to the aglycone quercetin form and also to the O-glucoside form down to nM levels. However, for the same molecule, its O-glucuronide form is showing very poor sensitivity (is not sensitive below 10uM). I have tried tuning the instrument for the compounds (at 50uM concentration - since that's the only concentration where I can see a tune signal- this is the same concentration used for the aglycone/O-glucoside/glucuronide forms for tuning. Below this, the instrument is not giving stable peaks for tuning). However, when I analyze the standard with lower concentrations and the optimized tune settings via the LC-mass spec route, I still cannot get a signal for the glucuronide forms. What could be the possible reason for this, when the aglycone or O-glucoside forms are so sensitive?

Thank you!