Same peak with 2 mass transitions from different M-1

[Edde]

I measure anthraquinones in urine by LC-MS/MS. When injecting standard of one of the anthraquinones, rhein, an unexpected mass transition, 239-211, appeared in the RT of rhein. The unexpected transition is same as another anthraquinone, danthron, whose RT is verified by standard to be different.

Rhein has a -COOH group attached to the danthron structure. Can the COOH break away in the ion source and so danthron is released and be detected at the same RT as rhein?

I appreciate your expert help and comment.

Thanks.

[sav]

Rhein has a -COOH group attached to the danthron structure. Can the COOH break away in the ion source and so danthron is released and be detected at the same RT as rhein?

Yes ([M-CO2-H]- etc.)
You can decrease fragmentation voltage.

[Alp]

If COOH does fragment off, it may be doing so in the source.
Try to reduce your entrance potential, or adjust the parameters that affect the very front end (eg orifice) of the instrument.

You can also try infusing a dilute solution of Rhein, see if you get the mass transition for Danthron in addition to that of Rhein.

Alp

[lmh]

Can I just ask whether you also have the "right" transition for rhein? If so, maybe you don't need to worry about having the "wrong" one too. SRM is very sensitive and source fragmentation is very common. It's certainly a good idea to try a little optimisation to get the right ion rather than excessive source fragmentation, but you may always find a small peak of the "wrong" transition. If you didn't, chromatography would lose half its appeal!

[Edde]

Thanks all for your comments.

It's really a good idea to test with an infusion for the danthron transition.

Can I just ask whether you also have the "right" transition for rhein?

Yes, the right transition is there.
I guess optimization to minimize source fragmentation is necessary for quantitation. But not as crucial in my case of qualitative analysis.

This is the first time I encounter source fragmentation. May I ask what chemical groups are susceptible to source fragmentation besides -COOH?