Isobaric compounds

[flbak]

Hi, new to the site.
How could one- using LCMSMS- differentiate isobaric compounds. Specifically, imipramine and norimipramine?

[tom jupille]

Are imipramine and norimipramine isobaric?

[mckrause]

Different molecular weight. Also will give different fragmentation pattern.

[PeterPopoff]

As already said these two are not isobaric.

The reason you are seeing two peaks in the Norimipramine MRM window (one at the RT of Norimipramine and the other at the RT of imipramine) is that when the imipramine is first ionized through ESI, a relevant amount of Norimipramine is formed in that first ionization where the M+1 ion is formed. Therefor (in the MRM window for Norimipramine) you see Norimipramine at its normal RT and at the same RT as imipramine.

Its important to note that you are not seeing imipramine and norimipramine in the same MRM window. They have different masses therefore you cannot see them both in an MRM window that only allows you to see one of them. This same thing happens with Nordoxepin and doxepin.

[chemboy831]

Peter, would you happen to have a reference for this phenomenon? Mechanism perhaps?
Thanks

[Klaus I.]

This phenomenon is called 'in-source fragmentation' and occurs when source parameters are too harsh for the molecule. The reduction of temperatures and voltages may help to avoid this type of fragmentation.

Edit: Several typos removed, I'm sorry