Chromatography Forum

Hi everybody!

I'm analyzing (quali & quanti) various medicinal plant extracts (mostly methanolic / aq-methanolic) for plant phenolics by means of LC (Agilent 1200) coupled with ESI QQQ MS/MS (Agilent 6410B). My current mobile phase is A: 1 % HCOOH, B: MeOH (various gradients). The column is (most often) Zorbax XDB-C18 30x2.1x1.8.

Due to nature of the samples, some amounts of Na+, NH4+ and K+ ions are bound to be present, leading to very nice clusters of cationized pseudomolecular ions ([M+H]+, [M+NH4]+, [M+Na]+, [M+K]+, [2M+Na]+...). When doing quali analysis, I have nothing against it (helps determining the Mw if there are a lot of fragments and/or impurities), but for quantification it's a problem if I want to use MRM mode.

What are your experiences? Any luck with suppressing the formation of alkali metal adducts? For some time, I'm thinking about putting some crown ether in the mob. phase... anyone tried it already? Or you can suggest some better additive?

Thx

In some cases it could help if you add some alkali salts during your extraction or, if not compatible with your procedure, you could try to dry them after extraction and re-suspend them with a salty solvent.
In this way you can promote the formation of just one specie instead of more species.

Well, that would be just a workaround. Unlike the [M+H]+, adducts with alkali metal ions usually don't fragment with low collision energies, and are completely destroyed (fragmented to tiny useless ions) with high energies. Thus, I wouldn't obtain good spectra nor I would be able to use MRM mode, only single-stage MS. That is why I need the most/all of my compound to be in [M+H]+ form.