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- Posts: 238
- Joined: Fri Sep 10, 2004 11:53 pm
I am being assured by a colleague that it is "because it's ionic", however I found that I quickly developed a debilitating headache during our conversation and was thus unable to continue to reason my side in greater depth.
Due to said colleague's strength of belief, I figured I would try to search for references, or gain other opinions on the matter.
Further evidence presented to me was given by the following observation made by my colleague:
- A method to analyse a surfactant molecule and side-products comprising the following functionality; quaternary alkylammonium ion, terminal or bridging phosphate, a small number of hydroxyls in an alkyl sidechain. Chain lengths are C6-C10 and the quaternary ammonium is formed with two methyl groups.
- Luna C8 column.
- Mobile phase consists of 10 mM ammonium acetate in water, methanol, THF and is run as a gradient to a maximum of 70% organic (30:40:30 aqueous:MeOH:THF). The aqueous pH is unspecified and unmeasured.
Now, because "peaks move around" when including or excluding ammonium acetate in the aqueous part of the mobile phase, this is interpreted as evidence of ion pairing.
Depending on the model considered, my initial thoughts on the subject are that 1) acetate ion is not hydrophobic enough to be retained on the column surface and 2) the mobile phase isn't of low enough dielectric constant to allow formation of a paired-ion.
Comments appreciated.