Phenoxyethanol and Parabenes

[temmentec]

For the last months I have been trying to analyse Phenoxyethanol and Parabenes. The seperation has worked perfectly but the assays are not correct. I have a mixture of Phenoxyethanol, Methylparaben, Ethylparaben, Propylparaben, Butylparaben and Isobutylparaben with known assays of each ingredient. I dissolve the mixture in Methanol with 0.1% Acetic Acid. Column: Nucleodur 100-5 C8, Mob. phase: A: Water pH 2.5 with Acetic Acid 50% ; B: Methanol 50%
Can someone explain to me why I don't find the assay even though I have a good separation?

Thanks for your help
Sue

[Consumer Products Guy]

This is a straightforward assay; we use acidified mobile phase and reverse-phase like you state, but we don't have any issues at all. We happen to use RP-18 and acetonitrile instead of methanol, but thats a minor difference. We use UV detection at 280 nm, please provide more details. HPLC for these is easier than GC after derivatization (which also works well).