Oxidation problem in protein analysis

[brazdzionis]

Hello,
I have a problem with oxidated forms in my analysis. We are using H2O + 0.1% TFA as solvent A and ACN + 0.1% TFA as solvent B, C18 waters symetry column. I'm analysing a specific protein all the time, and when I do my system routine check with reference standart, oxidised protein forms should be around 0.25 - 0.40 area%, but something has changed now and it's around 0.5-0.6%. I have rechecked everything, solvents, precolumn, column what causes may result this?

Apreciate your help.

[Kristof]

Have you considered storage conditions of samples? Temperature, pH, buffer composition, time?

[brazdzionis]

Yes these factors have been check already, something is deeper than that

[carls]

A detailed investigation of peptide oxidation in autosampler vials is given in the thread below. Many oxidation sources were tested. Maybe this will give you some ideas.

viewtopic.php?f=19&t=13785&hilit=peptide+oxidation