Perchloric acid in Methanol/ACN - UV absorption?

[JD]

I'm using a gradient separation and my mobile phases are as follows:

A = 0.1% HClO4 in water
B = 0.1% HClO4 in a mixture of MeOH/ACN

I'm monitoring 220 nm.

My question/problem is this: Is anyone aware of an incompatibility with perchloric acid and MeOH/ACN? It appears that the UV absorption of my "B" mobile phase increases over a relatively short period of time (< 24 hours). This affects the sensitivity of the method and also my mood.

Thanks in advance.

[Uwe Neue]

Maybe you are forming acetic acid, which would also explain the vinegary mood...

[Mark Tracy]

Uwe is probably right. If you have a ternary pump, put aqueous 1% HClO4 in the third channel, and run a constant 10%C in your gradient.

[JD]

I'll give it a try. Thanks for the suggestion.

JD