hello

I've been toning my Vandyke's with platinum. I mix up a litre of the toner at a time and then use it as a one shot.
I've noticed a very slight tone to my highlights. It's almost barely there but it concerns me. I am trying to find the culprit and wondering if using toner which was made last summer might be at fault?
I do remember reading somewhere that platinum toner does have a shelf life but what happens after that? Does is merely just loose strength or could it have a staining tendency.

cheers
david