To date, we have not found any benzidine based dyes which can not be collected by the above way.
Azo dyes have a tendency to be light and heat sensitive.
So far we have only found a problem with Direct Black 38 being light and heat sensitive and therefore we recommend protecting Direct Black 38 samples from light and heat and shipping them on blue ice. We have not had a problem with analysis when several benzidine based dyes are present on the same filter. If many were present on the same filter then we might have problems in obtaining analytical separation and therefore problems in accurate quantitation due to interferences. It is best to request the analysis that way or to call and specify what is needed. This may vary depending upon detector sensitivity to specific dye and presence or absence of interferences.