Non-pH-sensitive fluorophore for relative protein quantification - (Nov/18/2014 )

Hey there.
I'm conducting in vitro drug release studies utilizing FITC-tagged proteins to quantify (relative) release from Float-a-Lyzer dialysis tubes into a dialysate of neutral PBS. 
The problem I'm encountering stems from the pH-sensitive nature of FITC; my control and experimental solutions are at a pH range from 4 to 7.5, so my data isnt representative of actual protein release. 
I'm looking for a non pH-sensitive fluorophore to label these proteins (or other model proteins of ~70kDa) to circumvent this misrepresentation.
I'd really appreciate some input from the community.

Thanks,
Ethan L.

Undergraduate Research Assistant
Laboratory for Vaccine and Immunotherapy Delivery
University of Arkansas

Try looking through the molecular probes handbook (online at  lifetechnologies.com).