Have you noticed any false positives and/or negatives in VOC sampling while utilizing polyethylene bladders in tubing?

No. QED actually did a study on comparing Teflon bladders and tubing material to polyethylene material.  Specifically, we used two different types of bladders in our portable water pump – we have a blow-molded polyethylene and we also have a Teflon bladder.  In leaching studies we saw no difference for benzene and trichloroethylene between the two materials based on the residence time of water in the bladder and the time required for the polyethylene to reach equilibrium concentration with the water that it was sitting in.