offers a range of wet strengthened qualitative filters which contain a small quantity of a chemically stable resin to give improved high wet strength.
This does not introduce any significant impurities into the filtrate. 
The resin, however, does contain nitrogen so these grades should not be used in Kjedldahl estimations, etc. 
All wet strengthened grades are available in prepleated forms. 
Whatman provides a wide range of qualitative filters to meet your specific needs.