These cellulose filters are used in qualitative analytical techniques to determine and identify materials. Prepleated qualitative filters, also available, give improved flow rate and increased loading capacity compared to equivalent flat filters. Whatman provides a wide range of qualitative filters to meet your specific needs. Wet strengthened qualitative filters (Grade 91, Student Grade/Grade93, Grade 113, Grade 114) contain a small quantity of a chemically stable resin to give improved wet strength. This does not introduce any significant impurities into the filtrate but because the resin contains nitrogen, these grades should not be used in Kjeldahl estimations.