Whatman 1004-090 Quantitative Filter Paper Circles, 20-25 Micron, 3.7 s/100mL/sq inch Flow Rate, Grade 4, 90mm Diameter (Pack of 100) 9.0cm Grade 4 Used in qualitative analytical techniques to determine and identify materials Extremely fast filtering; provides excellent retention of coarse particles (20-25m) and gelatinous precipitates such as ferric hydroxide and aluminium hydroxide Used when high flow rates in air pollution monitoring are required and the collection of fine particles is not critical