The quantitative filter papers are made from the highest quality alpha cotton cellulose. The ash content is 0.01% ( Except No.4A with an ash content of 0.25%). The quantitative filter papers are double washed in hydrochloric acid and then in hydrofluoric acid and then rinsed with ultrapure water to neutralize. The No.4 is further treated with nitric acid before washing. The sizes available range from 55mm. to 500mm. Additional sizes are available on request.