Filter paper remains a cornerstone of laboratory filtration in biochemistry and molecular biology, offering cost-effective separation of particulates, precipitates, and biomolecules based on pore size and flow rate.

Types and Specifications

Quantitative filter papers (e.g., Whatman Grade 1-6) feature ashless (<0.01%) cotton cellulose with defined pore diameters (2.5-25 μm), while qualitative grades prioritize speed over purity for routine filtrations. Ultrafilter papers, prepared by impregnating standard sheets with 4% collodion (nitrocellulose in alcohol-ether), harden with formaldehyde to shrink pores to 1-10 nm, enabling separation of colloids from true solutions via pressure-assisted ultrafiltration. Medium (10-20 μm), fine (2-8 μm), and ultrafine (<2 μm) variants support gravity, vacuum, or Büchner funnel setups.

Biochemical Applications

In protein precipitation workflows, Grade 1 (11 μm) captures TCA/acetone pellets post-lysis, yielding clean supernatants for SDS-PAGE. Nucleic acid protocols use hardened ashless papers for plasmid DNA washing or RNA cleanup from polysaccharides. Membrane-based variants (nitrocellulose, 0.45 μm) clarify cell lysates before Western blotting or HPLC, while pulp/paper industry ultrafiltration recycles whitewater by rejecting colloids (TOC >90% removal).