What is the difference between 0.22 & 0.45 um membrane filters
Originally, sterilizing filters were designated as 0.45 micron in pore dimension, presumably on the basis of mercury intrusion measurements. The devising of the 0.2 micron rating arose from the halving of the 0.45 micron pore size, which was necessitated by a 1967 study by Bowman et al. relative to Pseudomonas diminuta (Brevundimonas diminuta) retention wherein that organism penetrated the “0.45 sterilizing filter”. The 0.45 number was reduced by half, to 0.22, to yield sterile effluent.
“However” Under proper filtration conditions, 0.45-micron rated membranes can sustain the required B. diminuta challenge to yield sterile effluent. Viscous media may be filtered advantageously using this more generously proportioned pore size.
Interestingly, Tanny et al. demonstrated that a 0.45-micron rated cellulose triacetate membrane was capable of producing sterile effluent against B. diminuta challenges of 2 *107/cm2 when the filtration was performed at 5 psi. The practicality of low pressures aside, sterile filtrate need not be the exclusive consequence of using “0.22 micron (or less)” membranes. In this case, a 0.45-micron rated membrane was the sterilizing filter because it produced sterile filtrate.