Drugs and the pharmaceutical sciences

4 Quigley
FIGURE 3 Diatoms in diatomaceous
earth.
charge to the internal surfaces of the filter sheet. The positive charge is referred to as zetapotential
and it allows the filter to retain particles that are smaller than the pore size by
attracting them to these positively charged sites (Fig. 4).
The components can be mixed together in different ratios to affect the performance
qualities of the filter sheet, the most important of which are flow capacity and particle
retention. Cellulose-based filter sheets can provide effective particle retention down to
0.1 mm. The retention rating of depth filters is stated as a nominal value, essentially
meaning that it will remove at least one particle of the stated size. Most manufacturers,
however, provide the rating based upon a certain percentage of particles of the stated size
under standard conditions. The rating is usually based upon the filter’s ability to retain a
FIGURE 4 Wet strength resin.