The Relationship among Pore-Size Ratings, Bubble Points, and Porosity - Pharmaceutical Technology

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The Relationship among Pore-Size Ratings, Bubble Points, and Porosity

Pharmaceutical Technology

Ambiguities in pore-size rating

Within any given pore-size designation available in the market, there exists some range to its quantified properties. In its manufacture, a membrane lot may, and indeed will, incur some variation in the in-process bubble-point measurements that are translated into pore size. Each pore-size rating is prepared using a different casting formula. Nonetheless, the preparations do not yield distinct quanta, although the variations within a class are less than those between classes. Each batch or even each filter within a batch of membranes is classified to a single pore-size value, although each of the individual filters comprising the group may have its own bubble point within the range of values that define the given rating.

As Schroeder points out, the pore size–retention correlation is not a step function (26). In its manufacture, a membrane lot will incur some variation. In using nonuniform standards, filter manufacturers might assign their pore-size labels somewhat ambiguously. One fabricator may, on the basis of flux, consider a membrane to be an "open" 0.1 μm. Another filter producer, using a somewhat different rating system might classify it as being a "tight" 0.2 μm. This could give rise to a labeled 0.1 μm, not so handicapped by reduced flows, being compared with a labeled 0.2 μm, not so advantaged by enhanced flows. The more "open" 0.1 μm may not flow faster than an average 0.2 μm but may do so against a "tight" 0.2 μm. Consequently, it becomes an unrewarding exercise to try to compare competitive membranes each rated by their own individual catalogue descriptions as perhaps being of the same pore size, yet exhibiting different flux rates or retentions under test.

Figure 6: Different flow rates of 0.2- and 0.1-μm rated filters.
As is usually the case in the physical sciences, reliable evaluations and meaningful comparisons must be derived by users from performance data obtained through experimental investigation founded on suitable experimental designs. Another possibility for initial comparisons is the standard ASTM organism challenge test designed for 0.2/0.22 μm-rated filters but extendible to 0.1- or 0.45 μm-rated membranes as well (27).

Consequences of rating ambiguities

As shown in Table II, Tolliver and Schroeder found that the retention of 0.198 m latex particles by five commercially available 0.2/0.22 μm-rated membranes differed significantly (21). The particle-retention mechanism was not complicated by adsorptive sequestrations because the solution contained surfactant. Given the size uniformity of latex particles, the variation in retentions based on sieving indicates different reference standards for the like-rated pore sizes. Nonstandard ratings also explain the Lindenblatt et al. (28) report that the flow rates exhibited by four 0.2/0.22 μm-rated membranes, as also of four 0.1 μm-rated filters differed markedly (see Figure 6) (28). Moreover, as indicated in Figure 7, the throughput volumes measured for the nonstandardized similarly rated pores also differed. Comparable data are offered by Jornitz et al. (29).

Figure 7: Different throughputs of 0.2- and 0.1-μm rated filters.


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