Reliability of retention performance is of paramount importance for membrane filters designed for sterile and virus filtration. To achieve dependable retention, filter suppliers employ multiple quality assurance measures, including an integrity test to ensure the absence of oversize pores or defects that can compromise the retention capability of the filter. Integrity testing is also routinely practiced immediately prior to and after use of the filter to detect any defects that may have been incurred during installation or use of the filter. Probably the most commonly applied non-destructive integrity test for membrane filters - especially sterilizing grade and virus filters - is the gas-liquid diffusion test, with air and water often used as the gas-liquid pair. In a gas-liquid diffusion test, the measured diffusive flow rate through a wetted membrane is compared to a theoretical or empirically established flow rate for an integral membrane. A measured flow rate value that exceeds the specification for an integral filter signals the presence of one or more defects. However, the sensitivity of the air-water diffusion test is limited by the fact that the diffusive flow rate for an integral membrane can span a range that is large compared to the flow contributed by a defect, resulting in a relatively poor signal-to-noise ratio.
In this talk, a novel non-destructive air-water integrity test for microporous and nanoporous membranes is described. The test is based on the principle of differing rates of gas diffusion of different gases through the liquid layer of a wetted membrane. Oxygen permeates through water faster that nitrogen, so with air as the challenge gas and water as the wetting fluid, the permeate stream will be enriched in oxygen. The permeate oxygen concentration is predictable, accurately measurable, and within a narrow and repeatable range for an integral membrane. A leak through the membrane will result in a deviation from the integral permeate concentration, signaling a defect. Compared to the conventional air-water diffusion test, this air binary gas (i.e., O2 and N2) test in which the permeate gas composition is measured (in addition to the diffusive flow rate) has a superior signal-to-noise ratio and was demonstrated to provide a significantly higher level of retention assurance for both sterilizing grade and virus filters. Since air and water are used as the gas-liquid pair, the air binary gas test also maintains the convenience, safety, and environmentally friendly aspects of the air-water diffusion test.