COMPARING HOW THE TESTS MEASURE CONTROLLED DEFECTS AND VIRUS RETENTION
The research included conducting integrity tests on virus filtration membranes using both the air–water diffusion test and
the binary gas test. The virus filtration membrane used was Millipore Viresolve Pro (asymmetric PES) membrane in flat sheet
or disc formats.
Researchers introduced defects by laser drilling 2–10 µm diameter holes in the center of the membrane discs (see Figure 4).
Figure 4: Laser hole drilled through a Viresolve Pro membrane disc.
For the air diffusion test, pressurized air was applied to the upstream side of the membrane and downstream air flow rate
was measured using a mass flow meter.
For the binary gas test, the CO2/C2F6 test gas was introduced to the membrane at 345 kPa, and a constant sweep gas rate at
a 4:1 ratio relative to the permeate flow rate was maintained through the vent port of the filter holder. Gas composition
was measured using fourier transform infrared spectroscopy (FTIR) (inDuct FTIR, MKS Instruments). Measurements were recorded
continuously until an essentially steady state permeate composition was achieved, typically within 15–20 minutes, which was
the time required to fully flush out the residual air from the volume downstream of the membrane, the sample lines leading
to the FTIR, and the FTIR sample chamber. A small volume custom cell was procured for the FTIR in order to minimize the total
internal volume downstream of the membrane and thereby reduce the overall test time.
After the initial air diffusion and binary gas testing, researchers challenged the membrane devices with a solution consisting
of a bacteriophage mixed with polyclonal human immunoglobulin in a buffer solution. The solution was filtered through the
membrane until flux had declined by 75% compared to the clean buffer. They then collected feed and permeate samples, determined
the infectious titer, and calculated the virus LRV.
Figure 5: Defect detection by the air diffusion test.
Figures 5 and 6 show the air diffusion and binary gas test results as functions of defect size. The solid lines are the model
predictions for the air diffusion and binary gas tests. The shaded regions in each graph show typical test value ranges for
integral membranes. These regions represent background noise against which a signal for a defect must be compared. Figure
5 shows that a 2 µm defect was not "visible" to the air diffusion test because the additional flow rate due to the defect
was not large enough to increase the total flow rate beyond the range typically measured for integral membranes. In contrast,
as shown in Figure 6, the elevated C2F6 concentration in the permeate is a clear signal for the same 2 µm defect. This result
was an unambiguous demonstration of the binary gas test's superior defect detection sensitivity. For defect sizes larger than
2 µm, both tests provided a strong signal for a defect.
Figure 6: Defect detection by the binary gas test.
The research also showed that the loss in LRV compared to an integral membrane due to a single defect can be predicted from
the binary gas value. This means that a maximum allowable binary gas value can be established for a desired level of LRV assurance
based on a worst case assumption of a single defect.
In addition to using the test in conjunction with controlled defects, researchers looked at defect detection sensitivity in
manufactured devices. Both tests were applied to a set of prototype Viresolve Pro devices. A portion of the two–layered membrane
used to manufacture the devices was tested for virus retention and the LRV of the three discs was determined to be 5.9. The
LRV of the devices, however, ranged from 5.0 to 5.9. While the air diffusion test did not differentiate among these devices,
the binary gas test showed clearly elevated values for the two devices with lower LRV values (see Table I).
Table I: Comparison of integrity test sensitivity between the air-water and binary gas tests on prototype Viresolve Pro devices.
Compared to the conventional air-water diffusion test, the binary gas test provides superior defect detection sensitivity
in virus filters. While the air diffusion test provided an LRV assurance of about 4.5–5.0 for the virus filters studied, the
binary gas test can provide an LRV assurance of greater than 6.0.
The greater sensitivity of the binary gas test is due to a much more favorable signal–to–noise ratio than the air-water diffusion
test. Unlike the gas–liquid diffusion test, the binary gas test has low sensitivity to membrane porosity, liquid layer thickness,
and membrane area. Other factors that can confound the sensitivity of the air diffusion test such as thinning of the liquid
due to membrane asymmetry or evaporation, liquid retention of membrane support layers (porous non–wovens, for example), and
membrane movement or compression have a much lower impact on the sensitivity of the binary gas test.
Sal Giglia*is principal applications engineer, and Mani Krishnan is director of single use technologies, both at EMD Millipore, email@example.com