Updated 100K microcon wet-wet permeability

A while ago I made a post about the permeability through the 100K microcons in the wet-wet EQ format. I had run three membranes, and stopped (and restarted) the centrifugation at three separate time points to be able to calculate three permeability values for each membrane. When I did this I got strange results; the first point had a low permeability, the third point had an extremely high permeability, and the central point was somewhere in between. After talking with Tom and Jess I think I now understand the reasons for the low and high values.

Since these tests are done in the EQ format we are using 15mL conical tubes to hold the microcon membranes. These conical tube do not have a uniform cross-sectional area, they taper to a “point”. During the initial stages of centrifugation the area of the annulus between the “cup” and the tube is smaller then it is later in the centrifugation (due to the tape of the tube). This smaller annulus area means that a small drop in water level within the “cup” will provide a larger rise in water on the outside of the cup initially. So, initially while the outside water level is in the “tapper” range of the tube, the area of the annulus is small; based on the equation derived by Jim this decreases the permeability.
Essentially the low permeability values for the first point was due to the configuration and geometry of the 15mL conical tube.

As for the third point, I looked back at my data and the water height within the “cup” was very close to equilibrium. Taking with Jess and looking at the equations describing permeability shows that the equation blows up and goes to infinity as the cup height approaches equilibrium height.

Today I retested three 100K microcons in the wet-wet format using a more uniform conical tube (more uniform cross sectional area with no drastic taper as with the previous test). I also only stopped centrifugation once to collect the water height within the “cup”.

The results show an increased permeability in the wet-wet format when compare to the wet-dry format, which is the same result we obtain previsouly. Each membrane was spun in the clinical centrifuge at 2400RPM (21.80psi) for 2min, they were then removed and the water height remaining in the cup was recorded. The tubes were then placed back into the centrifuge and spun for an additional 20min at the same speed to allow them to reach equilibrium.

Compared to the previous post in which I did the three timpoints, I believe this data shows a true representation of the 100K permeability in the wet-wet format.