% Adsorptive Loss
This is just a little update on the results of the pnc-Si, cellulose, and pes separations. When I added up the retentate and filtrate, it didn’t always equal the intensity of the starting sample (keeping in mind that I was unable to collect filtrate from PES and should re-try that separation).
I took the percent difference between the intensities of the retentate/filtrate sum and the starting sample in the table below. In all cases there is more loss in the 10mM samples, although we can’t necessarily attribute that different to more surface adsorption. Cellulose and pnc-Si seem to be on par, and from this first test PES has the most loss.
| 10mM Cell | 100mM Cell | 10mM PES | 100mM PES | 10mM pnc-Si | 100mM pnc-Si |
| 29.6% | 12.5% | 74.1% | 58.7% | 32.2% | 8.7% |
It seems we are having a hard time showing advantages over cellulose. Similar fractionations of proteins and similar loss. Cellulose was unable to fractionate gold, right?
What was the original goal of this protein loss work? I thought that we were just trying to show that as a system, our filters absorb no more protein than conventional cellulose and PES. The results seem reasonable, so if you have confidence in these experiments, then mission accomplished.
Considering that we have 1/1000th the surface area, this is nothing to brag about, as the material that is there binds protein like crazy. It illustrates that we need to coat it with something for low volume protein work.
From the early TEM work, we already know that it binds a complete monolayer, then stops, which makes sense to me. If you look at the amount of protein that is lost, does it correspond to a monolayer? Back of the envelope – a 5nm monolayer of protein on a planar surface is ~5ng/mm^2, so a rough estimate is fairly easy.
It is indeed good to be able to claim similar loss to cellulose since it is considered the low loss ultrafiltration material. We were really hoping that separations would be sharper and for large MWCOs at least, they are not. I’m hoping that for lower cut-offs we see an advantage. Data’s coming.
Our other advantage should be transport rates. Despite stagnant fluid layers we seem to be able to show this advantage time and again (beginning with the Nature paper and since in Barrett’s TECAN work). I’m worried that our game plan for this publication is not yet producing an advantage in performance over cellulose. So we may want to work up a figure showing we at least have faster transport of proteins below the cut-off.
Jim,
Right, I couldn’t do the gold separation with cellulose, but everything else is looking similar. I should have lower molecular weight cutoff data early this week.
Superficially it seems these losses are on the order of a microgram for pnc-Si and cellulose, and more like 100s of micrograms for PES. With your formula for a monolayer, Chris, I think we should expect closer to .1ug total adsorption given the size of the pnc-Si chips.