Choosing a Molecular Weight (MW) Cutoff

The cutoff of the membrane should be chosen based on the size and type of molecule being targeted. In general, the cutoff of the membrane should be at least 3-4 times the molecular weight of the molecule.

There are issues that present as the cutoff increases, so it is wise to use the smallest cutoff that will provide adequate collection of your molecule. Specifically at higher cutoff (55kDA+), outward fluid loss can begin to occur through the membrane. This is called ultrafiltration.

There are strategies to address ultrafiltration (add Dextran 500 to perfusion fluid when using 55kDA/100kDA membrane, using push pull system with peristaltic pump at outlet when using 500kDA+ membranes) but it complicates things and is best avoided if not needed.

Also, the cutoff of the membrane is providing a degree of selectivity in the sampling. Molecules above the cutoff are not being sampled, which equates to a cleaner sample with less post-sample prep required. Also, this helps to prevent collection of many larger MW enzymes that can cause the sample to degrade during collection and storage.

Standard Cutoff, Standard Setup
Small Molecule Metabolites/Neurotransmitters
6 kDa
20 kDa

High Cutoff, Dextran 500 Recommended for Perfusate

Small Molecules/Peptides
55 kDa

100 kDa

(Article Discussing use of Dextran 500 vs 70:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4004663/)

Ultra High Cutoff, Push-Pull Pump Setup Recommended
Larger Molecules, Peptides, Proteins
500 kDa
2000 kDa = 2 MDa

Need more information? Please check out our introductory information here.