How do you select the appropriate membrane size and type for buffer exchange and diafiltration?

1 What are buffer exchange and diafiltration?

Buffer exchange and diafiltration are both based on the principle of size exclusion. They use a semi-permeable membrane that allows smaller molecules to pass through, while retaining larger molecules in the sample solution. The membrane can be either flat or tubular, and can have different pore sizes and materials. The sample solution is driven through the membrane by applying pressure, either manually or using a pump. The smaller molecules, such as salts, solvents, or contaminants, are removed from the sample solution and replaced by a fresh buffer or solvent that flows in the opposite direction. This process can be repeated until the desired concentration, pH, or conductivity of the sample solution is achieved.

3 How to select the membrane size for buffer exchange and diafiltration?

The membrane size is one of the most critical factors for buffer exchange and diafiltration. The membrane size determines the selectivity and efficiency of the separation process. The membrane size is usually expressed as the nominal molecular weight cutoff (MWCO), which is the molecular weight of the smallest molecule that is retained by 90% or more by the membrane. The general rule of thumb is to choose a membrane size that is 3-5 times smaller than the molecular weight of the target molecule in the sample solution. For example, if you want to retain a protein of 100 kDa, you should use a membrane size of 20-30 kDa. This ensures that the target molecule is retained by the membrane, while the smaller molecules are removed. However, this rule of thumb may not apply to all cases, as some factors may affect the retention of the target molecule, such as the shape, charge, aggregation, or interaction with the membrane.

4 How to select the membrane type for buffer exchange and diafiltration?

The membrane type is another important factor for buffer exchange and diafiltration. The membrane type determines the compatibility and performance of the membrane with the sample solution and the buffer or solvent. The membrane type can be classified into two main categories: organic and inorganic. Organic membranes are made of synthetic polymers, such as cellulose acetate, polyethersulfone, or polyvinylidene fluoride. Inorganic membranes are made of ceramic, metal, or glass materials. Organic membranes are more widely used for buffer exchange and diafiltration, as they offer more flexibility, lower cost, and higher permeability. However, organic membranes may have some drawbacks, such as lower chemical resistance, higher fouling tendency, or lower mechanical strength. Inorganic membranes may overcome some of these drawbacks, but they are more expensive, less available, and harder to handle.