Jim Simmons is the Senior Director of Education at SpectrumLabs.com. He has over 30 years experience in membrane separations and spent 15 years as the Regional Sales Manager for the central United States. Jim now conducts the Spectrum Lab’s Training and Education Program.
In this installment, we will discuss Concentration Applications. Concentration has a wide array of applications; usually, the applications fall into either a general sample concentration or a harvest.
The channel is the path the sample follows as it moves along the membrane surface – the tangential flow. With hollow fiber filtration, the channel is formed by the inner diameter of the fiber, known as the fiber lumen. Tangential Flow Filtration (TFF) requires a suitable volume of liquid to carry the sample through the membrane channel. As concentration removes a portion of this volume, the degree to which a sample can be concentrated is limited in TFF. Factors that impact this limitation are:
- Initial concentration. If the sample is at a high concentration to begin with, you may be unable to concentrate it much further. If the initial concentration of the sample is very dilute, a higher concentration factor may be achieved.
- Adverse reaction of the sample as it concentrates. This could be aggregation or formation of precipitates, or any other adverse response specific to your sample.
- Pressure buildup. As the sample concentrates, the viscosity of the sample increases resulting in excessive pressure buildup.
These factors generally represent conditions that can limit the degree to which a sample can be concentrated using TFF – a limiting factor. This is the maximum achievable concentration. As it is generally desired to concentrate the sample as much as possible it is necessary to understand the indicators that tell us when the point of maximum concentration has been reached.
Concentration Factor (CF) is the degree to which the sample can be concentrate — 10L down to 1L = CF of 10.
The Loading Factor is the ratio of the untreated sample size in Liters to the surface area of the membrane in square meters. L = V(L) / M2
Permeate is the species that passes across the membrane. Permeate flow rate is measured in a normalized unit known as LMH (liters per square meter per hour). This number tends to decrease as concentration factor increases.
Therefore, as concentration factor increases, it is very important to monitor permeate LMH, feed, and retentate pressure. Increasing viscosity of the sample can result in excessive feed pressure and differential pressure. This can become the point of maximum concentration. In this case, one solution could be to increase the channel dimension, i.e.: increase fiber lumen from 0.5mm to 1.0mm, thereby allowing the more viscous solution to pass through the channel with less pressure buildup.
If permeate LMH declines so greatly that the process becomes slow and tedious, one solution could be to increase the membrane surface area, thereby reducing loading factor, allowing the concentration step to continue. Declining LMH is the indicator of a process characterized with operating pressure too great for the tangential flow rate. We will discuss that in a future article.
Harvest is a form of concentration of the sample. In this case, the permeate is often the product of interest. Permeate may go by other names in these applications, most commonly used are Supernate or Perfusate. By any name, this is the species that passes through the membrane wall – the soluble species. Collecting proteins or antibodies from cell culture is an example of a harvest process. In the last installment, we defined solute passage as how freely (or not) the soluble species passes through the membrane. Since the soluble species is the desired product of the harvest process – the concentrated retained species is generally waste – it is very helpful to understand the solute passage efficiency of the membrane selected for the process.
We refer to buffer exchange using TFF as diafiltration. Diafiltration applications can be used as a step in any of the concentration applications. Harvest processes generally begin with a concentration step, and often end with a diafiltration step. In the next installment, we will discuss the diafiltration step in detail.