S: Hello Patrick, why don’t we start from the beginning. What is ultrafiltration? 

P: Well Shannon, ultrafiltration is the filtration and separation of macro molecules in the sub-micro range. These particles are less than 0.1um in size, and the separation is performed using extremely fine porous membranes.

S: So very small molecules. But you don’t start by filtering the smallest molecules. How does ultrafiltration differ from general clarification in a process?

P: There are several methods by which one could clarify a sample, but if we are sticking to physical filtration, prior to an ultrafiltration step, one can usually be expected to clarify a sample using a microporous membrane to remove much larger particles from a sample. These are typically in the micro range greater than 0.1um, and up to 10um. They have defined pore sizes measured in microns (micrometers), as opposed to a MWCO. Use of these types of membranes will ensure better results when you try to concentrate your sample, as it removes larger unwanted molecules, and will contain more of the smaller molecule, or particle of interest that you are looking to concentrate.

S: You just mentioned that ultrafiltration membranes were used for filtering and separating macro molecules, what kind of molecules exactly?

P: Primarily these types of membranes are used for separating and concentrating proteins, viruses, extracellular vesicles, and other nano particles. Secondary applications would include diafiltration, buffer exchange, and sample washing.

S: So, would these types of membranes be widely used in many industries?

P: Ultrafiltration membranes are used heavily in Academia, Biopharma, and Diagnostic Industries. They are also key to many workflows relating to cell culture, structural biology, in vitro diagnostics, and environmental testing.

S: There are many considerations to separation using ultrafiltration. If someone needs to select an ultrafiltration product, what would some of those factors be?

P: Well, for the device you select there are many biochemical considerations, as well as physical ones. You will want to consider the membrane material, the MWCO of the membrane, the sample volume, the chemical makeup of the solution, and the type of device you are using to concentrate your sample.

S: That sounds like quite a lot to consider. How should a customer go about making a choice with the options provided in our portfolio, and the factors mentioned?

P: You’re right, there are many options among the considerations mentioned. If we were to tackle them in order.

We could start with the choice for a membrane material. We have several options for membrane materials, such as PES (Polyethersulfone), RC (Regenerated Cellulose), and Hydrosart (Treated Regenerated Cellulose). Different membrane types can perform equally well, but one type may be optimal for a specific sample type. Membranes often have their own unique characteristics relating to charge and chemical compatibility. PES has a slightly negative charge, and decent chemical compatibility. RC is a good all-around membrane, and has low non-specific adsorption, and Hydrosart is a more chemically robust, and improved version of the RC. 

S: And how does one decide on the MWCO for a membrane when trying to separate and concentrate a molecule of interest?

P: For the MWCO, when making a choice, we use a general rule that the MWCO of the ultrafiltration membrane should be ½ to 1/3 of the protein to be retained. We also have a selection guide to assist customers who happen to be working with applications relating to ssDNA, dsDNA, or with nanoparticles. Sometimes this rule isn’t as helpful when trying to make a conversion from a MWCO to base pairs, or to nano meters, but we usually encourage customers to reach out to discuss their individual applications, as we are able to assist with making recommendations.

S: And lastly, we can’t forget sample volume, as it helps with selecting which concentration device you will use.

P: This is true, the sample volume helps to define the workflow, as quite often it drives the decision for which device you will use for your concentration step. 

S: To give people an idea of the types of concentration devices that are available, we should give an overview of the ultrafiltration products we do offer for different workflows.

P: Well, we have a large product range for ultrafiltration consisting of our biggest product offering within the portfolio which is our Vivaspin centrifugal concentrators.

These employ lab bench centrifuges to drive separation through the ultrafilter contained in the tube. We offer several sizes that range in working volumes from 100ul up to 20mL. These are a quick and easy to option when it comes to setup and result.

S:  And what options do we have for customers who are working with larger sample volumes, like say 100mL, or even several liters?

P: Well, for volumes larger than 20mL we have our Vivacell pressurized concentrator

This concentration device is for volumes ranging from 20mL up to 100mL. An inert gas is used to pressurize these devices, and they can either be gently agitated using a benchtop shaker, or they can be centrifuged. The shaker option in combination with the pressurized gas allows for low shear stresses during concentration.

And for volumes larger than 100mL we also offer our VivaFlow SU TFF / CrossFlow Devices.        

The sample is pumped across an ultrafilter using a peristaltic pump. The sample is recirculated until a desired concentration factor is reached. As an added benefit, besides being able to process larger samples without fractioning, there is also the option of performing diafiltration, or buffer exchange while concentrating your sample.

S: That feels like a good summary, is there anything else you can think of that people should keep in mind?

P: Besides the general considerations for selecting the appropriate ultrafiltration device, the following should also be kept in mind: 

-          Different membranes may have different binding specificities. Customers should test devices to ensure that they’re using the best device for their sample 

-          Different membranes may offer different recoveries, even with similar MWCO’s

-          Lower MWCOs may increase recovery but reduce speed of concentration. So, be aware of the trade off in these situations.

-          Crude fractionation is only effective when there is a 10-fold or greater difference in MW of the molecule be separated. 

S: Well, I think that really covers the basics for ultrafiltration. Thanks for your time Patrick.

P. You’re welcome, Shannon.