How Filters Work: Mechanisms of Filtration Part 3
June 30, 2022
In the previous blog, we concluded that filters work differently in gas and liquid applications due to the presence of at least one additional removal mechanism – inertial impaction. Here we will look at another that can add a significant boost to the retention capabilities of filters in gas applications – diffusional interception.
Many years ago, in high school, I recall a science experiment. In this experiment, we were asked to describe the movement of small smoke particles held under a microscope. The movement was erratic; particles dancing randomly, rapidly changing direction without any visible reason. The cause, we were later told, was the phenomena of Brownian motion, where high speed gas molecules interact with the much larger smoke particles, causing them to change direction as their kinetic energy is transferred in the collision.
Imagine a particle passing through a small gas-filled tube. A heavier particle would be expected to pass through in a straight line but, if the particle is light enough, it is affected by Brownian motion and erratically changes direction as it passes through the tube. This erratic motion increases the likelihood of the particle impacting and embedding into the side of the tube. Our tube in this model is a simplified pore but represents millions of pores in a typical filtration matrix. As a result, filters with a pore size as small as 0.2 micrometers, that can be validated to robustly retain bacteria from liquid process streams, can be shown to retain even the smallest of viruses (20 nm). This can be critical for the control of virus contamination resulting from the many process gases that are required in processes such as mammalian cell culture.
Most Penetrating Particle Size (MPPS)
For gas applications, when we add together all of the potential removal mechanisms, we can estimate what is commonly called the MPPS – the most penetrating particle size; the size of particle that is most challenging for any filter to remove. When evaluating the theory, size exclusion through direct interception can be considered to be constant, inertial impaction increases with particle size and diffusional interception decreases with particle size. The minimum in this combined impact is somewhere around 0.3 micrometers, about the size of the smallest common bacteria. This particle size is used as the test particle for HEPA (high efficiency, particle-free air) filters, common in claims made of high-end vacuum cleaners and in air handling filters in pharmaceutical manufacture. It should be noted that this is not a suitable measure of performance for critical pharmaceutical filters – a subject we will visit in our next blog.
Electrostatic interaction is the final removal mechanism relevant to both gas and liquid applications in the biotech industry. As the name suggests, this enhances removal based upon attractive forces resulting from opposite charges between the particle being removed and the surface of the filtration matrix. As most particles tend to have a surface negative charge, a net positive zeta-potential on the surface of a filter media can significantly increase the ability of a filter to retain these charged species. Electrostatic interaction as a mechanism is commonly deployed to assist in the retention of waterborne contaminants such as diatoms and silicaceous particulate that can be difficult to filter. In the world of pharmaceutical processes, it can be used to retain endotoxin, a common and known process contaminant originating from bacteria that can be present after the sterilization process. These bacteria are killed using either steam sterilization, or gamma/x-ray irradiation. It is here that the lines between simple removal mechanisms and specific charge-based purification can often blur. However, from a filtration perspective, the modification of surface charge via a ligand, designed to have an affinity for a defined species, can be explored and is seen in a number of chromatography products.
Mark Ayles, Senior Marketing Manager
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