“AMC filters don’t work.” “We don’t need to trap AMC’s.” These facile sentiments impede progress in critical applications.

Certainly, being proactive with Airborne Molecular Contamination (AMC) is essential. Understanding, detecting, and eliminating or reducing AMC sources should be the first line of defense.¹ Sometimes, however, AMC must be trapped; you have to “head it off at the pass.” Successful removal of molecular contamination from air and gas streams requires knowledge of the chemical nature of the contaminating molecules.

Why are AMC trapping devices not ubiquitous? The field of molecular filtration and purification is still emerging from the shadow of emphasis on particle control. Further, many industry sectors still do not fully appreciate the importance of molecular level process control and the distinctions between particle removal and molecule removal.

While specific applications are distinct, AMC removal and gas purification, including the preparation of Clean Dry Air (CDA), have many common processes and suppliers. Both involve removal of unwanted molecular species before they reach the surface of the product.

MOLECULAR REMOVAL IS MORE COMPLEX THAN PARTICLE FILTRATION
For particles, the primary discriminator is particle size. A particle filter acts somewhat like a window screen to let small particles and gases through and block larger particles. Granted, High Efficiency Particle Air (HEPA) filters are more complex than a window screen, and they can trap or impede particles both larger and smaller than the pore openings between filter fibers. However, standard HEPA filters do not remove molecular contaminants that are several orders of magnitude smaller than the size rating for a HEPA filter. For AMC filtration, rather than removing contaminants on the basis of size alone, the chemical nature of the contaminant becomes very important.

TERMINOLOGY
The fact that molecular filtration is much more complex than particle filtration is reflected in the terminology. The terms filtration and purification are both widely used; some experts are adamant about the appropriateness of one term over another. Some use filters for devices that physically separate gas components but do not change their chemical nature as distinguished from purifiers for devices that alter the chemical being removed. We tend toward the view² that a filter is a device to perform purification. Other terms, such as trap, sieve, desiccant, or getter, may be used to describe certain filtering or purification functions.