Hard to Find Configurations Part 2

Welcome back to the continuation on the “Hard To Find Filters”. Part 2

In the last post I covered the first 2 points of the ideal filer.

In this post I will detail points 3, 4 and 5

Point 3 … be pure, down to nanograms, of the substance that you are measuring.

Ideally, the filter used to capture particles in air or liquid for analysis, should contain zero of the substance you are sampling for. In this manner the analyst can obtain the best limit of detection and thus the best limit of quantitation.

But of course, background contamination is an elusive goal for a variety of reasons. The most common filter material employed in the science of airborne particulates, is the Mixed Ester of Cellulose (MEC) or also known as Mixed Cellulose Ester (MEC).

As the name implies, the filter is composed of cellulose. Cellulose is a naturally occurring substance found in trees trunks and the cotton of the cotton plant.

Two major industrial uses for cellulose is the acetate and nitrate form. The former is used cellulose for the manufacture of clear plastic films or solid parts of optically clear plastic articles. In the last century, movie and still films consumed vast amounts of cellulose acetate.

The latter is in the production of guncotton. The munitions industry still uses millions of tons for this application.

When the invention of thin cellulosic membranes came to be (Circa 1920) and eventually routinely used in laboratories (circa 1943) and further commercialized in (1954) in the separation of particles in liquids and harvesting particles for chemical analysis, the need for the filter material to be pure, became more acute.

Over the span of 15 years after 1954, manufactures of membrane filters, improved their manufacturing process, and utilized purer forms of chemicals used in the production of the membranes.

As a result, the chemical purity of membrane filers improved by orders of magnitude.

By the 1970’s the purity reached a plateau as the level of purity reached acceptable levels and did not interfere with the analytical quantitation of contaminants collected on membranes. Around the mid 1980’s however, four important events coalesced to create a greater need for purity.

A: More sensitive analytical instruments hit the market making it possible to detect smaller and smaller amounts of a specific contaminant

B: Research on the toxicological effects of airborne contaminants, demonstrated that smaller amounts of inhaled contaminants would trigger deleterious effects in humans.

Concurrently, aerosol research showed that different parts of the inhalation pathway were reached by of different size particles during inspiration.

C: Some industrial processes, that generate airborne contaminants, are very short in duration. Short term exposure limits (STELs) have been implemented to ascertain safety of working with certain materials. The short duration of sampling time impacted the amount of contaminant that could be collected, further challenging the detection.

The net result of all this, was that regulated substances had their permissible airborne quantities, reduced, and the need to utilize cyclones to harvest specific size ranges from the total inhaled quantity, and short (15-30 min samples) became more common place.

All the above actions served to reduce the amount of material collected on the 37mm or 25mm filters commonly used today. If the meantime, background contaminants in the filter did not drop in tandem with the collected material, the limit of detection will suffer.

Point C in the above paragraph, has generated specialized sampling equipment that employ specialty cut filters. These filters have been hard to source, Air Sampling Devices has focused on this segment and makes available filters for, Marple™ impactors, Mark III™ Anderson stack samplers, and Respicon™, in several substrates.

These filters are Specialty Filters in their own right, as they are not widely used.

Making these filters easier to source, is the first concrete step in Air Sampling Devices’s mission to bring some relief to the consumers of industrial hygiene filtration products.

The follow-on actions will be described further down the post and will address other hard to procure filters.

The industrial hygiene community is faced with having to increase sensitivity.

Several ways to do that

1. Increase the volume of air aspired through the collection filter (increase the sample collected for a given time). That means more powerful pumps, means heavier pumps. 4 Liter per minute is the most common upper limit in 80% of pumps used today.
2. Hunt for a “cleaner” filter.
3. Pretreat the filters used in the sampling, to reduce the background.

The first method involves the expense of upgrading the pumps, in the “sampling fleet”. Upgrading can be expensive and unpopular by the wearer of the sampling equipment because of the increase in weight of the pump.

The second method is tedious and a bit of a hit and miss, as background contaminants are not uniform from lot to lot of filters. Not because the manufacturer is “dropping the ball” but because of the nature of the raw material. After all it is a natural product, at the mercy of its “homeland”, literally the ground that the tree or the cotton bush grew in.

The raw material suppliers (Cellulose esters) could be more helpful, but it is strictly economics. The amount of cellulose required for all membrane filter production in the world is probably in the low tens of tons per year. Only a small fraction of that is required to be purified to the degree required for use with modern instrumental analysis.

Manufacturers of membranes turned to the raw cellulose suppliers to ask if they would be able to supply a purer form of cellulose.

Their requests fell on deaf ears. Membrane manufacturers were such a small application when compared with the major demand from the two main applications, plastics and munitions, (which do not require high purity) and you can see that no raw material manufacturer of cellulose is interested in devoting effort to supply purified raw material. That lack of suitable raw material in turn leaves the membrane filter manufacturer in an impossible position.

With the first two of three options reaching their limit.

The third option is viable , but it is left to the individual user to perform the steps required.

Examples of that is, the US Air force developed a purification method to rid filters of low levels of chromium to allow the detection of as low as .01 micrograms of Chromium in one 37mm disk, with a good limit of quantitation.

Imagine doing that with each of the 26 metals on your own.

Thankfully for the time being not all the metals require such draconian measures, however lead (Pb) and cadmium (Cd) are starting to enter that realm of this needed purity.

It is Air Sampling Devices’ plan to follow that third option and delve into making ultrapure membrane filters and other substrates, available commercially.
The path to achieving this, is as follows :-

1. To identify contaminants that are challenged with too much background in presently available, off the shelf membrane filters.
2. To identify the necessary processes that can purify filters sufficiently.
3. Publish the purity of each batch sold, so that the consumer does not have to do QC on each batch purchased, without any confidence that it contaminant will be within the acceptable range for their needs.
4. Make the filters commercially available at an acceptable cost to the consumer.

Point 4 …. flow with no pressure drop.
Physically, point 4 is an impossibility, but it is possible to utilize larger pore-size filters to collect contaminants by following the guidelines that were laid out in Point 1 and 2.

Air Sampling Devices will do the necessary testing to prove the efficacy of larger pore filters for the quantitative collection of specific contaminants and demonstrate equivalency at the flow rate required by the specific method used to sample for specific contaminants.

Larger pore-size should reduce the pressure drop, at the equivalent air flow, without compromising capture capability.

Point 5. …. Have trivial cost
Answering this one is tricky, we will strive to keep the cost as low as feasible, but the promise will be that the price will be based on value added. If an attribute imparted to a filter enhances the value, the filter will be priced to reflect that added value.

Takeaway’s from this edition of “What’s in the Air”
Filters that have an order of magnitude larger pore size than 0.8 µm, can be used to capture 99.99% of particles > than 0.5 micrometer when sampled by standard air sampling cassettes.

Limits of detection can be enhanced, by using larger pore membranes wherein larger volumes of air (larger amount of a contaminant) can be collected without suffering pumps faulting out.

As permissible limits of contaminants keep dropping, the collection media’s natural background contamination can frustrate the detection of low levels of contamination.

Careful selection of sampling media can mitigate the background interference

Air Sampling Devices plans to commercialize pretreated filters that will have lower backgrounds for specific contaminants