Air Sampling 101

Centrifugal samplers are generally known to be efficient in collecting particles 15 µm and larger, but have trouble with smaller particles. Less than 10 percent of particles under 2 µm are deposited. The use of a strip instead of a standard Petri dish presents evaluation problems and colony counters may have difficulty counting the strip, requiring counts to be done manually. Because the sampler exhausts the air stream from the same opening used to create the vortex, the surrounding atmosphere is disrupted, affecting the accuracy of the collected sample. This single opening system also creates another problem relating to validation. With the air entering and leaving the same opening, flow rate quantification is only theoretical. The vortex itself can also affect sampling accuracy. As the particles enter the vortex through the rotating impeller the CFUs (colony forming units) will break apart showing higher counts and giving a false higher efficiency rating that is not representative of the actual atmosphere.

Impingement

With an all glass impinger, air is drawn through a curved suction tube (curved to simulate the nasal passage) and accelerated through a jet at the bottom. The bottom part of the impinger is filled with a suitable liquid that captures the particles. The liquid is filtered after sampling and then processed as usual. Impingement is an efficient method for collecting various sizes of microorganisms and is therefore suitable for determining the initial air quality of an area.

Although effective, impingement is not considered a convenient air sampling method. The glass containers are not disposable and must be prepared every time and depending on the area to be sampled, this can be very time consuming. Impingement can not be reliably validated because flow rate is very low and can’t be measured. While impingers are efficient for collecting microbes, the collection time must be carefully monitored to decrease the effects of evaporation of the collecting fluid as well as cooling of the sample. Evaporation and cooling can both negatively affect the retention rate of the microorganisms. Also, microbes grow while sampling and can be disrupted or separated during sampling, which can create higher counts similar to that of the centrifugal impactor.

Gelatin Membrane Filtration

The most innovative air sampling technology is the gelatin membrane filtration method. Air is sampled at a programmable flow rate and passes through the gelatin membrane filter which captures the microbes. The filter is 300 µm thick, making it capable of capturing even the smallest microorganism. It is very complex and porous, so microbes will collect not only on the surface, but will become entangled within the filter. This is the only sampling method with an absolute retention rate and the only method that also reliably captures airborne viruses. The filter is composed of 50 percent water which protects the captured microbes and completely eliminates the problems associated with desiccation. Another important feature is that all of the air entering the sampler must first pass through the filter; thereby ensuring microbes are not reintroduced into the atmosphere. The exhaust air is not located close to the filter sampling the air, so air flow is not affected.

Processing the filter is also convenient and easily read with most automated colony counters. After sampling, the filter is placed directly on a standard (90mm) agar plate and dissolves, leaving the microorganisms in direct contact with the agar. The filter can also be dissolved into a sterile solution for special evaluations, such as when inhibitors (i.e. disinfectants or antibiotics) are present in the air being sampled, very high colony counts are expected or the microorganisms collected are to be incubated on several different media types at the same time.