How to Select (and Compare)
Portable Air Samplers

Widely used in the pharmaceutical industry, these tools are finding a place in food and beverage manufacturing.

Written by Michael J. O’Grady, Senior Product Manager
for Microbiology and Analytics at EMD Chemicals Inc.

Microbial Air Sampling in the Food and Beverage Industry has historically been done with settling plates. But the information provided by settling plates is qualitative at best and does not provide true baseline data for microbial contamination. Portable microbial air sampling devices were developed to improve upon the settling plate method. Widely used in the pharmaceutical industry, portable air samplers are finding a place in food and beverage manufacturing.

Basic Principles

Microbial air samplers collect a predetermined volume of air and impact microorganisms against agar-based growth medium. Once the sample has been collected and the medium incubated, the results are typically expressed in colony forming units per cubic meter (cfu/m3). The most common impaction principles found in portable air samplers are:

Centrifugal Impaction: This was one of the first technologies employed in portable microbial air samplers. It utilizes a rotor device in the head of the air sampler to draw air–and microorganisms–in and onto a special strip containing growth medium. The centrifugal force causes particles and microorganisms to impact the medium at a rate dependent on the size of the particle. These instruments typically use a median particle size as the basis for the flow rate. Choosing a flow rate based on a median particle size can cause an overabundance of larger particles on the media. Past publications indicate that some systems employing centrifugal impaction overestimate total counts based on the variable particle separation rate.

Sieve Impaction: Sieve impaction is based on the aspiration of air through small holes at the top of the sampler. This is a proven technology and the basis for the well-known Anderson principle of air sampling. A Petri dish containing growth medium sits in a holder and a perforated lid locks in place over the medium. A fan mechanism is placed below and draws air in through the lid. The air directly impacts the Petri dish, forcing the microorganisms to stick to the surface of the agar. The impaction speed as well as particle size efficiency is a function of the diameter of the holes in the perforated lid and the speed of the fan. This allows for efficient impaction of particles as small as 1 micron in diameter at a single flow rate. Systems employing this technology typically aspirate air at higher flow rates, with volumes starting from 100L/min. These higher flow rates reduce the sample collection time. Some sieve impaction systems allow the use of standard Petri dishes, giving the user greater flexibility and availability of culture media.

Comparing Features

The next step is to determine which features best fit your needs. Features like rechargeable batteries, user-selectable sampling volumes and preset sampling volumes are common. Some additional features to consider include:

The Culture Media Format: The cost of culture media becomes the daily operating cost of these instruments; some instruments require a special format of the agar medium such as strips or cassettes. These media products can only be purchased directly from the manufacturer and are typically more expensive per test than a standard Petri dish. If your sampling frequency is high, this can add up to a significant operating cost over time, but many of the newer systems allow the use of standard plastic Petri dishes.

Delay/Remote Activation Function: Humans contribute a large amount of bioaerosols to the environment, so when collecting a sample you do not want the collector to be contributing to the total counts. The ability to delay sample collection or remotely activate the unit is an important feature for representative sampling. These features are not found on all microbial air samplers.

Flow Rate Accuracy: The flow rate accuracy for the different models of air samplers varies by manufacturer; the specifications can range from +/-2.5% to +/- 10% of the volume collected, depending upon the model. In critical environments such as clean rooms, the sampling times are typically a full m3 of air and the counts are very low. In this environment, accurate sample collection is important. Assuming a sampling volume of 1000 liters (m3) the difference in collection accuracy is as follows:

• At ± 2.5% accuracy the variance in the volume collection is ± 25 liters of air.

• At ± 10% accuracy the variance in volume collection is ± 100 liters of air.

39182-90 MAS-100 with built-in flow sensor.

This is a difference of ± 75 liters of air in overall volume collection accuracy between different models. It is possible for the target organism to be missed by the instrument with lower accuracy. Flow rate accuracy should be considered based on the manufacturing environment when choosing a system.

Calibration & Service: All manufacturers of microbial air samplers provide a certificate of calibration as well as offering calibration and service programs. All calibrations are not necessarily equal in accuracy. To guarantee the quality of these calibrations, the calibration system should be traceable to an accepted standards organization. The greater the number of calibrations in the chain between the air sampler and the primary standard, the greater the uncertainty of the measurement. Manufacturers that maintain calibration standards, which are calibrated directly by a standards organization, can minimize the level of uncertainty and provide very accurate calibrations. You should ask to see a calibration certificate and understand the quality of the measurement before purchasing a microbial air sampler. Some manufacturers sell calibration systems. This is desirable if you have several air samplers and wish to cut the yearly cost of calibrations. The cost and ease of use of these devices vary greatly; check with the individual manufacturer for more information on calibration systems. Also ask the manufacturer about their service program and average turnaround time. If you only have one sampler, you do not want to be without it for long. Some manufacturers will let you borrow a system before making your decision. This can allow you to more accurately evaluate how the features of these systems fit with your manufacturing environment. Some users have chosen to run short performance evaluations during the selection period and there are two approaches you can take. The following are key points to consider prior to starting:

Media Selection & QC: Make sure the media you have selected will grow the target organism(s). Typically, Tryptic Soy Agar is used for a general aerobic plate count; more specific medium for yeast and mold determination may also be used. Verify the performance of the media prior to starting the evaluation.

Comparing Baseline Results: Remember that microorganisms are not typically evenly distributed in the air. Airborne microorganisms are normally found attached to dust or particulates and not flying around by themselves.

39152-80 MAS-100 Eco Air Sampler and the 39182-82 DA-100 portable calibration device.

The easiest approach to use when comparing air samplers is to simply run each sampler independently and collect data at the points in your process where you currently sample. Once sufficient data points have been collected a baseline for total counts can be derived; these are then compared to determine the performance similarity.

Direct Comparison

When an existing air sampler is in use, some companies require a direct performance comparison of the old system to the new system. This can be accomplished by running both systems simultaneously at the same sampling point and directly comparing the results. This direct comparison is more difficult than the baseline comparison previously discussed. The user must have an understanding of the individual systems and set up the test properly to achieve representative results. When directly comparing samplers, the device-specific properties of each must be taken into account when designing the experiment. This is especially important when comparing the old versions of portable air samplers to any of the newer, more accurate ones on the market. Issues involving the overestimation of viable counts by centrifugal impaction systems must also be taken into account.

Sampling Time vs. Sampling Volume: When comparing two air samplers with different flow rates, the results can vary depending on whether the same air volume or the same total sampling time is used for both devices; the total sampling time rather than sampling volumes should be identical. To ensure equivalent representative samples, both units must be set to start and stop at the same time. Once the plates are read the results can then be multiplied to get to cfu/m3. When running systems with the same flow rates, identical sampling volumes may be used.

Placement Samplers During Measurement: When choosing the sampling points to be tested, some consideration should be given to avoiding areas with potential of extreme variability during the comparison (i.e. busy hallways). The air inlets for both systems should be oriented in the same direction and should be located at the same approximate level.