A Guide to Buying the Right Microbial Air Sampler

What is a microbial air sampler?

An active microbial air sampler, also known as a bio-impactor, is an instrument routinely used at various sampling locations for environmental monitoring within a pharmaceutical manufacturing facility. It samples a predetermined volume of ambient air for the presence of viable microbial contaminants. The air is impacted onto an agar plate, where the cells or spores are captured. After incubation of the plate, growth is analyzed to determine whether the level of contamination is within limits or not. Worldwide regulations emphasize the importance of active microbial air monitoring as part of a comprehensive contamination control strategy to ensure product safety.

What are the criteria when selecting a microbial air sampler?

Quite a few criteria should be considered when you choose a suitable active air sampler. The priorities can differ from buyer to buyer. They depend on the features of the instrument, the characteristics of your facility and the products you manufacture, and how you intend to use the sampler. We have grouped the selection criteria into the following broader categories:

• Technical properties and features
• Cleanroom suitability
• Ease of use
• Data integrity and compliance
• Supplier service and maintenance

Technical properties and features of a microbial air sampler

• Collection efficiency: A microbial air sampler must be validated to demonstrate that it collects viable particles efficiently according to EN 17141 or ISO 14698. This should be performed by a competent external body on behalf of the supplier or manufacturer. The validation method has two parts: physical and biological efficiency. Physical efficiency describes an air sampler’s ability to collect particles of various sizes and is defined by the d50 value. Biological efficiency, determined as percent recovery, describes the sampler’s ability to capture organisms from the air in a way that these remain viable after impaction. Biological and physical efficiencies are influenced by the impaction speed, which must be high enough to collect small particles but low enough not to harm the microorganisms. The better the air sampler is at impacting small particles without significantly impairing the viability of captured organisms, the more reliable your viable air monitoring results will be. While most samplers do well with larger particles bigger then 2 µm diameter, some struggle with particles smaller than 2 µm, which are extremely important in areas grade A and B. Buyers should ask suppliers for detailed validation data and compare efficiencies.
• Flow rate accuracy: The most critical operating parameters of a microbial air sampler are flow rate accuracy and sampling time, which together determine the volume of sampled air. Periodic calibration of these sampling parameters should be traceable to ISO 17025 accredited laboratories, ensuring the validated accuracy of the calibration tools in comparison to standardized references. Advanced samplers feature sophisticated technology such as a mass flow sensor system, which helps to ensure that the flow rate stays at the same steady and repeatable level during sampling, or components that carry out self-checks for leaks. Check which flow rates the air sampler offers. Most operate at a fixed rate of 100 liters per minute as suggested in EN 17141, while some offer the option to set other values. 200 L/min reduces the sampling time by half, but should be validated for efficiency. Ask your supplier for data.
• No-touch sampling start: If you want to move away or even leave the room before sampling starts, the instrument will have to offer either remote or time-delayed starting of the sampling process.
• Battery properties: If you operate your active air sampler on battery power, battery capacity and lifespan matter. Determine how many samples the battery can take before recharging and how many charging cycles you can expect from the battery over its lifespan.

Cleanroom suitability of a microbial air sampler

• Non-viable particles: It is important to consider that microbial air samplers themselves may emit non-viable particles during sampling—to what extent depends mainly on the operating principle of the blower unit. Different maximum levels of non-viable airborne particles apply to cleanrooms of the various grades, so high emission levels could limit the areas in which the air sampler can be used. The supplier should be able to hand you documentation that sheds light on the sampler’s particle emission levels.
• Size, weight, robustness: If you need to transfer your microbial air sampler frequently, its portability and weight should be considered. A small footprint is advantageous particularly in sampling environments where space is at a premium. On the other hand, the sampler should stand stably to avoid it being moved—or worse, knocked over—during sampling. Generally speaking, your viable air sampler will probably serve you longer if it is made of robust materials. Try to find the right balance of these properties for your intended usages.
• HEPA filter: Transferring active air samplers from lower to higher grade areas is known to be a significant source of contamination because viable particles may be trapped inside. Any sampler moved to a Grade A area should offer the option to have a particle-retaining HEPA filter mounted to its air outlet. Of course, this filter should not impact the airflow and the sampling efficiency.
• Airflow: The microbial air sampler itself should also be designed to minimize disruption to unidirectional airflow as required by EU GMP Annex 1, something smoke studies can visualize.
• Easy sanitization: Simple cleaning and sanitization is a further consideration. Aluminum or stainless steel housing, a design that minimizes dirt traps, and sanitizable or sterilizable components are preferred sampler features, as is the possibility to VHP decontaminate the inner parts of the sampler.

Ease of use of the microbial air sampler

• Convenient handling: To enter settings, state-of-the-art microbial air samplers feature a touchscreen that can be operated conveniently even when wearing gloves. It should be intuitive to use. Ease of use, however, is more than a convenience issue. The more complex it is to operate your microbial air sampler, the more likely handling errors will occur. Human error can lead to a false-negative result and possibly a compromised finished product, or to a false positive result that triggers an unnecessary and costly out-of-specification investigation.
• Minimal manual actions: What can help to minimize human errors is a configuration possibility to limit the actions that operators perform on the air sampler, ideally to pressing a single button. An automatically closing sampler lid reduces manual activities further. Lids are made of stainless steel or aluminum, with some suppliers offering single-use lids that do not require autoclaving. It is worth finding out if the supplier thoroughly quality-checks these lids for their perforations. Clogged or poorly drilled holes can impair air sampling.
• Self-check features: Advanced microbial air samplers possess self-check features that reveal a leak or errors such as a missing agar plate or the wrong perforated lid type being used, which further reduces the likelihood of human errors.
• Plate-size adaption: While most air samplers are primarily designed to operate with 90 mm diameter Petri dishes, many offer the option to adapt the plate holder to 55 mm dishes. Size adaptation should be easy to do without using a tool, and plates should be simple to insert and retrieve.

Data integrity and compliance of a viable air sampler

• Data transfer & storage: Data integrity and traceability requirements are constantly on the rise, so it is advisable to think ahead and take this into account when deciding on a new microbial air sampler to purchase. If your facility is not yet digitalized, this might change in the coming years. In highly digitalized environments, the smooth transfer of data into a LIMS or a data management system for environmental monitoring may already be a necessary requirement, so the connectivity features of the viable air sampler must support this. A few instruments allow the sampling data to be scanned in via a barcode shown on the display and transferred via Wi-Fi communication. The exported data should include operator ID, date, time, location, sample volume and Petri dish.

Supplier service and maintenance

• Spectrum of services: Occasionally, your microbial air sampler will need special attention. It must be qualified (IQ, OQ and PQ) before implementation for routine use, and it requires calibration at regular intervals, at least once a year. While some of these tasks are best performed by the user, others are usually sourced to the supplier or to third-party service providers. Find out which services are available. A comprehensive service and support spectrum would include IQ/OQ, maintenance, calibration and technical assistance. It makes sense to set up a plan what is going to be done by whom, when and where.
• Calibration: As the task that verifies instrument constants such as flow rate and timer, calibration is crucial for the continued reliable operation of the microbial air sampler, and is performed along with its lids. Some suppliers sell an anemometer with which users can perform calibration themselves. If you outsource calibration, consider whether this will be performed at your facility or whether you have to send the sampler in. In the latter case, find out how long you will have to do without the air sampler. Depending on how well or not you pack it, there is also a certain risk of damage to the instrument during shipment. When calibration is completed and your instrument returned, you should be able to get a calibration protocol and a sample certificate.
• Maintenance: If the instrument malfunctions, you will need maintenance support, which can usually be agreed in a separate contract from calibration. The provider should have technical support to help with any technical questions. Here, too, you will want to know whether maintenance staff is based in your region and can visit your facility or if your only option is to send in your instrument.

Microbial air samplers for specific purposes or environments

Finally, it is worth noting that some suppliers offer product variants of their microbial air samplers (or conversion options) for specific purposes or environments, for example

• Samplers with multiple sampling heads for simultaneous air sampling
• Samplers for use in isolators that allow moving parts to be placed outside the critical area
• Samplers with ATEX certification for use in explosion hazard areas
• Samplers for microbial monitoring of compressed gases

Although often quite different in appearance, these sampler variants are usually based on the technology of the supplier’s standard model. Many of the discussed criteria apply to these instruments as well.

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