HomeViable Air MonitoringAir Outlet Particle Filter for Viable Air Sampler MAS-100 NT®

Air Outlet Particle Filter for Viable MAS-100 NT® Air Sampler

Roland Durner 1, Tony Ancrum2, Anne-Grit Klees2

1MBV, Switzerland, 2Merck KGaA Darmstadt, Germany

Content obtained from MBV AG - Microbiology and Bioanalytic - Industriestrasse 9 - CH-8712 Stäfa

Accurate Microbial Air Sampling for Highly Controlled Areas and Cleanrooms

Viable air samplers are used for the quantitative determination of airborne contamination in cleanrooms and isolators. To maintain the cleanliness of the environment, the airflow should neither disturb the unidirectional airflow nor pose a contamination risk to the environment. As portable air samplers are sometimes moved and used in different environments with different levels of microbial contamination, there might be a risk of cross-contamination from higher to lower-contaminated environments.

ISO class 5 GMP grade A environments

If instruments are used in different cleanroom classes, there is a minor risk that particles may be carried over inside an instrument. The MAS-100 NT® air sampler is equipped with a brushless motor that generates a minimal amount of particles, so the use of a filter can be avoided even for GMP Grade A usage. However, an H13 air particle filter can be mounted at the exhaust air, when needed (see Figure 1).

  1. Air inlet
    Perforated lid to accelerate air onto agar plates

  2. Blower unit
    Brushless motor drives air blower controlled
    by flow sensor

  3. Filter adapter
    H13 particle filter mounted at the air exhaust

  4. Control unit
    Display, control panel, electronics and battery
MAS®-100 NT equipped with a H13 particle filter

Figure 1.MAS-100 NT® air sampler equipped with a H13 particle filter

In this article, we demonstrate the cleanliness ability of the MAS-100 NT® air sampler with and without in-built filters under several environmental setups.

Experimental Setup: Air Particle Measurement Using MAS-100 NT® Air Sampler

Experimental set-up of particle measurements

Figure 2.Experimental set-up of particle measurements. 1) MAS-100 NT® instrument in reference environment; 2) Sampling adapter collects exhaust air and directs it to the particle counter, 3) Biosafety workbench with low particle count environment; 4) Particle counter ACS Plus in biosafety workbench

The standard MAS-100 NT® air sampler is compared to a MAS-100 NT® air sampler equipped with an H13 filter. The particle emission under different environmental conditions is measured using an ACS Plus particle counter, which is directly mounted to the air outlet of the air sampler.

In Figure 2 the MAS-100 NT® sampler is positioned in a non-controlled reference environment, whereas in other configurations the instruments are placed beside the ACS Plus particle counter (KM Optoelektronik GmbH, Germany) on a clean bench.

Filtration Efficiency of MAS-100 NT® microbial air sampler in environmental monitoring

1. Cross-contamination of cleanrooms

After the test system was established, the following basic assumption was to be tested: Is there a need for a filter to prevent the transfer of particle contamination between different environments?

A 1000 L air sample was collected using a MAS-100 NT® in the reference environment with or without a filter. Next, the instrument was transferred into a clean environment and the same procedure was followed. Particle counts were taken from the exhaust air at the outlet every 5 seconds and the test was repeated 5 times for both setups. In tests without a filter, a total of 126 –296 particles were found in the first 5 seconds. After starting the blower, 0 – 3 particles were found for the next 5 seconds, and no particles were found thereafter (Figure 3). A total of 1042 particles were detected between all 5 repetitions and about 12% of them had particle sizes larger than 1 µm. This data demonstrates the minimal transfer of particles between different environments. The fact that there were no particles detected after 10 seconds proves that the particles were not generated by the blower motor or other instrument components.

When the MAS-100 NT® was equipped with a HEPA air outlet filter, 0 – 6 particles were counted in the first 5 seconds, after which 0 particles with particle size smaller than 1 µm were counted (Figure 3). The filter dramatically reduced the transfer of particles and virtually eliminated the risk of them being CFU as those tend to be larger than 1 µm.

Transfer of particles from low to high contaminated

Figure 3.Transfer of particles from low to high contaminated to low contaminated environments

2. Filtration efficiency of the complete assembly

10 filters on two different units of MAS-100 NT® (5 filters each) were tested for filtration efficiency of the air sampler, filter adapter, and HEPA filter (the complete assembly). The filter is rated H13 and expected a filtration efficiency of 99.95% or better for particle sizes of 0.3 μm or larger.

For every test sample, a reference sample was taken by measuring the particle count of the exhaust air from the MAS-100 NT® sampler running in the reference environment. As the particle load was very high, only 10 samples for 10 seconds were assessed and extrapolated the particle count to 1000 L. This reference count was compared to a 1000 L sample taken from the identical MAS-100 NT® sampler with a particle filter running in the same environment. For every 3 minutes, the particle count was recorded to assess if there was a trend. In all cases, the filter assembly passed and even exceeded, the required performance threshold (see Table 1) for H13 filters. Throughout the 36 minutes (approx. 1 m3) of sampling interval, there was no trend for any filter (data not shown). The filter assembly, therefore, meets the criteria to protect an environment from the carry-over of particles from another environment with a lower particle rating.

Table 1.Filtration efficiency of the HEPA H13 filter assembly for the MAS-100 NT® air sampler.

For reference, the permissible particle concentrations of ISO Class 5 are also given

3. Long-term filtration efficiency

It is recommended to recalibrate a MAS-100 NT® air sampler annually. Hence, under normal usage, a filter can be used for a complete calibration interval. The assumption was that 10 samples of 1000 L per day would be taken under intense usage for 300 days per year. This amounts to about 21 days of permanent operation. Two MAS-100 NT® instruments with filters were run continuously for 21 days in the uncontrolled reference environment. From time to time 1 m3 of the exhausted air was sampled for particles from both devices and compared to the reference sample taken immediately before or after without a filter. The data in Table 2 shows no detectable deterioration of filtration efficiency during this time. All filters meet or exceed the H13 filter specification. In addition, there was no detectable filter clogging and airflow calibration remained within specification.

Table 2.Filtration efficiency over a simulated 1-year usage lifetime for two MAS-100 NT® air sampler

MAS-100 NT® viable air sampler with HEPA air exhaust filter eliminates particle carry-over

The portable MAS-100 NT® microbial air sampler is designed for quantitative analysis of microbial contamination in the air. The brushless motor does not generate particles, which exceed the limits of a cleanroom ISO 5. This study investigated the need for filtration of the exhaust air, especially when air sampling is performed with the same instrument and in environments with a range of contamination levels.

  • There is a minimal risk of particle carry-over between different environments from air trapped inside a microbial air sampler.
  • Particle carry-over is essentially eliminated by the insertion of a HEPA filter at the air outlet of the MAS-100 NT® viable air sampler
  • The filter has no significant influence on the airflow calibration and therefore on the microbial sampling efficiency
  • Stressing the filter for a simulated year of heavy usage in an uncontrolled environment does not reduce the filtration efficiency or clog the filter

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