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

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

Viable Air Samplers are used for quantitative determination of airborne contamination in cleanrooms and isolators. To maintain the cleanliness of the environment, the air flow should neither disturb the unidirectional air flow nor pose a contamination risk to the environment. As portable air samplers are sometimes moved and used in various environments with different levels of microbial contamination, there might be a risk of cross-contamination from higher to low contaminated environments. The MAS-100 NT^® is equipped with a brushless motor which generates a minimal quantity of particles, so that no filter is required even for GMP Grade A usage. A H13 particle filter can be mounted at the exhaust air, when needed (Figure 1).

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

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

Test procedure

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

In Figure 2, the MAS-100 NT^® sampler was positioned in a non-controlled reference environment, whereas in other configurations the instruments were placed beside the ACS Plus particle counter (KM Optoelektronik GmbH, Germany) in the cleanbench.

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

Cross-contamination of cleanrooms

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

A 1000 L air sample was collected with a MAS-100 NT^® in the reference environment with or without filter. Thereafter, the instrument was transferred into the clean environment and started again. Particle counts were taken from the exhaust air at the outlet every 5 s. The test was repeated 5 times for both setups. In tests without filter we found a total of 126–296 particles in the first 5 seconds after starting the blower and between 0 and 3 particles for the second 5 seconds and no particles thereafter (Figure 3). A total of 1042 particles were detected between all 5 repetitions, about 12% of them larger than 1 µM. The data demonstrate that there is 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.

If the MAS-100 NT^® was equipped with a HEPA air outlet filter 0–6 particles were counted in the first 5 seconds and 0 thereafter (Figure 3), all of them smaller than 1 µM. 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.

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

Filtration efficiency of the complete assembly

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

For every test we took a reference sample by measuring the particle count of the exhaust air of a MAS-100 NT^® sampler running in the reference environment. As the particle load was very high, we took 10 samples of 10 s each 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^® instrument with particle filter running in the same environment. 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 (Table 1) for H13 filters. Throughout the 36 minutes (approx. 1 m³) sampling interval, no trend was evident for any filter (data not shown). The filter assembly therefore meets the need to protect an environment from carry-over of particles from another environment with lower particle rating.

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

	0.3 µM	0.5 µM	1.0 µM	3.0 µM	5.0 µM	Total Particles
ISO 5 particle limit	10200	3520	832	N/A	N/A	N/A
ACS Plus: Clean air in workbench (mean, n=2)	15	8	5	2	7	37
ACS Plus particle Counter: Non-controlled air (mean, n=2)	762,314	186,569	45,432	4,462	5,923	1,004,700
Reference: MAS-100 NT® without filter (mean, n=10)	701,746	163,906	42,185	4,109	4,956	916,901
MAS-100 NT® with filter (mean, n=10)	69	6	1	0	0	76
Filter retention efficiency	99.99%	100%	100%	100%	100%	99.99%
Max particle count with filter	119	14	6	0	1	140
Min particle count with filter	6	1	0	0	0	0

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

Long-term filtration efficiency

It is recommended to recalibrate a MAS-100 NT^® system annually. Therefore, under normal usage a filter should be able to be used for a complete calibration interval. We assumed 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 permanent operation. Two MAS-100 NT^® instruments with filter were run continuously for 21 days in the uncontrolled reference environment. From time to time, 1 m³ of the exhausted air were sampled for particles from both devices and compared to the reference sample taken immediately before or after without filter. The data in Table 2 shows that there was no detectable deterioration of filtration efficiency during this time. All filters met or exceeded 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®

Filtration Efficiency	Instrument 1		Instrument 2
	Day 1	Day 21	Day 1	Day 21
0.3 µM	99.980%	99.976%	99.955%	99.984%
0.5 µM	99.995%	99.994%	99.959%	99.999%
1.0 µM	100.000%	100.000%	99.937%	100.000%
3.0 µM	100.000%	100.000%	100.000%	100.000%
> 5.0 µM	100.000%	100.000%	100.000%	100.000%
Total particles	99.985%	99.981%	99.955%	99.988%

Summary

The MAS-100 NT^® is a portable instrument which is designed for quantitative analysis of microbial contamination in 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 with the same instrument is performed 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 was essentially eliminated by the insertion of a HEPA filter at the air outlet of the MAS-100 NT^® viable air sampler.
• The filter had 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 did not reduce the filtration efficiency or clog the filter.

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