What is the minimum number of samples collected on 0.8 micron mixed cellulose ester (MCE) filters loaded in conducting cassettes with extension cowls?

• A. Two
• B. Four
• C. Five
• D. Three

Answer: (D)

The key idea is using replication and a background check to ensure accurate, reliable results from air sampling with close-faced cassettes and extension cowls. When you collect air through a 0.8 µm mixed cellulose ester filter in a conducting cassette, the extension cowl helps minimize wall losses and inlet contamination, giving a cleaner representation of what’s in the air. However, because concentrations can change over time and there can be small equipment or background contamination, you don’t rely on a single sample. You typically run two air samples from the location to provide replication and a third filter used as a background/blank check to verify that any measured mass isn’t coming from the setup itself. This combination gives you enough data to assess precision and detect contamination, which is why three is the minimum. Four or five would be more than the minimum required for this QA purpose, unless the study design calls for more replication.