Explain QA/QC concept of field blanks in filter-based sampling.

• A. Field blanks are used to measure actual samples in parallel.
• B. Field blanks are unused filters that travel to the site and back to assess contamination during handling, transport, or lab processing.
• C. Field blanks are used to calibrate instruments in the field.
• D. Field blanks are discarded before sampling.

Answer: (B)

Field blanks are used to detect contamination that can be introduced during handling, transport, or laboratory processing, not from the sampled air. In filter-based sampling, an unused filter (or equivalent sampling medium) is carried to the field and back through the same handling steps as a real sample but is not exposed to the ambient air. By subjecting this blank to exactly the same chain of custody, opening, packaging, transport, storage, and lab analysis as a real sample, any contaminants found in the blank reveal where non-air sources of contamination are entering the process.

If the field blank shows little to no contamination, you gain confidence that the sampling and analysis system is not adding bias to the actual samples. If the blank shows a measurable signal, it indicates background contamination from handling, cross-contamination, or lab procedures that could affect the real samples, so those data may need adjustment or flagged QA/QC-wise.

The other statements don’t fit because field blanks are not used to measure actual air samples, they don’t calibrate instruments in the field, and they aren’t discarded before sampling.