What is the significance of field blanks in QA/QC results?

• A. They help detect contamination introduced during sampling; they should be non-detect.
• B. They are used to calibrate instruments.
• C. They are the primary sample for analysis.
• D. They determine the allowable clearance level.

Answer: (B)

Field blanks establish a baseline for the measurement system by running a clean sample through the entire field process. They carry no analyte, so any signal detected reflects background or contamination from sampling, handling, or the instrument itself. This baseline is essential because it effectively sets the instrument’s zero level in the field, allowing you to identify and correct for background noise or carryover, and to ensure that measurements of real samples are accurate.

In practice, a field blank should come back non-detect, which confirms there’s no introduced contamination affecting the results. If a blank shows a signal, it flags issues that could bias real sample readings and may require recalibration, adjustment, or re-collection of data. The key idea is that blanks help calibrate the instrument’s baseline in the field and verify the integrity of the entire QA/QC process.

The other options don’t fit as well: field blanks are not the primary sample for analysis, they aren’t used to determine the allowable clearance level, and while they reveal contamination risks, their main purpose in QA/QC is to establish and verify the instrument’s baseline in the field.