What is the purpose of a field blank in QA/QC and how is it used?

• A. A field blank detects contamination from sampling and handling.
• B. A field blank contains a known amount of analyte to check recovery.
• C. A field blank is used to calibrate the instrument before analysis.
• D. A field blank is a duplicate of the sample to test precision.

Answer: (A)

The main idea is that a field blank acts as a detector for contamination that can be introduced during the sampling and handling process. It is a sample of clean matrix (like distilled water) that goes through the exact same collection, transport, and handling steps as a real sample. By analyzing this blank, you can see whether contaminants are picking up from bottles, field equipment, or the environment during sample collection. If the blank shows any analyte, it signals that contamination entered the process and could bias the actual sample results, prompting investigation, data flagging, or re-collection.

This isn’t about checking recovery, which would involve adding a known amount of analyte to a matrix (a spike). It isn’t used to calibrate instruments, which relies on calibration standards with known concentrations. It also isn’t a duplicate of the sample to test precision, which would be a replicate used to assess reproducibility rather than contamination during sampling.