Which of the following surfaces would NOT be used to check calibration?

The choice involving a surface with high iron content is a valid option for not being suitable for checking calibration. Calibration of lead detection equipment typically relies on surfaces that allow for consistent and accurate measurements of lead levels. A surface with high iron content could interfere with the lead detection process, as the properties of iron might affect the readings due to electromagnetic interference or other chemical interactions.

In contrast, surfaces with a known level of lead are standard for calibration since they provide a reliable benchmark to ensure that the equipment is functioning properly and accurately detecting lead levels. A surface with no lead can also be used effectively for calibration as a control or baseline measure to confirm that no false positives are detected. Lastly, surfaces that have been previously tested for lead can be useful for calibration if their lead levels are known and documented, supporting consistent testing practices.

Thus, the presence of high iron content on a surface makes it unsuitable for reliable calibration checks in lead risk assessment practices.