What does a calibration curve refer to?

• A. A zero baseline measurement to set the instrument's baseline signal before running standards
• B. A plot of signal vs known concentrations
• C. A plot of known concentrations versus signal used to determine the concentration of an unknown sample
• D. Unknown sample concentration derived from curve

Answer: (C)

A calibration curve shows how the instrument’s signal changes with known concentrations, which lets you determine the concentration of an unknown sample. You create it by measuring the response for a set of standards with known concentrations and plotting the signal against those concentrations. Then you measure the unknown, see where its signal falls on the curve, and read off or calculate its concentration from the curve’s equation. This is why the best description highlights using a plot of known concentrations to derive the unknown’s concentration—the curve exists to quantify unknowns. The baseline-only option describes just setting a zero point, not the curve itself; the option about “unknown concentration derived from curve” points to the result rather than the defining purpose; and the plot of signal versus concentration is effectively the same relationship, but it’s the use for determining an unknown that distinguishes the calibration curve.