Detection Limit (LOD — Limit of Detection) is the lowest quantity or concentration of an analyte that can be reliably distinguished from zero — the minimum detectable signal that is statistically distinguishable from the background noise with a specified confidence level (typically 99%).

Detection Limit Calculation

• 3σ Method: $LOD = 3 imes sigma_{blank}$ — three times the standard deviation of blank measurements.
• Signal-to-Noise: $LOD$ at $S/N = 3$ — the concentration giving a signal three times the noise level.
• ICH Method: $LOD = 3.3 imes sigma / m$ where $sigma$ is blank SD and $m$ is calibration slope.
• Practical: The LOD from theory may differ from the practical detection limit — verify experimentally.

Why It Matters

• Contamination Monitoring: For trace metal analysis (ICP-MS, TXRF), LOD determines the lowest detectable contamination level.
• Specification: The detection limit must be well below the specification limit — typically LOD < 1/10 of the spec.
• Semiconductor: Advanced nodes require sub-ppb (parts per billion) detection limits for critical contaminants.

Detection Limit is the minimum measurable signal — the lowest analyte level that can be reliably distinguished from blank background.