detection limit

**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.