quantification limit

**Quantification Limit** (LOQ — Limit of Quantification) is the **lowest concentration of an analyte that can be measured with acceptable accuracy and precision** — higher than the detection limit, LOQ is the concentration at which quantitative results become reliable, typically defined as 10σ of the blank. **LOQ Calculation** - **10σ Method**: $LOQ = 10 imes sigma_{blank}$ — ten times the standard deviation of blank measurements. - **ICH Method**: $LOQ = 10 imes sigma / m$ where $sigma$ is blank SD and $m$ is calibration slope. - **Signal-to-Noise**: $LOQ$ at $S/N = 10$ — sufficient signal for quantitative reliability. - **Accuracy/Precision**: At the LOQ, accuracy should be within ±20% and precision (CV) should be ≤20%. **Why It Matters** - **Reporting**: Results below LOD are reported as "not detected"; between LOD and LOQ as "detected but not quantified"; above LOQ as quantitative values. - **Specifications**: The LOQ must be below the specification limit — cannot reliably determine if a sample passes if LOQ > spec. - **Method Selection**: If LOQ is too high, a more sensitive method is needed — drives instrument selection. **Quantification Limit** is **the reliable measurement floor** — the lowest level at which quantitative results have acceptable accuracy and precision.