General Analytical Chemistry: Validation, Calibration, and Quality Control
Core concepts
General analytical chemistry underpins all quantitative measurement. Candidates should master calibration (external standard, internal standard, standard addition), figures of merit (linearity, sensitivity/slope, LOD/LOQ, selectivity, robustness), and statistical treatment of data (mean, standard deviation, confidence intervals, outlier tests).
LOD and LOQ are commonly estimated from the calibration curve: LOD = 3.3 × (SD of response or residual SD)/slope; LOQ = 10 × SD/slope. Signal-to-noise ratios of 3:1 (LOD) and 10:1 (LOQ) are classic benchmarks. Standard addition is used when matrix effects are severe or no blank matrix exists; the spike is extrapolated back to the x-intercept to find the original concentration.
Traps in questions
- Applying the 3:1 and 10:1 S/N rules as universal-they are conventions, not laws.
- Confusing precision (random error) with accuracy (systematic error); a precise method can be biased.
- Using external calibration in a strongly suppressing matrix without checking recovery.
- Forgetting that robustness tests deliberate small variations (pH, temperature, flow rate), while ruggedness tests between-lab/analyst reproducibility.
Rule of thumb
Always report results with appropriate significant figures and units, and verify calibration linearity with the correlation coefficient (r ≥ 0.99) plus residual inspection. Worked example: slope = 2.0 × 10⁴ units/(mg/L), residual SD = 100 units → LOD = 3.3 × 100 / 2.0 × 10⁴ = 0.0165 mg/L.