Laboratory Analysis, Monitoring and QA/QC: Defensible Data
Laboratory Analysis, Monitoring and QA/QC tests whether you can produce defensible data. Expect sampling design, preservation, calibration, method performance, chain of custody and acceptance criteria.
Sampling and preservation
Grab samples represent a moment; composite samples represent a period. Water samples for BOD/COD are preserved at 4°C with H2SO4 to pH <2; dissolved metals need filtration through 0.45 µm and acidification; DO is fixed in the field. Air PM10/PM2.5 uses pre-weighed filters, conditioned before and after sampling, with flow and leak checks. Every batch needs field blanks, trip blanks, duplicates and chain-of-custody records.
Analysis and QC
BOD is 5 days at 20°C; COD is dichromate closed reflux; TSS is retained on a glass-fibre filter dried at 103–105°C. Calibration curves use at least five points; verify with a second-source standard. Method blank must be below LOQ; duplicate relative percent difference (RPD) is usually ≤20%; spike recovery 80–120%; CRM results must fall within certified limits. LOD is about 3σ of blank response and LOQ about 10σ. NABL accreditation follows ISO/IEC 17025:2017 and requires traceability, uncertainty budgets and proficiency testing.
Worked example
Duplicate TSS results are 48 and 52 mg/L. Mean = 50; RPD = |48−52|/50 × 100 = 8%. If the laboratory criterion is ≤20%, the pair is acceptable. A spike of 10 mg/L added to a sample containing 0.5 mg/L gives 9.2 mg/L: recovery = (9.2−0.5)/10 × 100 = 87%, within 80–120%.
Traps
LOD is not LOQ; RPD is not simple percent difference from a standard; preservation is part of the method; BOD without seed control is invalid; total and dissolved fractions differ; calibration verification is not a full recalibration; data below LOQ should not be reported as precise values.