Laboratory Analysis, Quality Assurance, and Data Interpretation
Laboratory Methods
The candidate should know the principal laboratory methods for lead: atomic absorption spectroscopy (AAS) and inductively coupled plasma (ICP) for paint chips, dust wipes, soil, and water. Results are reported in the units appropriate to the matrix: mg/cm² for paint, µg/ft² or µg/100 cm² for dust, ppm (mg/kg) for soil, and µg/L (ppb) for water. Reporting units are a frequent source of error.
Quality Assurance Concepts
- Accuracy and precision: accuracy is closeness to the true value; precision is reproducibility.
- Blank samples: field blanks detect contamination introduced during collection or handling.
- Duplicate samples: measure precision.
- Spiked samples: measure recovery and matrix effects.
- Calibration and control charts: track instrument performance over time.
- Laboratory accreditation: NLLAP recognition is the relevant credential for lead laboratories.
Data Interpretation
Interpret results against the applicable standard or clearance level, and consider the limit of detection and uncertainty. A result below the detection limit is not necessarily zero. When comparing to a clearance standard, use the correct units and the correct comparison value.
Traps in Questions
Watch for unit mismatches (ppm vs. mg/cm², µg/ft² vs. µg/100 cm²), treating a non-detect as zero, and confusing accuracy with precision. Also be alert to questions that assume any laboratory result is acceptable without a blank or duplicate.
Rule of Thumb
Before interpreting any number, confirm three things: the matrix, the reporting unit, and the comparison standard. If any of the three is unclear, the interpretation is unreliable.