Laboratory Analysis
This section covers Laboratory Analysis. The exam tests whether you can produce defensible compliance data: correct containers, preservation, holding times, approved methods, chain of custody, and QA/QC.
Core rules
- Sample integrity begins at the tap. Flush to a representative condition, use the correct container, preserve as required, and record chlorine residual and temperature where applicable.
- Holding times and preservation are method-specific. Exceeding either can invalidate a compliance result even if the instrument reading looks reasonable.
- Use approved methods and calibrated instruments. Chlorine residual is commonly measured by DPD; turbidity by nephelometry; pH by a calibrated meter with fresh buffers.
- QA/QC includes duplicates, blanks, spikes, and control charts. Chain of custody documents who collected, handled, and received each sample.
- Laboratory certification and method approval matter for compliance samples; field measurements also need calibration records.
Worked example
A matrix spike adds 1.0 mg/L to a sample with a background of 0.2 mg/L. The measured result is 1.1 mg/L. Recovery = (1.1 − 0.2) ÷ 1.0 × 100 = 90%. If the acceptance range is 80–120%, the spike passes. Reasoning: subtract the native background before dividing by the spike added; do not compare the raw 1.1 mg/L to the 1.0 mg/L spike.
Traps
Missing a holding time; using the wrong preservative; collecting a chlorine sample without accounting for detention; failing to calibrate turbidity or pH; losing chain of custody; reporting a result without the associated QC; confusing dilution factor with recovery.