Laboratory Analysis, Sampling, and Monitoring
Laboratory Analysis, Sampling, and Monitoring tests whether you can collect representative samples, preserve them correctly, and interpret results. Expect questions on bacteriological sampling, chlorine residual, turbidity, holding times, chain of custody, QA/QC, and approved methods.
Core rules and concepts
- Use approved methods and certified laboratories. Follow holding times, preservation, and temperature requirements; otherwise results are invalid.
- Bacteriological samples are collected in sterile bottles, usually 100 mL, from a representative tap after removing aerators and flushing.
- Chlorine residual is measured at the time of collection because it changes quickly. Record free and total chlorine as required.
- Turbidity is measured in NTU; calibrate instruments and use proper sample handling.
- Chain of custody documents who collected, handled, and received the sample.
- QA/QC includes duplicates, blanks, spikes, and calibration checks. A duplicate measures precision, not accuracy.
Worked example
A lab reports nitrate as 11.3 mg/L NO3. The MCL is 10 mg/L as nitrogen (NO3-N). Convert: 11.3 ÷ 4.43 = 2.55 mg/L NO3-N. The result is below the 10 mg/L MCL. Reporting the number without checking the units would incorrectly suggest a violation. Always confirm whether nitrate is reported as NO3 or NO3-N.
Traps
Mailing bacteriological samples late; using a non-sterile bottle; measuring chlorine after the sample has sat; confusing NO3 with NO3-N; accepting a result without checking holding time; treating a duplicate as a calibration standard.