Sampling and Analysis
Core concepts
Sampling integrity determines whether data are defensible. Know representative sampling: correct location, adequate flushing, appropriate container, preservation, holding time, and chain of custody. Microbiological samples require sterile containers, dechlorination where residual would inhibit growth, and temperature control. Chemical samples may need acid preservation or refrigeration depending on the analyte. Understand QA/QC elements: blanks, duplicates, spikes, and calibration standards, and why a method blank detects contamination.
Traps in questions
- Sampling at a dead-end or poorly flushed tap and calling it representative.
- Missing holding time or preservation requirements, which invalidate results.
- Forgetting to dechlorinate a bacteriological sample.
- Confusing precision (repeatability) with accuracy (closeness to true value).
- Overlooking chain-of-custody documentation as part of sample validity.
Worked example / rule of thumb
If a duplicate pair differs far more than expected, suspect sampling technique or sample heterogeneity before blaming the lab. Rule of thumb: a result is only as good as the sample; document location, time, preservative, and handler for every sample.
Exam focus
Expect questions on container type, preservation, holding time, and QC purpose.