Sampling, Monitoring, Chain of Custody, and Data Quality
What the exam tests
This domain tests whether a sample result is defensible. Expect sampling plans, containers and preservation, holding times, chain of custody, quality assurance and quality control, blanks, duplicates, calibration, detection and reporting limits, and data validation.
Core rules
- A sampling plan or QAPP defines objectives, locations, methods, containers, preservation, holding times, and QC.
- Representative sampling requires a strategy matched to the matrix and question: grab samples for variable conditions, composites for average conditions, and discrete samples for source identification.
- Chain of custody documents who collected, handled, and received each sample, with dates, times, signatures, and custody seals. Corrections should be single-line, initialed, dated, and explained.
- QC commonly includes field blanks, trip blanks for volatile organics, duplicates, matrix spikes, and calibration checks.
- Data validation compares results with method criteria, holding times, preservation, blanks, and laboratory QC before the data are used.
Worked example
A groundwater metals sample is collected but not acidified with nitric acid to pH below 2 until three days later. Even though the laboratory’s six-month holding time has not expired, preservation was not performed at collection as required. The result should be flagged as suspect and the sample re-collected if the data are needed for compliance. The reasoning separates holding time from preservation: both must be met, and preservation begins at collection.
Traps
Treating the chain-of-custody form as the entire chain; accepting a cooler temperature above the method limit; using a trip blank for every analyte rather than for volatile organics; confusing detection limit with reporting limit; compositing samples for volatile analytes; and correcting custody records with correction fluid or overwriting.