Laboratory Analysis and Process Control
Results are only as good as the sample behind them. Sampling plans specify location, container, preservation, holding time, and chain of custody. Typical problems: a grab sample where a composite is required, chlorine residual not measured in the field (it dissipates), samples not chilled, or a coliform sample taken from a leaking or unsterilized tap.
QA/QC vocabulary is heavily tested. Calibration with standards sets instrument response. A method blank detects contamination from reagents and glassware; a field blank detects contamination during sampling and transport; a duplicate measures precision; a matrix spike measures recovery (accuracy) in that particular matrix; a known standard checks the whole method. Precision is agreement among replicates; accuracy is closeness to the true value.
Common methods: turbidity by nephelometry in NTU; pH by electrode with two- or three-point buffer calibration; alkalinity by titration to about pH 4.5; hardness by EDTA titration or calculation; free and total chlorine by DPD colorimetry or amperometric titration; fluoride by ion-selective electrode or ion chromatography; nitrate by IC or cadmium reduction; coliforms by presence/absence or membrane filtration of a 100 mL sample. Process control uses jar tests, streaming current, and control charts with statistical action limits.
Traps: calling a duplicate an accuracy check; reporting a result above the calibration curve without dilution; forgetting that turbidimeters need their own standards; and ignoring that a positive routine coliform sample triggers repeat sampling within 24 hours.
Worked example: 100 mL filtered, 22 colonies counted = 22 CFU/100 mL; a positive routine total coliform sample must then be checked for E. coli and followed by repeat samples.