Sampling, Sample Preparation, and Traceability
Sampling, Sample Preparation, and Traceability
This domain tests the chain from a lot to a reported result: representative sampling, comminution, homogenization, preservation, contamination control, and an unbroken traceability chain to a certified reference material or SI unit.
Core rules
- The sampling plan defines the lot, increment size, number of increments, compositing and acceptance criteria. Probability sampling reduces selection bias.
- Gy's theory: sampling variance rises with particle size and analyte heterogeneity, and falls with sample mass and number of increments. Do not take a convenient grab portion.
- Control contamination: use acid-washed PTFE for trace metals, glass for many organics, and avoid metal tools where iron or zinc matter.
- Preserve samples by cooling, keeping them dark, acidifying to pH < 2 for metals, limiting holding time, and filling VOC vials headspace-free.
- Traceability covers balance calibration, certified standard purity, volumetric glassware, dilution sequence and CRM recovery. Chain of custody records who handled the sample.
- Drying and digestion must be quantitative; report on the correct basis, such as dry mass or as-received.
Worked example
A 2.505 g sample is dissolved and made to 250.0 mL; 10.00 mL is diluted to 100.0 mL and gives 4.20 mg/L. Mass in the measured solution = 4.20 mg/L × 0.1000 L = 0.420 mg. Dilution factor = 250.0/10.00 = 25.00, so sample mass = 0.420 × 25.00 = 10.5 mg. % = 10.5 mg / 2505 mg × 100 = 0.419%. The traceability chain includes the balance, flask, pipette and standard.
Traps
- Sampling only the top or fine fraction.
- Adsorption of metals onto container walls or loss of volatiles.
- Inverting a dilution factor.
- Reporting as-received when the method requires dry basis.
- Broken chain of custody or uncorrected CRM recovery.