Uncertainties in Analytical Processes

In order to identify the possible sources of uncertainty in an analytical procedure it is helpful to break down the analysis into a set of generic steps:

1. Sampling

– Homogeneity.

– Effects of specific sampling strategy (e.g. random, stratified random,    proportional etc.)

– Effects of movement of bulk medium (particularly density selection)

– Physical state of bulk (solid, liquid, gas)

– Temperature and pressure effects.

– Does sampling process affect composition? E.g. differential adsorption in sampling system.

2. Sample preparation

– Homogenisation and/or sub-sampling effects.

– Drying.

– Milling.

– Dissolution.

– Extraction.

– Contamination.

– Derivatisation (chemical effects)

– Dilution errors.

– (Pre-)Concentration.

– Control of speciation effects

3. Presentation of Certified Reference Materials to the measuring system

– Uncertainty for CRM.

– CRM match to sample

4. Calibration of Instrument

– Instrument calibration errors using a Certified Reference Material.

– Reference material and its uncertainty.

– Sample match to calibrant

– Instrument precision

5. Analysis (data acquisition)

– Carry-over in auto analysers.

– Operator effects, e.g. colour blindness, parallax, other systematic errors.

– Interferences from the matrix, reagents or other analytes.

– Reagent purity.

– Instrument parameter settings, e.g. integration parameters

– Run-to-run precision

6. Data processing

– Averaging.

– Control of rounding and truncating.

– Statistics.

– Processing algorithms (model fitting, e.g. linear least squares).

7. Presentation of results

– Final result.

– Estimate of uncertainty.

– Confidence level.

8. Interpretation of results

– Against limits/bounds.

– Regulatory compliance.

– Fitness for purpose.

Reference

EURACHEM/CITAC Guide Quantifying Uncertainty in Analytical Measurement Third Edition Editors S L R Ellison (LGC, UK) A Williams (UK)

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