• Table of Content
• Foreword
• Scope
• Uncertainty
• Analytical Measurement
• The Process
• Step 1: Specification
• Step 2: Identification
• Step 3: Quantification
• Step 4: Calculation
• Reporting Uncertainty
• Appendix A: Examples
  • Example 1: Calibration Standard
  • Example 2: Standardising NaOH
  • Example 3: An Acid/Base Titration
  • Example 4: In-house Validation Studies
  • Example 5: Determ. Cadmium Release
  • Example 6: Crude Fibre in Animal Feed
  • Example 7: Lead in Water - double IDMS
• Appendix B: Definitions
• Appendix C: Analytical Proc.
• Appendix D: Analyse U Sources
• Appendix E: Statistics
• Appendix F: MU at the Limit
• Appendix G: Common Sources
• Appendix H: Bibliography

Appendix C: Uncertainties in Analytical Processes

C.1 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
2. Sample preparation
   
3. Presentation of Certified Reference Materials to the measuring system
4. Calibration of instrument
5. Analysis (data acquisition)
6. Data processing
7. Presentation of results
8. Interpretation of results
   

C.2 These steps can be further broken down by contributions to the uncertainty for each. The following list, though not necessarily comprehensive, provides guidance on factors which should be considered.

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.
• 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

• 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.