Measurement uncertainty: Approaches to the evaluation of uncertainties associated with recovery†

Abstract

A number of approaches for evaluating recovery and its contribution to uncertainty budgets for analytical methods are considered in detail. The recovery, R, for a particular sample is considered as comprising three elements, R̄_m, R_s and R_rep. These relate to the recovery for the method; the effect of sample matrix and/or analyte concentration on recovery; and how well the behaviour of spiked samples represents that of test samples. The uncertainty associated with R, u(R), will have contributions from u(R̄_m), u(R_s) and u(R_rep). The evaluation of these components depends on the method scope and the availability, or otherwise, of representative certified reference materials. Procedures for evaluating these parameters are considered and illustrated with worked examples. Techniques discussed include the use of certified reference materials and spiking studies, and the use of extraction profiling to predict recoveries. All the approaches discussed evaluate the recovery and its uncertainty for the analytical method as a whole. It is concluded that this is a useful approach as it reduces the amount of experimental work required. In addition, most of the required data are frequently available from method validation studies.

References

1. M. Thompson, Analyst, 1995, 120, 117N.
2. W. Horwitz and A. Albert, Analyst, 1997, 122, 615.
3. S. L. R. Ellison, W. Wegsheider and A. Williams, Anal. Chem., 1997, 69, 607A.
4. J. S. Kane, Analyst, 1997, 122, 1283.
5. S. L. R. Ellison and A. Williams, Accred. Qual. Assur., 1998, 3, 6.
6. S. L. R. Ellison, Accred. Qual. Assur., 1998, 3, 95.
7. ISO, Guide to the Expression of Uncertainty in Measurement, International Standards Organisation, Geneva, 1993.
8. EURACHEM GUIDE: Quantifying Uncertainty in Analytical Measurement, Laboratory of the Government Chemist, London, 1995.
9. M. Pueyo, J. Obiols and E. Vilalta, Anal. Commun., 1996, 33, 205.
10. A. Williams, Anal. Proc., 1993, 30, 248.
11. Analytical Methods Committee, Analyst 19951202303.
12. S. L. R. Ellison, Accred. Qual. Assur., 1998, 3, 95.
13. S. L. R. Ellison and A. Williams, Accred. Qual. Assur., 1998, 3, 6.
14. W. Horwitz, Pure Appl. Chem., 1988, 60, 885.
15. AOAC recommendation, J. Assoc. Off. Anal. Chem., 198972694.
16. ISO 5725: 1994, Accuracy (Trueness and Precision) of Measurement Methods and Results, International Standards Organisation, Geneva, 1995.
17. S. L. R. Ellison and V. J. Barwick, Accred. Qual. Assur., 1998, 3, 101.
18. S. L. R. Ellison and V. J. Barwick, Analyst, 1998, 123, 1387.
19. V. J. Barwick and S. L. R. Ellison, Anal. Commun., 1998, 35, 377.
20. K. D. Bartle, A. A. Clifford, S. B. Hawthorne, J. J. Langenfeld, D. J. Miller and R. Robinson, J. Supercrit. Fluids, 1990, 3, 143.
21. S. L. R. Ellison and A. Williams, in The use of recovery factors intrace analysis, ed. M. Parkany, Royal Society of Chemistry, Cambridge, UK, 1996.
22. Soil Quality—Determination of polycyclic aromatic hydrocarbons (PAH), Draft British Standard, 1998..
23. T. J. Farrant, Practical Statistics for the Analytical Scientist, A Bench Guide, Royal Society of Chemistry, Cambridge, UK, 1997.