Uncertainty budgets can be used for a variety of situations, e.g. reporting the total uncertainty... more Uncertainty budgets can be used for a variety of situations, e.g. reporting the total uncertainty, calculating tolerance limits or method optimisation. In this paper it is demonstrated how the use of uncertainty budgets can help in reducing the total uncertainty of an analytical method, i.e. method optimisation. In this example it has been possible to reduce the total uncertainty of a concentration determination of hydrogen peroxide (by titration) from 2.7 x 10(-2) M to 3.93 x 10(-3) M (or to about a 1/7) by changing the traceability chain and working with more pure reagents.
A number of approaches for evaluating recovery and its contribution to uncertainty budgets for an... more 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, R 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(R 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.
The paper describes experiments for the evaluation of uncertainties associated with a number of c... more The paper describes experiments for the evaluation of uncertainties associated with a number of chromatographic parameters. Studies of the analysis of vitamins by HPLC illustrate the estimation of the uncertainties associated with experimental ''input'' parameters such as the detector wavelength, column temperature and mobile phase flow-rate. Experimental design techniques, which allow the efficient study a number of parameters simultaneously, are described. Multiple linear regression was used to fit response surfaces to the data. The resulting equations were used in the estimation of the uncertainties. Three approaches to uncertainty calculation were compared -Kragten's spreadsheet, symmetric spreadsheet and algebraic differentiation. In cases where non-linearity in the model was significant, agreement between the uncertainty estimates was poor as the spreadsheet approaches do not include second-order uncertainty terms.
A strategy is presented for applying existing data and planning necessary additional experiments ... more A strategy is presented for applying existing data and planning necessary additional experiments for uncertainty estimation. The strategy has two stages: identifying and structuring the input effects, followed by an explicit reconciliation stage to assess the degree to which information available meets the requirement and thus identify factors requiring further study. A graphical approach to identifying and structuring the input effects on a measurement result is presented. The methodology promotes consistent identification of important effects, and permits effective application of prior data with minimal risk of duplication or omission. The results of applying the methodology are discussed, with particular reference to the use of planned recovery and precision studies.
Page 1. Accred Qual Assur (2000) 5:104113 Q Springer-Verlag 2000 PRACTITIONER&#x... more Page 1. Accred Qual Assur (2000) 5:104113 Q Springer-Verlag 2000 PRACTITIONER'S REPORT Vicki J. Barwick Stephen LR Ellison Mark JQ Rafferty Rattanjit S. Gill The evaluation of measurement uncertainty from method validation studies ...
The measurement uncertainty associated with the determination of 60 Ni in aqueous samples by ICP-... more The measurement uncertainty associated with the determination of 60 Ni in aqueous samples by ICP-MS has been calculated using a cause-and-effect approach. A cause-and-effect diagram was constructed to aid in the identi®cation of the sources of uncertainty associated with the method. The uncertainty estimate was calculated from a combination of existing quality control data and specially planned experiments. The uncertainty budget was based initially on precision data, followed by separate evaluation of the method bias and the effect of parameters not suf®ciently covered by these estimates. The construction of the cause-and-effect diagram, its reconciliation with existing data and the estimation of the individual components of the uncertainty budget are described in detail. The expanded uncertainties for three different nickel concentrations (3,10,35 ng g À1 ) were calculated as 1.1, 1.5 and 5.3 ng g À1 , respectively. These were calculated using a coverage factor of two approximating to a 95% level of con®dence. The dominant contributions to the uncertainty budget were method precision, instrument drift and bias measured as method recovery. The uncertainties associated with the concentration of the working standard and sample dilution were found to be insigni®cant. #
Uncertainty budgets can be used for a variety of situations, e.g. reporting the total uncertainty... more Uncertainty budgets can be used for a variety of situations, e.g. reporting the total uncertainty, calculating tolerance limits or method optimisation. In this paper it is demonstrated how the use of uncertainty budgets can help in reducing the total uncertainty of an analytical method, i.e. method optimisation. In this example it has been possible to reduce the total uncertainty of a concentration determination of hydrogen peroxide (by titration) from 2.7 x 10(-2) M to 3.93 x 10(-3) M (or to about a 1/7) by changing the traceability chain and working with more pure reagents.
A number of approaches for evaluating recovery and its contribution to uncertainty budgets for an... more 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, R 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(R 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.
The paper describes experiments for the evaluation of uncertainties associated with a number of c... more The paper describes experiments for the evaluation of uncertainties associated with a number of chromatographic parameters. Studies of the analysis of vitamins by HPLC illustrate the estimation of the uncertainties associated with experimental ''input'' parameters such as the detector wavelength, column temperature and mobile phase flow-rate. Experimental design techniques, which allow the efficient study a number of parameters simultaneously, are described. Multiple linear regression was used to fit response surfaces to the data. The resulting equations were used in the estimation of the uncertainties. Three approaches to uncertainty calculation were compared -Kragten's spreadsheet, symmetric spreadsheet and algebraic differentiation. In cases where non-linearity in the model was significant, agreement between the uncertainty estimates was poor as the spreadsheet approaches do not include second-order uncertainty terms.
A strategy is presented for applying existing data and planning necessary additional experiments ... more A strategy is presented for applying existing data and planning necessary additional experiments for uncertainty estimation. The strategy has two stages: identifying and structuring the input effects, followed by an explicit reconciliation stage to assess the degree to which information available meets the requirement and thus identify factors requiring further study. A graphical approach to identifying and structuring the input effects on a measurement result is presented. The methodology promotes consistent identification of important effects, and permits effective application of prior data with minimal risk of duplication or omission. The results of applying the methodology are discussed, with particular reference to the use of planned recovery and precision studies.
Page 1. Accred Qual Assur (2000) 5:104113 Q Springer-Verlag 2000 PRACTITIONER&#x... more Page 1. Accred Qual Assur (2000) 5:104113 Q Springer-Verlag 2000 PRACTITIONER'S REPORT Vicki J. Barwick Stephen LR Ellison Mark JQ Rafferty Rattanjit S. Gill The evaluation of measurement uncertainty from method validation studies ...
The measurement uncertainty associated with the determination of 60 Ni in aqueous samples by ICP-... more The measurement uncertainty associated with the determination of 60 Ni in aqueous samples by ICP-MS has been calculated using a cause-and-effect approach. A cause-and-effect diagram was constructed to aid in the identi®cation of the sources of uncertainty associated with the method. The uncertainty estimate was calculated from a combination of existing quality control data and specially planned experiments. The uncertainty budget was based initially on precision data, followed by separate evaluation of the method bias and the effect of parameters not suf®ciently covered by these estimates. The construction of the cause-and-effect diagram, its reconciliation with existing data and the estimation of the individual components of the uncertainty budget are described in detail. The expanded uncertainties for three different nickel concentrations (3,10,35 ng g À1 ) were calculated as 1.1, 1.5 and 5.3 ng g À1 , respectively. These were calculated using a coverage factor of two approximating to a 95% level of con®dence. The dominant contributions to the uncertainty budget were method precision, instrument drift and bias measured as method recovery. The uncertainties associated with the concentration of the working standard and sample dilution were found to be insigni®cant. #
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