Uncertainty budgeting accuracy

In this Section we aim to make the CRM user aware of the uncertainty budgets that need to be considered with the use of CRMs. Certified values in CRMs are the property values (mass fraction, concentration, or amount of substance) and their uncertainty, the uncertainty being in many instances a specified confidence interval for the certified property. As we discussed before, this uncertainty value is not always a complete uncertainty budget for an analytical process from sampling to production of data. But even when disregarding the subtle differences in the certificates, the way a CRM is used has serious consequences on the uncertainty budget that has to be applied to a user s result. This is summarized in Table 7.2. These uses may affect accuracy claims as well as traceability claims. It is the user s obligation to establish com-... 

In their broadest application, CRMs are used as controls to verify in a direct comparison the accuracy of the results of a particular measurement parallel with this verification, traceability may be demonstrated. Under conditions demonstrated to be equal for sample and CRM, agreement of results, e.g. as defined above, is proof. Since such possibilities for a direct comparison between samples and a CRM are rare, the user s claims for accuracy and traceability have to be made by inference. Naturally, the use of several CRMs of similar matrix but different analyte content will strengthen the user s inference. Even so, the user stiU has to assess and account for all uncertainties in this comparison of results. These imcertainty calculations must include beyond the common analytical uncertainty budget (i) a component that reflects material matrix effects, (2) a component that reflects differences in the amount of substance determined, (3) the uncertainty of the certified or reference value(s) used, and 4) the uncertainty of the comparison itself AU this information certainly supports the assertion of accuracy in relation to the CRM. However, the requirement of the imbroken chain of comparisons wiU not be formally fulfilled. 

Wolff-Briche, C.S.J., Harrington, C.F., Catterick, T, and Fairman, B., Orhodox uncertainty budgeting for high accuracy measurements by isotope dilution inductively coupled plasma mass spectrometry, Anal. Chim. Acta, 437, 1 (2001). 

These references are mandatory for regulatory compliance, and contain the essential certified value via a traceable route. Equally important from a QA perspective is a defined uncertainty budget. Given this budget figure, and the stated accuracy of the spectrophotometer (assuming the same level of confidence), simple addition of these two uncorrelated... 

The following example of a high-accuracy INAA determination of Cr in SRM 1152a Stainless Steel illustrates the use of an uncertainty budget and the resulting observation of a satisfactory measurement outcome (Zeisler et al. 2005). The experiment had been designed to... 

In general, the decay properties of the radionuclides are known with greater accuracy than are the concentrations that the analyst is likely to measure. The uncertainties associated with the chronometric atom concentration ratios represent almost the entire uncertainty budget associated with the age determination. However, these error hars do not translate directly to identical error hars on the derived sample ages that is, a 5% error har on the value of a concentration ratio does not necessarily translate into a 5% error har on the age, instead being dependent on the functional form relating the age and the ratio. 

Single-collector instruments also prove very usefid for mass content determinations via isotope dilution, as carefiil estimation of all quantities that influence the uncertainty budget demonstrates that the precision on the isotope amount ratio is typically not the dominant factor for high-precision measurements. Often, the accuracy of a mass content measurement will hardly improve through the use of MC-ICP-MS instruments as other influence quantities, such as uncontrolled spectral interferences and sample inhomogeneities, typically deserve more attention. 

IDMS enables high accuracy and small measurement uncertainties, when applied properly. A full uncertainty budget can easily be calculated. 

The accuracy of a fixed capital estimate tends to be a function of the design effort involved. As the project definition is refined, the estimates evolve from the various preliminary phases, ie, order of magnitude, predesign, factor estimates, etc, into the more detailed estimates used for budget authorization, project control, and contracts. At the same time, the uncertainty in the estimate decreases from 50% to as Htfle as 5%. 

Uncertainty The test plan should address the accuracy that is expected in the data. Equipment, personnel, time, budget, and detector features all affect the accuracy. The expected or possible accuracies are unique to the laboratory and the job. Despite this variability, it is natural to want to know what accuracies should be expected. As a starting point for discussions. Table 10.2 attempts to provide typical accuracies for a few common tests. 

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