Certified reference materials uncertainty

Traceability of measurement results is essential in the establishment of a certified reference material. As stipulated in ISO Guides 30 and 35, a certified reference material can only be certified if there is an uncertainty statement with a traceability statement. Basically, traceability means anchoring. In classical analytical chemistry, that SI system is often the best choice as a reference (= anchoring poinf). However, there is a wide range of parameters either defined by a method or defined by the... 

Certified Reference Materials for Microanalytical Methods 1135 4.3.4.3 Uncertainty Budget of INAA... 

Comparison of test values with a conventional true value ( reference value ) of a (certified) reference material (RM, CRM). In method development and validation of analytical procedures, the comparison of experimental results with standards of diverse kind (laboratory standards, certified reference materials, primary standards) plays an essential role. The decision as to whether an experimental result hits the reference value depends not only from the result itself but also from its uncertainty interval. 

In analytical chemistry, we do not have a standard mole. Therefore, solutions made up to a well-defined concentration using very pure chemicals are used as a basis from which we can compare other solutions or an instrument scale. This process is calibration . For some analyses, the chemical used may be a Certified Reference Material which has a well documented specification, e.g. in terms of the concentration of a particular species and the uncertainty of the specified value. However, it is not sufficient just to calibrate the apparatus/equipment used, it is important that the complete method of analysis is validated from extraction of the analyte from the sample to the final measurement. 

Reagent Purity The purity of many reagents is specified by the manufacturer as being not less than a specified value. Any assumptions about the degree of purity will contribute to the uncertainty. The nature of any impurities may also be important. Pure substance Certified Reference Materials will have a stated purity, plus an estimate of the uncertainty associated with the purity value. 

Provided the sample matrix and analyte concentration are appropriate, matrix Certified Reference Materials (CRMs) can make ideal proficiency testing samples. The assigned value is the certified value given on the certificate accompanying the CRM. The certificate will also give an uncertainty estimate for the certified value, and the use of CRMs allows the traceability of analytical data to be established. However, matrix CRM availability is limited and the materials are often expensive. Hence, Certified Reference Materials are seldom used as PT samples. 

Certified Reference Material A reference material characterized by a metrolog-ically valid procedure for one or more specified properties, accompanied by a certificate that states the value of the specified property, its associated uncertainty and a statement of metrological traceability. 

Certified Reference Material Reference material, whose property values are certified by a procedure that establishes its traceability to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty statement (certificate) at a specified level of confidence. 

Homogeneity. Homogeneity assures that the analysis of all subsamples of the reference material taken for measurement will produce the same analytical result within the stated measurement uncertainty. This is particularly important in the case of certified reference materials. Reference material producers therefore must specify the minimum amount of sample for which homogeneity has been measured and is valid. Finally, the ease of re-homogenizing the material after packaging must be taken into consideration. 

Accuracy, uncertainty, and traceability. A certified value is the best approximation of the true concentration of the analyte. During the certification process, a variety of analytical methods may be used to determine this true value. Uncertainty estimates ultimately based on this process, together with information about the material s homogeneity can give a certified reference material traceability, needed for true international comparability. 

The bias part of the uncertainty also has two parts the bias itself and the uncertainty of the nominal value. If only one certified reference material is available we have to include the uncertainty of the determination of the bias in addition. This is done by using the standard deviation of this bias determination. All components are combined as shown in the slide. 

We Start with an example where only one certified reference material is available with a certified value of 11.5 and an expanded uncertainty (95%) of 0.5. First we convert the expanded uncertainty into a standard uncertainty by dividing it by 1.96. Now we have the standard uncertainty of the reference value. 

Usually certified reference materials are prepared in batches and the property values (including the uncertainty) are determined in representative samples. 

According to the ISO guide 35, the production of a certified reference material is an integrated process of correct preparation, demonstration of homogeneity and stability of the properties as well as accurate and traceable characterization of these properties. In addition all the components of uncertainty of the sample on the desk of the usef should be accounted for. 

One of the important steps in the production process is the measurement of the homogeneity. This step ensures that all the units of the certified reference material cany the same (within the uncertainties) property value. 

The uncertainty of the assigned value, as stated before, is also very important. Uncertainty of the assigned value is the uncertainty of the average concentration of 1 unit (of the n prepared) after storage for time T and after transport, i.e. when the certified reference material arrives at the desk of the user. In general this can be obtained by combining the uncertainties of the value assignment measurement, the... 

Summarizing the production and in order to help the selection of high quality certified reference materials, in general the traceability of the certified property should be stated, the uncertainty according to ISO guidelines should be stated, both of the above should also be demonstrated and it is preferred that the certified reference material is produced according to ISO guide 35. 

Note that no allowance has been made for the uncertainty of the certified reference material. Analysts often assume that the value of the reference material has been determined with much greater precision than their experiments. Suppose the certificate does give an expanded uncertainty of 0.0004 ppm (95% probability). The standard uncertainty is u = 0.0004/2 = 0.0002 ppm with, in the absence of any other information, infinite degrees of freedom. The t value can now be recalculated as... 

The value and measurement uncertainty of a certified reference material. 

Some examples include evaluation of uncertainty components associated with published values (i.e., the analyst did not measure them), uncertainties in a certificate of a certified reference material, manufacturer s statements about the accuracy of an instrument, or perhaps even personal experience. The latter could be viewed as an opportunity for anyone to just make up an uncertainty, but experience does count for something, and it is indeed usually better than nothing. Leaving out a component because of lack of exact knowledge immediately underestimates the uncertainty. 

There are some existing texts that cover the material in this book, but I have tried to take a holistic view of quality assurance at a level that interested and competent laboratory scientists might learn from. I am continually surprised that methods to achieve quality, whether they consist of calculating a measurement uncertainty, documenting metrological traceability, or the proper use of a certified reference material, are still the subject of intense academic debate. As such, this book runs the risk of being quickly out of date. To avoid this, I have flagged areas that are in a state of flux, and I believe the principles behind the material presented in this book will stand the test of time. 

Certified reference materials (CRMs) are mainly applied to validate the analytical procedure developed for routine analysis in order to determine the accuracy of analytical data, the recovery for selected elements, the uncertainty of trace element determination and the detection limits. Otherwise, in solid-state mass spectrometric techniques, such as SSMS, LA-ICP-MS, GDMS, SNMS or SIMS, one point calibration using CRMs has been established as an important calibration strategy to obtain reliable analytical data. The one point calibration is performed using the experimentally determined relative sensitivity coefficients (RSCs) on a suitable CRM with a similar trace/matrix composition. An RSC of a chemical element is defined as the ratio of the measured element concentration (experimentally determined) divided by the certified element concentration (accepted or recommended value of element concentration) in a given matrix. 

Analyzing control materials alongside the test samples greatly improves proficiency in mycotoxin analysis. Certified reference materials (CRMs) represent ideal control materials, due to their statement of uncertainty and traceability, and they should be routinely used as much as possible. Unfortunately, as outstanding as the improvements made in the last decade have been, even though the list of CRMs in the area of mycotoxins is rather long, it is still insufficient. A list of the available reference materials in the mycotoxins area is reported in Table 1 the issue has been reviewed by Boenke (27). 

One alternative is to compare the results of the method with results from an established reference method. This approach assumes that the uncertainty of the reference method is known. Second, accuracy can be assessed by analyzing a sample with known concentrations (e.g., a certified reference material) and comparing the measured value with the true value as supplied with the material. If such certified reference material is not available, a blank sample matrix of interest can be spiked with a known concentration by weight or volume. After extraction of the analyte from the matrix and injection into the analytical instrument, its recovery can be determined by comparing the response of the extract with the response of the reference material dissolved in a pure solvent. Because this accuracy assessment measures the effectiveness of sample preparation, care should be taken to mimic the actual sample preparation as closely as possible. 

Sahuquillo, A., Lopez-Sanchez, J.F., Rubio, R., Rauret, G., Thomas, R.P., Davidson, C.M. and Ure, A.M. (1999) Use of a certified reference material for extractable trace metals to assess sources of uncertainty in the BCR three-stage sequential extraction procedure. Anal. Chim. Acta, 382, 317—327. 

Fig.3 Issues arising when considering the meaning of a certified value and its uncertainty in a certified reference material...

The role of certified reference materials (with assigned value and uncertainty) in obtaining traceability and avoiding bias is obvious. 

Abstract Talking about traceability means talking about a property of the result of a measurement , about the value of a standard , about stated references and about an unbroken chain of comparisons . It describes by which comparison, and to which other value, the result of a measurement has been obtained, i.e. is traceable to . It is about the underlying structure of the measurement process of the result of a measurement and therefore about the authority of the result. Since values carried by (certified) reference materials have also been obtained by measurement, the definition of traceability equally applies. Traceability in the context of reference materials is also about the authority of the values carried by the (certified) reference materials and is, therefore, of key importance for the authority of the reference materials themselves. Hence, values of results of measurements constitute part of the traceability chain and their uncertainties are an intrinsic accompanying phenomenon. Uncertainties need a traceability chain against which they can be evaluated, and a traceability chain is an... 

The certificate of a reference material is only proof of traceability if the certified values are accompanied by uncertainties. The certified reference material... 

Abstract Since the uncertainty of each link in the traceability chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time, the chain lifetime is limited. The lifetime in chemical analysis is dependent on the calibration intervals of the measuring equipment and the shelf-life of the certified reference materials (CRMs) used for the calibration of the equipment. It is shown that the ordinary least squares technique, used for treatment of the calibration data, is correct only when uncertainties in the certified values of the measurement standards or CRMs are negligible. If these uncertainties increase (for example, close to the end of the calibration interval or shelf-life), they are able to influence significant-... 

According to the definition [1] the traceability chain is the unbroken chain of comparisons or calibrations from the result of a measurement or the value of a measurement standard to the national or international standards, all having stated uncertainties. The uncertainty of each link in this chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time. Therefore, the calibration intervals of measuring equipment used in testing (analytical) laboratories [2, 3] and of measurement standards used for their calibration are very important. The same applies to the shelf-life of a certified reference material (CRM) as a measurement standard. So, taking into account these... 

---