Certified reference material . See

You could decide to use control charts and to analyse certified reference materials (see chapter 13 and 14). Control charts are a simple, but effective tool for internal quality control. Internal quality control is one of a number of concerted measures that analytical chemists can take to ensure that the data produced in the laboratory are fit for their intended purpose. ... 

Establishing the traceability of the measurement is one of the most important requirements in ISO/IEC 17025. Measurement traceability ensures that the measurements in different laboratories are comparable in space and time all over the world. Calibration and the use of certified reference material (see chapters 9, 10 and 14) is the central tool in establishing trace-ability. Therefore a laboratory must have programmes and procedures for both. 

Every analytical laboratory should have its own reference materials for internal quality assurance, and such materials should conform to the same standards of appropriateness, homogeneity and long term stability as are required for certified reference materials (see section "preparation of reference materials). In practice, however, most analytical laboratories do not use their own "in-house" reference materials for internal quality assurance but rather rely on internationally available reference materials. In the opinion of the present author, this is undesirable since much larger amounts are required for internal quality assurance than for externai quality assurance, and thereby the available stocks of expensively prepared certified reference materials will be consumed much too quickly. 

For the analysis of antibiotic residues in foods, there is a limited number of methods that have been evaluated by collaborative study and a lack of certified reference materials (see other chapters on specific methods of analysis and quality assurance for additional details). Hence, the assessment of accuracy is most frequently based on analytical recovery from fortified materials when developing and validating these methods. 

In addition, a system for making sure staff are appropriately qualified and trained for the work that they are doing must be in place. This will enable an auditor to see clearly the demonstrated competence of the staff and how this has been checked. The requirements for all major items of equipment must be listed, to ensure that the equipment in use is suitable for the task, is in working condition and, where necessary, is calibrated. For all of the instrumentation there needs to be a documented schedule for maintenance. Measurements must be traceable, that is, the laboratory must be able to show how the calibration of measurement instruments is traceable to National or International Standards. Where this presents practical problems, as in some chemical measurements for example, interlaboratory comparison and the use of reference materials (and preferably Certified Reference Materials) will be required. 

From the available literature it becomes clear that method evaluation studies do not surpass the level of within-laboratory performances. Although several of these (see Table 3.3.1) reveal satisfactory levels of quality and environmentally relevant limits of detection, a genuine quality assurance of these methods is still lacking. There are no reports of interlaboratory studies and certified reference materials for surfactants are not available on the market yet. It can therefore be concluded that there remains much to be done in the field of improving and evaluating quality of analytical measurements of surfactants in biota. 

Use of certified reference materials for this purpose is possible, but actually it is not an appropriate use of the material. It is better and more economical to use in-house materials of adequate homogeneity and stability (see chapter 13). 

Let s see what is the influence for the two cases where the certified reference materials are useful in the laboratory (i.e. calibration and vahdation). In the case of calibration, the properly value of the certified reference material is used to calibrate the analytical instrument used for the measurement, thus it is used in order to obtain the measurement result. In this way the properly value of the certified reference material is part of the traceability chain, as shown in the shde, and is directly involved in the establishment of the measurement traceabihty. 

In the above discussion, standard reference materials (SRMs) were mentioned often. A reference material (RM) is a material or substance suitable for use in calibrating equipment or standardizing solutions. A certified reference material (CRM) that a vendor indicates, via a certificate, is an RM. A standard reference material (SRM) is one that is distributed and certified by a certifying body, such as NIST. The SRM is the material to which all calibration and standardization materials should be traceable. A standard material becomes one when it is compared to or prepared from another. Ultimately, it all rests on the SRM — meaning all standard materials are traceable to an SRM (see Figure 5.10). 

X is the mean determined value and n is the number of measurements for which the SD was calculated. If SD data of the certified reference materials are not available, 95% confidence limits may be used as an estimate of CRM SD (see second form of formula for z score) [21]. 

One or more of these bias components are encountered when analyzing RMs. In general, RMs are divided into certified RMs (CRMs, either pure substances/solu-tions or matrix CRMs) and (noncertified) laboratory RMs (LRMs), also called QC samples [89]. CRMs can address all aspects of bias (method, laboratory, and run bias) they are defined with a statement of uncertainty and traceable to international standards. Therefore, CRMs are considered useful tools to achieve traceability in analytical measurements, to calibrat equipment and methods (in certain cases), to monitor laboratory performance, to validate methods, and to allow comparison of methods [4, 15, 30]. However, the use of CRMs does not necessarely guarantee trueness of the results. The best way to assess bias practically is by replicate analysis of samples with known concentrations such as reference materials (see also Section 8.2.2). The ideal reference material is a matrix CRM, as this is very similar to the samples of interest (the latter is called matrix matching). A correct result obtained with a matrix CRM, however, does not guarantee that the results of unknown samples with other matrix compositions will be correct [4, 89]. 

To determine Sb in marine sediments by ETAAS, a direct method was developed based on quantitating the analyte in the liquid phase of the slurries (prepared directly in autosampler cups). The variables influencing the extraction of Sb into the liquid phase and the experimental setup were set after a literature search and a subsequent multivariate optimisation procedure. After the optimisation, a study was carried out to assess robustness. Six variables were considered at three levels each (see Table 2.13). In addition, two noise factors were set after observing that two ions, which are currently present into marine sediments, might interfere in the quantitations. In order to evaluate robustness, a certified reference material was used throughout, BCR-CRM 277 Estuarine Sediment (guide value for Sb 3.5 0.4pgg ). Table 2.13 depicts the experimental setup. 

Method validation is important to ensure that the analytical method is in statistical control. A method may be validated by the so-called method evaluation function (MEF) (Christensen et al., 1993), which is obtained by linear regression analysis of the measured concentrations versus the true concentrations. A true concentration in a solution can be obtained by use of a high purity standard obtained from another manufacturer or batch than the one used for calibration. Both the high purity standard and the solvent are weighed using a traceable calibrated balance. If certified reference material is available this is preferred. The method evaluation includes the most important characteristics of the method as the following elements (see Figure 2.7) ... 

Selective extraction procedures have been used for many years in order to obtain fractions of samples for subsequent analysis. The analytical data have then been used to elucidate the species present in the original sample. Concern has been expressed about the meaning of the results, because most results are operationally defined (see Chapters 1 and 10). Some current research is resulting in answers to the problems associated with selective extractions. Again, the need for certified reference materials is evident (see Chapter 6). 

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