Matrix CRMs

Further afield, in 1978 the Japanese National Institute for Environmental Studies (NIES) started the production of a series of biological and environmental matrix CRMs, certified for a number of trace elements (Okamoto and Fuwa 1985). Recently also the certification of metal species in some materials was reported (Okamoto and Yoshinaga 1999). 

Most CRMs are so-called matrix-CRMs , identifying that they have been made from material sampled in nature. For these materials, it is impossible to come up with property values traceable to SI, as preparation steps carmot directly be related to that. At best, a comparison can be made among methods, from which usually one is the technically best established method, and the results of such a comparison may flow into the establishment of the property values and their respective uncertainties. 

Table 3.6 presents examples for environmental and biological natural matrix CRMs from BCR, NIST and NRCC certified for elements using the above described analytical multi-method approaches. 

Tab. 3.6 Examples for BCR, NIST and NRCC natural biological and environmental matrix CRMs/ SRMs certified for elements...

Certified reference materials (CRMs) to validate measurements of organic constituents were introduced in the early 1980 s, more than a decade after the development of the first natural matrix CRMs for inorganic constituents. There are three types of CRMs to support measurements of organic constituents ... 

The BCR has also produced a large number of environmental matrix CRMs for PAHs, PCBs, pesticides, and PCDDs/PCDFs as shown in Table 3.12. These matrices include both natural contaminant level matrices as well as natural matrices spiked with low and high levels of contaminants. When viewed together the NIST and BCR CRMs provide a wide range of environmental matrices in which a considerable number of analytes have been assigned certified and reference values. 

Tab. 3.12 BCR natural matrix CRMs for the determination of organic contaminants in environmen-...

Fig. 3.3 General scheme used by the BCR for the certification of natural matrix CRMs...

BCR Analytical Approach for the Certification of PAHs in Natural Matrix CRMs Prior to the certification analyses for the CRM, each participating laboratory has to prepare standard solutions of the analytes to be determined from certified reference compounds (purity >99.0 %) to calibrate their instruments for response and response linearity (multiple point calibration), detection limit, and reproducibility. In the case of PAH measurements, reference compounds of certified purity are used as internal standards, which are not present at a detectable concentration in the matrix to be analyzed (e.g. indeno[i,2,3-cd]fluoranthene (CRM 267), 5-methylchrysene (CRM 081R), benzo[f ]chry-sene (CRM 046), picene (CRM 168), and/or phenanthrene-dio). 

Many more common problems start because the new users do not really understand their analytical systems, a problem described as the Nintendo scientist syndrome by Jenks (1995). Inexperienced scientists are often not sufficiently discriminating in their selection and use of CRMs. But incorrect choice can also be due to the unavailability of suitable matched matrix CRMs, or surprisingly often because the laboratory believes it cannot aftbrd the ideal product. 

The importance of particle size is directly proportional to the sub-sample size recommended by the analytical method. The larger the sub-sample size the larger the acceptable particle size. For sub-sample sizes of ig or greater a soil sieved through a imm screen is generally acceptable. Therefore if the sample is relatively coarse, e.g up to 2mm particles and the matrix CRM is an uniform sub-micron powder, it may be necessary to use a much larger sample from the material under test than for the CRM. 

One approach to providing secondary RMs is the NIST Traceable Reference Material (NTRM) program for gas standards (Jenks et. al. 1998 NIST 1997). The NTRM program is to be extended to metals, trace elements and pure substances. It may be possible to extend this concept to other types of RM, but it is difficult to see how it can easily be applied to the production of complex matrix CRMs. 

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. 

In many cases the bias can vary depending on changes in the matrix. This can be reflected when analysing several matrix CRMs... 

Skills and installation to process the material in a suitable form, especially for matrix CRMs... 

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]. 

The development and improvement of analytical methodologies for mycotoxins has been greatly improved by the increased availability of matrix matched certified reference materials (CRMs) (Boenke, 1995) (Table 11.6). The type of matrix CRMs and concentration of the specified mycotoxin are based on the natural occurrence pattern of the toxin in specific foods and feeds. The recent availability of suitable CRMs, while being a prerequisite for the implementation of regulations and standards, will also be invaluable in many ways for the validation of new methods, solving trade disputes and for harmonising proficiency schemes. 

Sound knowledge of method bias and recovery from thorough investigation of interferences and matrix effects and by use of matrix CRMs, and/or spiking studies. Consideration is given to any mismatch between standards and samples, such as concentration and matrix differences and differences in the way the analyte is incorporated into the matrix. 

Considering the metrological structure, a proficiency testing scheme was organised with laboratories working in water analyses. It was decided to choose a deionised water spiked with heavy metals (at 5 and 20 g/L for cadmium and lead, respectively) for the round robin test. A matrix CRM (groundwater containing cadmium and lead) was also sent to laboratories. Laboratories have been asked to analyse, in duplicate, the spiked water... 

For the METREAU project it was chosen not to organise a proficiency testing with a drinking water to avoid a possible matrix gap with the matrix CRM. It is well known that, for the production of groundwater matrix CRM, it is difficult to choose a matrix sample representative of an average natural water. For water analyses, this is a major point to determine if a matrix CRM is appropriate. For the METREAU project, it would have been risky to get different matrixes since uncontrolled... 

Matrix CRM was used to control the quality of routine determinations performed by laboratories, and so to evaluate the accuracy of analyses The magnitude of bias was estimated through the analysis of the matrix CRM using pure certified standard for calibration... 

Table 2 Concentrations of lead and cadmium in the matrix CRM, determined by LNE (and confirmed by SP for lead)...

Table 1 Major chemical species present in the matrix CRM...

Forty six field laboratories (mainly from France) have participated in the inter-laboratory comparison. They have analysed the water sample using their regular calibration solution and then have repeated this analysis using the certified standards. They also have analysed the matrix CRM. Laboratories have produced two results (duplicate) per sample. Techniques used were mainly atomic absorption spectroscopy with furnace but also ICP-OES and ICP-MS. 

Fig. 7 Repartition of results for lead analysis in the matrix CRM by the 45 laboratories...

Table 5 Performances of the participants in the proficiency testing for cadmium and lead analysis in matrix CRM...

The groundwater matrix CRM has been analysed in duplicate by laboratories, using certified calibration solutions. Results are presented in Table 5. 

Fig. 9 Results of lead determi ug/l Matrix CRM mean and uncertainty (2x SD) for Lead analysis...

A metrological structure was implemented in the METR-EAU project to evaluate the benefits for laboratories of using certified pure solutions for calibration and matrix CRM for method validation. 

Analysis of a matrix CRM has shown a noteworthy bias in lead analysis of about 10%. This bias was observed for all routine methods used by laboratories. Complementary experiments have indicated that a strong matrix effect was responsible for this bias. This underlined bias has demonstrated that for this matrix water, only a primary method can provide the true value of the sample of an inter-laboratory comparison. The mean value of laboratory results cannot be used as a reference value. 

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