Matrix matched CRMs

Primary calibrators, indeed all calibrators, must be commutable that is, they must behave during measurement in an identical manner to the native analyte material being measured. Matrix reference materials made by mixing a pure reference material with the components of the matrix are unlikely to be entirely commutable, and for this reason some authorities (EURACHEM for one [EURACHEM and CITAC 2002]) advise against using matrix-matched CRMs for calibration, recommending instead their use to establish recoveries, after calibration by a pure reference standard. 

In contrast to SIMS, in SNMS - where the evaporation and ionization processes are decoupled -the matrix effects are significantly lower, because the composition of sputtered and post-ionized neutrals corresponds more closely to the composition in the solid sample (compared to the sputtered secondary ions in SIMS), which means the RSCs of elements vary by about one order of magnitude. Consequently, a semi-quantitative analysis by SNMS can also be carried out if no suitable matrix matched CRM is available. This is relevant for thin film analysis, especially for the determination of elemental concentration profiles in depth, for studying the stoichiometric composition of thin films and interdiffusion effects. 

For food safety and quality control, a number of other CRMs would be very much welcomed. Many requests are made for matrix matched CRMs and PTMs. Especially official food and feed control laboratories and moreover the Community and National Reference Laboratories are dependent on fit-for-purpose validated analytical methods and real matrix reference materials. 

Matrix-matched CRMs are increasingly needed for a range of environmental matrices, elements or compounds, including materials certified for of>erationally-defined parameters (e.g. extractable trace element contents). Finally, training on the preparation of LRMs and use of CRMs should be systematised in the framework of accreditation systems. Such courses already exist [46] but are not fitted for purpose with respect to accreditation they should involve advice on how to develop LRMs versus CRMs, use RMs in sectorial proficiency testing (e.g. environmental monitoring) etc. 

In most analytical procedures, calibration is carried out by means of a calibration curve using com-pound(s) prepared with chemicals of an appropriate purity and verified stoichiometry. Matrix effects must often be taken into account and, consequently, the calibration solutions should be matrix-matched. CRMs of pure compounds may be used for calibration. However, matrix CRMs should in principle not be used for the purpose of calibration unless no other suitable calibrants are available, with the exception of those methods (e.g., spark source mass spectrometry, wavelength-dispersive XRF, etc.) that require calibration with CRMs of a similar, fully characterized matrix (e.g., metal alloys, cements). For such methods, accuracy can only be achieved when certified RMs are used for the calibration. 

Only a direct matrix match of sample and CRM, and the CRM s use as a direct calibrant will allow the user to demonstrate accuracy and subsequently traceability close to the uncertainties established during the CRM certification ( note matrixmatching may not be necessary with matrix-independent techniques). This reality places a significant burden on the CRM producers, since large uncertainties in the certified values may degrade the perceived value of the CRM. 

Use as a matrix matched calibrant (direct or via working standards) to ensure traceabiiity of results to an external reference (the CRM)... 

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]. 

Systematic effects are estimated by repeated measurements of a CRM, suitably matrix matched. Any difference between the CRM and a routine sample for which the measurement uncertainty is being estimated should be considered and an appropriate uncertainty component added. Suppose a concentration measurement is routinely made in a laboratory that includes measurement of a CRM in the same run as calibration standards and unknowns. The bias (6) is given by... 

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. 

Traceability is one major factor that can be achieved via CRMs as main means in the held of chemical metrology. In general, CRMs are applied for the validation of analytical methods. Standard solutions are then used for instrument calibration. Nonetheless, CRMs should not be understood as the solution for all problems in chemical measurement. It goes without saying that the matrix of a CRM should match the analytical problem as exactly as possible. It is clear that there are not CRMs available for all matrices and analytes. Thus, it is important to have the best matrix match. 

When an FF-RM and especially a CRM is desired for validation of a measurement procedure, such material is not always easily available. This is especially the case for CRMs with naturally incurred contaminants or residues. Unfortunately these are still rare and often not matrix-matched. Examples of urgent areas of need of such FF-RMs are given in the following section. 

ILLUSTRATION OF THE INFLUENCE OF MATRIX MATCHING FOR INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION AS METHOD OF FINAL DETERMINATION (MASS FRACTIONS IN mg kg- ) OF TRACE ELEMENTS IN A LIGHT SANDY SOIL - BCR CRM 142R... 

Calibration errors justified that two sets of Zn results be withdrawn (no verification of commercial calibrant nor matrix-matching, and use of a CRM for calibration). 

The sample matrix problem has stimulated the development of two pragmatic solutions matrix reference materials and interlaboratory comparisons. The matrix-matched, certified reference material (CRM) is a unique type of chemical standard commonly used to validate complete measurement methods and sometimes for instrumental cafibration (e.g., in XRF). Such standards must be available for each required analyte/matrix combination. Similarly, interlaboratory comparisons are vmdertaken for each relevant analyte/matrix combination in order to establish comparability of data between laboratories. These comparisons range from rovmd robin studies, which collaboratively test a new method, to formal PT schemes that assess agreement between laboratories on an ongoing basis. 

An important prerequisite for the use of CRMs as calibrants, at least for optical methods and particularly all AAS modes, is that they should match the matrix and level of analyte contents of the materials to be analyzed as closely as possible, so that potential matrix effects will be compensated if calibrant and sample material are affected by the applied method, e.g. the temperature program for furnace techniques, in the same way. Further it is very important for all methods that the CRMs used should not show a nugget effecf, i.e. particles with extremely high analyte content that can lead to a high analyte heterogeneity (Kurfiirst 1991 Kurfiirst et al. 

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. 

In trace organic analysis there is usually an extraction or clean-up process, rather than a sample dissolution. Here not only must the matrix effect be considered, but also the recovery yield of the extraction. Frequently an external spike standard is added, but there is often no way of knowing if the recovery of the spike standard matches the analyte in question. There is considerable evidence that the U S E P A method for VOA analysis (Minnich 1993) is subject to such error, as reported by Schumacher and Ward (7997). The analyst must always consider the possibility of such an error, especially when using CRMs to control methods that are applied in routine mode. 

A common way to assess the matrix bias is to analyse a drinking water (therefore containing a matrix) in the proficiency testing. For this analysis, laboratories calibrate their instruments using standard, commercial or in-house solutions. If a CRM is available, a possible matrix effect can be corrected by adjusting operational instrument parameters to match the certified value. 

The matrix of the CRMs may not match with that of the sample. 

The Technical Architecture and Development activity is meant to assure that the technology landscape is dehned and matches well with the prescribed needs of the CRM effort. It moves through the matrix in a manner intended to make certain members across the organization view the final design and deployment as benehcial for the internal functions as well as the key customers. 

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