Certified Reference Materials matrix matched

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

Matrix-matched Certified Reference Materials or spiked samples should be used to determine the linearity of a method. 

Where matrix interferences exist, the method should ideally be validated using a matched matrix certified reference material. If such a material is not available it may be acceptable to use a sample spiked with a known amount of the standard material. 

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. 

As might be anticipated, the reduction in flame temperature has a deleterious effect upon the incidence and extent of matrix interferences when such boat techniques are used. As a consequence, precise matrix matching is necessary for accurate results, or the standard additions method may be employed.6 If the user is in any doubt as to whether matrix matching alone is sufficient, the adequacy of this approach may be confirmed by the analysis of certified reference materials and/or by applying the standard additions technique as well to a selection of samples to make sure that both techniques give the same results. For bismuth, cadmium, lead, silver, and thallium, detection limits by AAS are a few ng ml -1 or better.6 For arsenic, selenium, and tellurium they tend to lie in the range 10-30 ng ml-1, depending upon the source used. 

It can be assumed that with the development and study of new methods, the ability to determine M (S), the method bias component of uncertainty, cannot be done given that it can be evaluated only relative to a true measure of analyte concentration. This can be achieved by analysis of a certified reference material, which is usually uncommon, or by comparison to a well-characterized/accepted method, which is unlikely to exist for veterinary drug residues of recent interest. Given that method bias is typically corrected using matrix-matched calibration standards, internal standard or recovery spikes, it is considered that the use of these approaches provides correction for the systematic component of method bias. The random error would be considered part of the interlaboratory derived components of uncertainty. 

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. 

Suitable reference material should match matrix composition and analyte concentration of the unknown samples as close as possible. Of the relatively large number of suppliers only very few offer reference materials for trace analysis in seawater (see Howard and Sta-tham, 1993). Such material collected in the off-shore North Atlantic mixed layer (NASS) and Nova Scotia estuarine (SLEW) and nearshore coastal waters (CASS) respectively, is available from the National Research Council of Canada (Ottawa, Canada KIA OR6). These samples have been certified for 13 elements (As, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni, U, Se, Zn). 

Use of matrix-matched calibration solutions. Either protein is added to the solutions or certified reference materials are used (if available). 

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 pure element standards can be also used as chemical composition standards. The certified properties are in this case the contents of all metallic traces at ultra trace level. BAM offers, e.g. BAM B Primary Cul with statements (certified values) of the content of 65 trace metal elements. This reference material is suitable for matrix matching in metal analysis, e.g. where using methods of atomic spectroscopy. 

The existence of reference materials and appropriate calibration procedures are two essential issues to be considered in quantifying the components of a sample. Quantitation has been substantially improved by the commercial availability of an increasing number of certified solid reference materials, especially for low concentration levels. Recently [42], NIST archival leaf standards were used as matrix-matched standards for reliable quantitative elemental analysis of Spanish moss samples. LA-ICP-MS was used with mixing standards in order to produce at least three data points for each calibration curve the results thus obtained were compared with those provided by microwave digestion ICP-MS/AES. Standard addition was also examined and found to be an effective method in the absence of matrix-matched standards. 

However, calibration of LA-ICP-MS measurements for accurate determination of trace elements is still a problem, which is mainly due to fractionation processes and lack of certified standard reference materials with a matrix composition that is sufficiently similar to that of the various sample types that need to be analyzed [39]. A great variety of different calibration methods have therefore been developed, including matrix-matched and non-matrix-matched calibration techniques [39-42], but also techniques relying on the use of standard solutions, with corresponding aerosol introduction or direct ablation of the liquid, have been suggested [43, 44]. As a result of the different difficulties and limitations of these calibration methods, LA-ICP-IDMS is a promising alternative for calibration of measurements in powdered and liquid samples. 

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