Standards certified reference materials

Water samples (drinking water, rain, sea, river or waste water and others) have been characterized by ICP-MS with multi-element capability in respect to metal impurities (such as Ag, Al, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, Hg, K, Na, Sb, Se, Mg, Mn, Mo, Ni, Pb, Tl, Th, U, V and Zn) in many laboratories in routine mode with detection limits at the low ng I 1 range using ICP-QMS, and below by means of ICP-SFMS. Drinking water samples are controlled in respect of the European legislation (Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption). For quality control of analytical data, certified standard reference materials e.g. drinking water standard (40CFR 141.51), river water reference material SLRS-4 or CASS-2 certified reference sea-water material and others are employed. 

Most of the instrumental direct methods, which are employed in routine analyses, are highly matrix-dependent, therefore methods must be validated with certified standard reference materials. Standard plant reference materials (SRMs), with certified elemental contents can be obtained from NIST, National Institute of Standards and Technology of the United States Department of Commerce or from BCR, Community Bureau of Reference, Commission of the European Community (Colinet et al., 1982 Criepink, 1990 Horwitz and Albert, 1991). 

No certified standard reference materials for bismuth were available. Therefore the method was first evaluated by analyzing particulateloaded filters to which a known amount of bismuth was added before the digestion. The average recovery was 104.0%... 

Reference Materials Many vendors supply certified standard reference materials which address either a single instrument or a group of instruments. As these materials are ejq)ensive, it is often advisable to perform only the primary tests with these materials and perform secondary tests with a stable and well-split material supplied by the user. For best relevance, the size range and distribution t e of this material should be similar to those of the desired application. It is essential that the total operational procedure be adequately described in full detail (S. Rothele and W. Witt, Standards in Laser Diffraction, 5th European Symposium Particle Char., Nuremberg, March 24—26,1992). 

The main problem with in-situ chemical analysis of biogenic material is the absence of suitable solid certified standard reference materials. Fractionation of the chemical elements can occur during sample ablation (by laser or electron beam), during transport to the mass spectrometer and, in the case of LA-ICP-MS, in the plasma (e.g. Kuhn Gunther... 

Technology (NIST), formerly the National Bureau of Standards founded in 1918, is responsible for maintaining both scientific and commercial metrology in the United States. Its mission is to promote American innovation and competitiveness and supplies industry, academia and government with certified standard reference materials, including documentation for procedures, quality control, and materials for calibration. The German Institute for Standards (DIN) was founded in 1917 while in the United Kingdom the BSI was formed in 1901. 

The performance of any analytical procedure must be assessed through analysis of certified reference materials. As geological materials are varied, often with complex heterogeneous matrices, artificially produced standards are seldom sufficient. Homogenized, certified standard reference materials are available for a wide range of rock, soil, mineral, and biomaterial types and must be used to validate analytical protocols. Interlaboratory comparisons of measurements of the elemental and isotopic composition of reference materials are frequently published in dedicated journals such as the Geostandards Newsletter. 

If available, run one or more certified standard reference materials (CRMs) to check the accuracy of the overall method (use eqn [7]). 

Certified (Standard) Reference Materials (CRM/ SRM) Real-world samples, as similar as possible to the unknown samples to be analyzed, for which the concentra-tion(s) of analyte(s) have been certified by expert laboratories using several independent analytical methods under strict international conditions (Section 2.2.1). Such materials are rare and expensive, and if available are used only occasionally to check on the performance of the method and/or to validate reference materials prepared in-house. 

Perhaps the greatest problem in trace analysis is assurance of the accuracy of the results (i.e., the avoidance of systematic errors). Systematic sources of error are possible in every step of an analytical process. The most reliable method for detecting systematic errors is continuous and comprehensive quality assurance, particularly by occasional analysis of (certified) standard reference materials. Strictly speaking, an analytical method cannot be calibrated if suitable (i.e., representative) standard reference materials adequately representing the matrix of the expected test samples are not available. However, internal laboratoiy reference materials can then usually be prepared, whose matrix largely resembles the matrix of the test portions expected. If problems occur in the preparation of such reference samples, the standard addition method (SAM) can be applied, in which internal laboratory standards are added stepwise to the test sample (analyte and matrix)... 

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. 

As evident in the preceding formula, lead concentration measurements by IDMS must also consider the isotopic compositions of lead in the unspiked sample (Huemann 1988). These include natural isotopic variations among samples and isotopic fractionation during the analysis. The latter correction, which is common to all elemental analyses by IDMS, is addressed with standard techniques using certified standard reference materials (e.g., NIST SRM 981). The former correction, which is necessary for few heavy elements besides lead, requries separate isotopic analyses of unspiked samples. This in turn necessitates additional analyses for lead concentration measurements. 

The use of a definitive analytical method can also be used to establish standard reference materials. Definitive methods are ones that can produce exacting quantitative data without the need to compare measurements to a calibration standard. The gravimetric analysis method is a definitive technique. Isotope-dilution mass spectrometry, which is extensively used by NIST and other agencies producing certified standard reference materials, is also considered to be a definitive method of analysis. As discussed in Chapter 7, isotope dilution quantitation can be effectively used with ICP-MS. Therefore, a laboratory with ICP-MS instrumentation can produce reference materials in specific sample matrices for selected elements by using the isotope dilution technique.These standard reference materials still must be considered secondary standards, because they are usually not traceable to existing certified standards. 

Certified Standard Reference Materials Available from the U. S. National Institute of Standards and Technology and the National Research Council of Canada... 

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