Reference materials — standards

Analysis sheet for Simulated Rainwater (SRM 2694a). Adapted from NIST Special Publication 250 Standard Reference Materials Catalog 1995-96, p. 54 U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. 

Consider the situation when the accuracy of a new analytical method is evaluated by analyzing a standard reference material with a known )J,. A sample of the standard is analyzed, and the sample s mean is determined. The null hypothesis is that the sample s mean is equal to p. 

Because of the risk of lead poisoning, the exposure of children to lead-based paint is a significant public health concern. The first step in the quantitative analysis of lead in dried paint chips is to dissolve the sample. Corl evaluated several dissolution techniques. " In this study, samples of paint were collected and pulverized with a Pyrex mortar and pestle. Replicate portions of the powdered paint were then taken for analysis. Results for an unknown paint sample and for a standard reference material, in which dissolution was accomplished by a 4-6-h digestion with HNO3 on a hot plate, are shown in the following table. 

The %w/w lead in a lead-based paint Standard Reference Material and in unknown paint chips is determined by atomic absorption using external standards. 

Analysis of Standards The analysis of a standard containing a known concentration of analyte also can be used to monitor a system s state of statistical control. Ideally, a standard reference material (SRM) should be used, provided that the matrix of the SRM is similar to that of the samples being analyzed. A variety of appropriate SRMs are available from the National Institute of Standards and Technology (NIST). If a suitable SRM is not available, then an independently prepared synthetic sample can be used if it is prepared from reagents of known purity. At a minimum, a standardization of the method is verified by periodically analyzing one of the calibration standards. In all cases, the analyte s experimentally determined concentration in the standard must fall within predetermined limits if the system is to be considered under statistical control. 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Physical or artifactual standards are used for comparison, caUbration, etc, eg, the national standards of mass, length, and time maintained by the National Institute of Standards and Technology (NIST) or the standard reference materials (SRMs) collected and distributed by NIST. Choice of the standard is determined by the property it is supposed to define, its ease of measurement, its stabiUty with time, and other factors (see Fine chemicals). 

Standard reference materials provide a necessary but insufficient means for achieving accuracy and measurement compatibiUty on a national or international scale. Good test methods, good laboratory practices, well-qualified personnel, and proper intralaboratory and intedaboratory quaUty assurance procedures ate equally important. A systems approach to measurement compatibiUty is ikustrated in Figure 2. The function of each level is to transfer accuracy to the level below and to help provide traceabiUty to the level above. Thus traversing the hierarchy from bottom to top increases accuracy at the expense of measurement efficiency. 

Vitreous siUca aimealed at 1100°C has been designated NIST Standard Reference Material 739 (LI and L2). Its expansion coefficient, a, may be calculated for 300—700 K from the following expression (144), where Tis the absolute temperature in Kelvin. 

ISCC-NBS Centroid Color Charts, NBS Standard Reference Material No. 2106, National Institute of Standards and Technology, Washiagton, D.C.,... 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

The set of Glass Standard Reference Material NIST SRM 610 - 614 was used for quantification of amount up to 45 trace elements in the synthetic oxide single crystals and natural quartzites. 

Because of the complex nature of the discharge conditions, GD-OES is a comparative analytical method and standard reference materials must be used to establish a unique relationship between the measured line intensities and the elemental concentration. In quantitative bulk analysis, which has been developed to very high standards, calibration is performed with a set of calibration samples of composition similar to the unknown samples. Normally, a major element is used as reference and the internal standard method is applied. This approach is not generally applicable in depth-profile analysis, because the different layers encountered in a depth profile of ten comprise widely different types of material which means that a common reference element is not available. 

The authors demonstrated the importance that correct use of the MDGC-IRMS System is essential for the achievement of precise and accurate measurements. Table 10.4 reports the GC-IRMS measurements of some standard reference materials, obtained with different cut conditions. As can be seen from this table, premature cuts result in 8 C values which are significantly higher than the true values, while delayed cuts give lower 8 C values. This fact indicates that the beginning of the peak is enriched in C, while the end is depleted. 

Table 10.4 GC-IRMS measurements of standard reference materials obtained under different cut conditions (25)...

Throughout this book the use of a number of standard analytical samples is recommended in order that practical experience may be gained on substances of known composition. In addition, standard reference materials of environmental samples for trace analysis are used for calibration standards, and pure organic compounds are employed as standard materials for elemental analysis. 

Only one isotope, Pu-244, is presently available as a NBS Standard Reference Material and can be obtained from the DOE New Brunswick Laboratory. A second NBS/SRM as a 1 1 mixture of... 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

Procedures for determining the spectral responslvlty or correction factors In equation 2 are based on radiance or Irradlance standards, calibrated source-monochromator combinations, and an accepted standard. The easiest measurement procedure for determining corrected emission spectra Is to use a well-characterized standard and obtain an Instrumental response function, as described by equation 3 (17). In this case, quinine sulfate dlhydrate has been extensively studied and Issued as a National Bureau of Standards (NBS) Standard Reference Material (SRM). 

Standardization. Standardization in analytical chemistry, in which standards are used to relate the instrument signal to compound concentration, is the critical function for determining the relative concentrations of species In a wide variety of matrices. Environmental Standard Reference Materials (SRM s) have been developed for various polynuclear aromatic hydrocarbons (PAH s). Information on SRM s can be obtained from the Office of Standard Reference Materials, National Bureau of Standards, Gaithersburg, MD 20899. Summarized in Table VII, these SRM s range from "pure compounds" in aqueous and organic solvents to "natural" matrices such as shale oil and urban and diesel particulate materials. 

Table VII. Standard Reference Materials (SRM s) for the Determination of Polycyclic Aromatic Hydrocarbons (PAH s) ...

Sharpless, K.E. et al., Valne assignment of retinol, retinyl palmitate, tocopherol, and carotenoid concentrations in standard reference material 2383 (baby food composite), J. AOAC Int., 82, 288, 1999. 

Cali JP and Stanley CL (1975) Measurement compatibility and standard reference materials. Annu Rev Mat Sci 5 329-343. 

Cali JP, Mears TW, Michaelis RE, Reed WP, Seward RW, Stanley CL, Yolken HT, and Ku HH (1975) The role of standard reference materials in measurement systems. NBS Monograph 148, Washington DC. 

Lee H-B and Chau ASY (1987) Analytical reference materials Part VII. development and certification of a sediment reference material for total polychlorinated biphenyls. Analyst 112 37-40. Meinke WW (1971) Standard reference materials for clinical measurements. Anal Chem 43(6) 28A-47A. 

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