Chemical methods reference materials

Epstein MS (1991) The independent method concept for certifying chemical-composition reference materials. Spectrochim Acta 466 1583-1591. 

However the chemical community has not been dormant in the period since the Convention of the Meter came into being. A vast range of analytical methods has been developed with ever increasing sensitivity and selectivity. Comparability of measurement results has been achieved in many sectors by the use of collabora-tively studied methods, reference materials, check samples and proficiency testing schemes. It is only recently that steps have been taken to develop traceability to international standards. In developing this traceability much can be learnt from the way traceability has developed for physical measurements. 

The main parts that support the traceability in chemical measurements are primary analytical methods, reference materials and valid analytical methods suitable for some available instruments for a group of materials, according to their nature, range of measurements in a specific matrix. These elements should serve to establish an uninterrupted chain of comparisons in chemical measurements and its uncertainty estimation. 

Analytical standards imply the existence of a reference material and a recommended test method. Analytical standards other than for fine chemicals and for the NIST series of SRMs have been reviewed (6). Another sphere of activity ia analytical standards is the geochemical reference standards maintained by the U.S. Geological Survey and by analogous groups ia France, Canada, Japan, South Africa, and Germany (7). 

Annular tautomerism does not occur in isothiazoles or benzisothiazoles. Substituent tautomers can sometimes be distinguished by chemical methods, but it is important that reaction mechanisms and the relative rates of interconversion of tautomeric starting materials or isomeric reaction products are carefully investigated. Physical methods only will be considered in this section, and references to original publications can be found in a comprehensive review (76AHC(S1)1). 

Prior to the evaluation of Li[Mn2]04 as a rechargeable cathode material, the ideal spinel framework [Mn2]04, (commonly referred to as A — Mn02, after Hunter) was chemically synthesized by acid digestion of Li[Mn2]04 [121]. The formation of A— Mn02 by chemical methods differs from the electrochemical reaction because it dissolves 25 percent of the Mn cations from the original spinel framework ... 

Ueiano GA and Geavatt CC (1977) The role of reference materials and reference methods in chemical analysis. CRC Crit Rev Anal Chem 6 361-411. 

Much of the early work with certified reference materials was linked to the derivation of reference methods and there was a period in which primary or definitive (i.e. very accurate but usually very complex) and secondary (or usable) methods were reported e.g. steroid hormones (Siekmann 1979), creatinine (Siekmann 1985), urea (Welch et al. 1984) and nickel (Brown et al. 1981). Although there are some application areas, such as checking the concentrations of preparations listed in a pharmacopoeia, where a prescribed, defined method has to be used, in practice such work is limited. However, this approach to chemical analysis is no longer widely used and will not be further discussed. The emphasis now is placed on using RMs to demonstrate that a method in use meets analytical criteria or targets deemed to be appropriate for the application and to develop figures of merit (Delves 1984). 

Some of the intended categories of use of radioisotopic reference material have been reviewed recently by Fajgelj et al. (1999). They include assignment of property values, establishing the traceability of a measurement result, determining the uncertainty of a measurement result, calibration of an apparatus, assessment of a measurement method, use for recovery studies and use for quality control purposes. It should be noted however that, in general, natural matrix reference materials are not recommended for calibration purposes. This should preferably be done with pure chemical forms of the element labelled with the isotope of interest. Calibrated isotopic sources of this kind are available from a number of commercial suppliers and are not the subject of this review. 

Chemical Process Safety, offer guidance on auditing methods and practices. Additional reference material can be found in CCPS s Guidelines for Technical Management of Chemical Process Safety (Ref. 1). 

In analytical chemistry, we do not have a standard mole. Therefore, solutions made up to a well-defined concentration using very pure chemicals are used as a basis from which we can compare other solutions or an instrument scale. This process is calibration . For some analyses, the chemical used may be a Certified Reference Material which has a well documented specification, e.g. in terms of the concentration of a particular species and the uncertainty of the specified value. However, it is not sufficient just to calibrate the apparatus/equipment used, it is important that the complete method of analysis is validated from extraction of the analyte from the sample to the final measurement. 

Becker, D., Christensen, R., Currie, L., Diamondstone, B., Eberhardt, K. R., Gills, T., Hertz, H., Klouda, G., Moody, J., Parris, R., Schaffer, R., Steel, E., Taylor, J., Watters, R. and Zeisler, R., Use of NIST Standard Reference Materials for Decisions on Performance of Analytical Chemical Methods and Laboratories, NIST Special Publication 829, National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA, 1992. 

Section 5.2 introduced the subject of metrological traceability and calibration and the use of pure chemical substances and reference materials in achieving trace-ability. Reference materials are used as transfer standards. Transfer standards are used when it is not possible to have access to national or international standards or primary methods. Transfer standards carry measurement values and can be... 

Chemical analyses of seawater are uniquely difficult given the poorly known speciation and the low concentration of many of the analytes of interest. Analyses of suspended and sedimentary marine particulate materials present their own distinct challenges, primarily due to potential interference by predominant mineral phases of different types (e.g., opal, carbonate, and aluminosilicate). Of all the analytical methods applied to marine waters and particles, at present only a small fraction can be systematically evaluated via comparison to reference materials that represent the appropriate natural concentrations and matrices. 

Reference Material Material or substance whose property values are sufficiently homogeneous and well-established so as to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. (Note A reference material may be in the form of a pure or mixed gas, liquid, or solid. Examples include synthetic mixtures such as calibration solutions used in chemical analysis as well as materials based on natural environmental samples such as sediments.)... 

The availability of reference materials or standards will not solve all the analytical problems faced by the marine community. In addition to using reference materials, the use of agreed-on common collection and analytical methods can also improve the chemical data being collected by oceanographers. Standardization of these methods minimizes the variability that may result from differences in laboratory procedure. A major disadvantage of method standardization, however, is that it can discour-... 

Many of the analytes of interest for solid phase chemical reference materials are the same as those in seawater, but the need for and the preparation of reference materials for suspended particulate matter and sediments is quite different. The low concentrations of many seawater species and the presence of the salt matrix create particular difficulties for seawater analyses. However while sediments frequently have higher component concentrations than seawater, they also have more complicated matrices that may require unique analytical methods. A number of particulate inorganic and organic materials are employed as paleoceano-graphic proxies, tracers of terrestrial and marine input to the sea, measures of carbon export from the surface waters to the deep sea, and tracers of food-web processes. Some of the most important analytes are discussed below as they relate to important oceanographic research questions. 

A wide range of high quality, non-certified carotenoid and chlorophyll chemical standards are commercially available (e.g., Sigma-Aldrich, DHI, and Roth). The availability of a mixed pigment reference standard and biological matrix reference materials would improve analytical performance in individual laboratories, facilitate method and laboratory in-... 

Kenneth Johnson is a Senior Scientist at the Monterey Bay Aquarium Research Institute. His research interests are focused on the development of new analytical methods for chemicals in seawater and application of these tools to studies of chemical cycling throughout the ocean. His group has developed a variety of analytical methods for analyzing metals present at ultratrace concentrations in seawater. His expertise lies in trace metal analysis and instrumentation. The creation of reference materials to calibrate these instruments is important for the production of long-term, high-precision datasets. Dr. Johnson has participated on the NRC Committee on Marine Environmental Monitoring and the Marine Chemistry Study Panel. 

Physical models of fuel cell operation contribute to the development of diagnoshc methods, the rational design of advanced materials, and the systematic ophmization of performance. The grand challenge is to understand relations of primary chemical structure of materials, composition of heterogeneous media, effective material properties, and performance. For polymer electrolyte membranes, the primary chemical structure refers to ionomer molecules, and the composition-dependent phenomena are mainly determined by the uptake and distribuhon of water. 

Calibration Most process analyzers are designed to monitor concentration and/or composition. This requires a calibration of the analyzer with a set of prepared standards or from well-characterized reference materials. The simple approach must always be adopted first. For relatively simple systems the standard approach is to use a simple linear relationship between the instrument response and the analyte/ standard concentration [27]. In more complex chemical systems, it is necessary to adopt either a matrix approach to the calibration (still relying on the linearity of the Beer-Lambert law) using simple regression techniques, or to model the concentration and/or composition with one or more multivariate methods, an approach known as chemometrics [28-30]. 

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