Reference materials and calibrants

The second area of VAM activity, developing the tools needed for reliable analytical measurements, has always represented the most extensive part of the programme. The emphasis lies with the development of reference methods and the production of reference materials and calibration standards. In the early days, distribution of reference materials was seen as a key part of VAM, aimed at encouraging and facilitating their use. Today, however, the importance of reference materials is widely accepted and the distribution function is undertaken as a completely separate, commercial function by LGC-Promo-chem. A very important aspect of the second area is the collaborative development of validated methods, protocols and guides working in close collaboration with both UK laboratories and overseas organisations. 

Out-of-laboratory measurements are undertaken across a broad range of industrial and analytical sectors for a variety of reasons in clinical and medical diagnostics for the control of chemical and petrochemical production processes and to monitor emissions and discharges to the environment. The validity of data derived from such measurements is clearly of vital importance, for example to demonstrate compliance with environmental legislation. However, it is particularly difficult to obtain valid and reliable measurements outside the laboratory. The inability to control the environment in which the measurements are made and the use of untrained operators both have potential to impact significantly on the reliability of data. The situation is made worse because of the lack of adequate QA and QC procedures, the shortage of reference materials and calibration standards, and... 

Galvanostatic experiments have been carried out at room temperature on DEFCs containing Pd/MWCNT, Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C or Pd/C anodes as well as on DMFCs and DGFCs containing a Pd/MWCNT anode. In all cases, the cathode was a Fe-Co Hypermec K-14 material and the sohd electrolyte was a Tokuyama A-006 membrane. " " The anode compartments were filled with 28.3 mmol of EtOH, 40.7 mmol of MeOH or 6.8 mmol of glycerol. Accurate mass balance determinations were performed by means of both C H NMR spectroscopy and ionic chromatography, using reference materials and calibration curves. 

Set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring system and the corresponding values of quantities represented by a material both in form of reference materials and samples. In a wider sense, calibration represents a set of operations that establish relationships between quantities in the sample domain with quantities in the signal domain, viz y = f(x) and... 

In addition, a system for making sure staff are appropriately qualified and trained for the work that they are doing must be in place. This will enable an auditor to see clearly the demonstrated competence of the staff and how this has been checked. The requirements for all major items of equipment must be listed, to ensure that the equipment in use is suitable for the task, is in working condition and, where necessary, is calibrated. For all of the instrumentation there needs to be a documented schedule for maintenance. Measurements must be traceable, that is, the laboratory must be able to show how the calibration of measurement instruments is traceable to National or International Standards. Where this presents practical problems, as in some chemical measurements for example, interlaboratory comparison and the use of reference materials (and preferably Certified Reference Materials) will be required. 

Chemical and Physical Standards, Calibrants, Certified Reference Materials and Reagents... 

An outline laboratory QA procedure would need to include the following. In addition to the use of freshly made synthetic calibration standards, in-house standards, internationally recognized reference materials, and method blanks, as described above, it should include ... 

Ideally both the control materials and those used to create the calibration should be traceable to appropriate certified reference materials or a recognised empirical reference method. When this is not possible, control materials should be traceable at least to a material of guaranteed purity or other well characterised material. However, the two paths of traceability must not become coincident at too late a stage in the analytical process. For instance, if control materials and calibration standards were prepared from a single stock solution of analyte, IQC would not detect any inaccuracy stemming from the incorrect preparation of the stock solution. 

The ability to make analytical measurements depends intimately on the availability of well-defined standards and calibrants. Many measurements of analytes in seawater (such as dissolved organic carbon and dissolved organic nitrogen) cannot be compared among laboratories because of the lack of appropriate reference materials and blanks for instrument calibration and testing. Intercomparison exercises are critical (NRC, 1993, p. 75). 

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. 

To establish linearity and the linear range, a blank solution, plus at least 6, but preferably 10, independent solutions should be prepared from a traceable reference material and presented to the instrument at least as duplicates and in random order. The range of these solutions then defines the linear range, assuming, of course, statistical analysis of the calibration data supports this contention. How is the range known without doing experiments As stated above, there may be enough prior information to have a... 

For application in the isotope dilution technique and for tracer experiments using isotope enriched spikes, Merck (Darmstadt, Germany) have launched the production of new sets of isotopically enriched materials, such as 53Cr or mCd enriched spike calibration solutions, which were characterized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium).52... 

Method. Whether to use stomatal density or stomatal index from leaf fossils for paleoelevation reconstruction depends on the availability of modern reference material and the quality of the fossil cuticle. Ideally both should be used on the same material to increase the confidence in the provided estimates. In cases, however, when the quality of the fossil cuticle does not allow epidermal cells to be recognized with the necessary accuracy, and stomatal index can not be determined, stomatal density measurements are the only available option. To use stomatal index, a training set of modem and herbarium material grown under different C02 pressure provides the necessary calibration. Because stomatal density is influenced by more factors than just the C02 partial pressure, the actual response of stomatal density of the chosen fossil taxon has to be confirmed by analyzing leaves from actual elevation transects. Stomatal density is physiologically more informative than stomatal index as SD is strongly related to maximum stomatal conductance. 

It consists of three levels. At the top of the structure a network of national laboratories provides the primary chemical measurement standards and ensures that these are linked up with the international reference framework for chemical measurements. Via primary reference materials and reference measurements, a secondary level consisting of accredited chemical calibration laboratories, including verification authorities in the regulated area, is connected to the national standards level. 

This secondary or intermediate level has an important multiplier function. It is firmly linked to the national standards and provides traceable calibration means (mainly certified reference materials) and test samples to the workshop level, which consists essentially of chemical testing laboratories (including medical laboratories) which are required to give evidence to their customers that their measurement results are traceable to recognized references. In the case of medical laboratories, the traceability requirement also has a legal background. 

Meeting these stringent requirements enables the chemical calibration laboratories to act as providers of calibration means at the secondary level. This task requires that the value assignment to the reference materials and other calibration means provided to the user is more accurate than the measurement results at the user level need to be. Chemical calibration laboratories also exist in other countries, for example in the Netherlands and in the U.K. 

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