Certified reference material calibrants

A basic tool in ensuring the traceability of a measurement is the calibration. Calibration determines the performance characteristics of an instmment, system, or reference material. It is usually achieved by means of a direct comparison against measurement standards or certified reference materials. Calibration establishes-raceability but it is also necessary to determine the accuracy of the instrument readouts. 

ISO Guide 32 (1997) Calibration in analytical chemistry and use of certified reference materials. International Organization for Standardization, Geneva. 

Quality control Calibration, (certified) reference materials... 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

From the chemical point of view, in cases where matrix effects appear and no suitable certified reference materials are available, the calibration may be performed in the sample matrix itself by means of standard addition. 

Stoll et al. [142] have described a rapid continuous-flow determination of total inorganic carbon in seawater samples. The method runs on an autoanalyser Traacs 800 spectrophotometric system and is calibrated versus certified reference materials readily available. A typical analysis speed of 45 samples per hour can be reached with an accuracy of 2-3 xM and a precision of 2.5 xM. 

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. 

The development and characterization of Certified Reference Materials is an expensive process. Because of this, emphasis on the use of Certified Reference Materials is usually directed more towards the initial validation of a method it is rarely economical to use a reference material for routine quality control although it can be used to calibrate other, cheaper, secondary materials which can be used for routine quality control. 

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... 

Josephs RD, Krska R, MacDonald S, Wilson P, Pettersson H (2003) Preparation of a calibrant as certified reference material for determination of thc fusarium mycotoxin zearalenone. J AOAC Int 86 50-60... 

Quantification is usually achieved by a standard addition method, use of labeled internal standards, and/or external calibration curves. In order to allow for matrix interferences the most reliable method for a correct quantitation of the analytes is the isotope dilution method, which takes into account intrinsic matrix responses, using a deuterated internal standard or carbon-13-labeled internal standard with the same chemistry as the pesticide being analyzed (i.e., d-5 atrazine for atrazine analysis). Quality analytical parameters are usually achieved by participation in interlaboratory exercises and/or the analysis of certified reference materials [21]. 

Stability of calibrants and analytes is another frequently overlooked aspect of quality assurance, which is particularly relevant to surfactants. This aspect is discussed in Chapter 4.4. Very few intercalibration studies have been performed for the surfactant types of analytes (cf. Chapter 4.5). Currently, no certified reference material is available for surfactants. The European Commission has recently tendered for production of a reference material with certified surfactant concentrations [2]. We can conclude that quality assurance in quantitative surfactant analysis is still in its infancy when compared to analysis of PCB or chlorinated dioxins. Notwithstanding this, several important achievements have been made during recent years regarding improvement of the accuracy and reliability of qualitative analysis of surfactants, which will be the subject of the following chapters. 

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. 

Since certified reference materials for seawater nutrient analysis are currently unavailable, individual laboratories must prepare their own standard solutions for instrument calibration. Standard stock solutions are prepared at high concentrations (mM) so that they can be used for months without significant alterations in concentration. Working low-concentration standard solutions are unstable and need to be prepared daily by diluting stock solutions with distilled water or low-nutrient seawater. In this case, the accuracy of nutrient analysis at a given laboratory is highly dependent upon the accuracy of the daily preparation of the calibration solutions. 

At present, there are no widely distributed certified reference materials containing all of the radionuclides in the uranium and thorium decay series. Such reference materials are needed to calibrate instruments that make radionuclide measurements and to compare analytical results from different laboratories. The most critical need is for reference materials in the 235U decay series 231Pa, 227Ac, and 223Ra. 

Denise LeBlanc is the Group Leader for the Marine Sciences Group and the Manager of the Certified Reference Materials Program at the Institute for Marine Biosciences (IMB) of the National Research Council of Canada. The Certified Reference Materials Program manufactures instrument calibration standards and certified reference materials for shellfish toxins, PCBs, PACs, and trace elements in marine sediments, in biological tissues, and in seawater. Her experience resides in the manufacture and long-term production of reference materials. 

Establishing the traceability of the measurement is one of the most important requirements in ISO/IEC 17025. Measurement traceability ensures that the measurements in different laboratories are comparable in space and time all over the world. Calibration and the use of certified reference material (see chapters 9, 10 and 14) is the central tool in establishing trace-ability. Therefore a laboratory must have programmes and procedures for both. 

Where traceability of measurements to SI units is not possible and/or not relevant, the same requirements for traceability to, for example, certified reference materials, agreed methods and/or consensus standards, are required as for calibration laboratories. 

Starting with the calibration, the certified reference material can be used as a calibrant (directly or via working standards prepared using them) and in this way traceability of the results to the property values of the certified reference material is ensured. Thus this is a very valid way for certified reference materials to improve the measurements in a laboratory. 

Thus summarizing the certified reference material should be used for calibration of an instrument or validation of a measurement method but preferably not used for measurement control. 

Let s see what is the influence for the two cases where the certified reference materials are useful in the laboratory (i.e. calibration and vahdation). In the case of calibration, the properly value of the certified reference material is used to calibrate the analytical instrument used for the measurement, thus it is used in order to obtain the measurement result. In this way the properly value of the certified reference material is part of the traceability chain, as shown in the shde, and is directly involved in the establishment of the measurement traceabihty. 

In the above discussion, standard reference materials (SRMs) were mentioned often. A reference material (RM) is a material or substance suitable for use in calibrating equipment or standardizing solutions. A certified reference material (CRM) that a vendor indicates, via a certificate, is an RM. A standard reference material (SRM) is one that is distributed and certified by a certifying body, such as NIST. The SRM is the material to which all calibration and standardization materials should be traceable. A standard material becomes one when it is compared to or prepared from another. Ultimately, it all rests on the SRM — meaning all standard materials are traceable to an SRM (see Figure 5.10). 

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