Metrology reference methods

Table 1 Metrological properties of methods used by the Choles- CV coefficient of variation, HDLC high-density lipoprotein cho-terol Reference Method Laboratory Network (CKMLN). 2 KM lesterol, SD standard deviation, LDLC low-density lipoprotein Secondary reference method, DCM designated comparison meth- cholesterol, NA not available od, IDMS isotope dilution mass spectroscopy, AK Abell-Kendall,...

Table 2 Comparison of Cholesterol Reference Method Laborato- tute, NIST National Institute of Standards and Technology (US ry Network (CRMLN) traceability model to the International Or- NMI), IDMS isotope dilution mass spectrometry, ACL accredited ganization for Standardization (ISO) model. BIPM International calibration laboratory, AK Abell-Kendall Bureau of Weights and Measures, NMI National Metrology Insti-...

Another way of providing a link between chemical laboratories and the SI units can use RMs, reference methods and standard measuring devices which are made available by National Metrology Institutes. 

The above sections have highlighted the importance of data comparability and trace-ability in the context of WFD chemical monitoring. Let us now examine in detail what references need to be considered for the development of a sound metrological system. Firstly, as a reminder, traceability is defined as the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties (ISO, 1993). The ways in which these elements can be applied to chemical measurements were discussed some years ago (Valcarcel and Rios, 1999 Quevauviller, 1999 Walsh, 2000) and those discussions still continue. In this context the basic references are those of the SI (Systeme International) units, i.e. the kg or mole for chemical measurements. Establishing SI traceability of chemical measurements may, in principle, be achieved in relation to either a reference material or to a reference method (Quevauviller and Donard, 2001). The unbroken chain of comparison implies that no loss of information should occur during the analytical procedure (e.g. incomplete recovery or contamination). Finally, traceability implies, in theory,... 

Complex environmental monitoring measurements are generally based on successive analytical steps, such as extraction, derivatization, separation and detection. This succession multiplies the risk that the traceability chain is broken because of a lack of appropriate references (e.g. pure calibrants, reference materials). Reference methods usually refer to methods with high metrological qualities (they may actually be standardized methods), and examples are provided by primary methods that are directly traceable to SI units without the need to use an external calibration. 

Jakopic, R., Sturm, M., Kraiem, M. et al. (2012). Certified reference materials and reference methods for nuclear safeguards and security. INSINUME Sixth International Symposium on In Situ Nuclear Metrology as a Tool for Radioecology. Brussels, Belgium Joint Research Centre, Presentation. 

With its specificity and ideal compensation for losses, SIDAs have the potential to become official reference methods for folates. A SIDA ean be traced back to a gravimetric i.e. primary) measurement and, therefore, is considered a primary method, which is a method having the highest metrological qualities, for which a complete uncertainty statement ean be written down in terms of SI units, and whose results are, therefore, aeeepted without reference to a standard of the quantity being measured (Quinn 1997). In ease of being validated intensively for the absence of systematic errors, it has the potential to be accurate, i.e. producing the true value. 

Andreis E, Kiillmer K, Appel M (2014) Applieation of the reference method isotope dilution gas chromatography mass spectrometry (ID/GC/MS) to establish metrological traceability for caUbration and control of blood glucose test systems. J Diabetes Sei Teehnol 8(3) 508-515... 

The methodical elaboration is included for estimation of random and systematic errors by using of single factor dispersion analysis. For this aim the set of reference samples is used. X-ray analyses of reference samples are performed with followed calculation of mass parts of components and comparison of results with real chemical compositions. Metrological characteristics of x-ray fluorescence silicate analysis are established both for a-correction method and simplified fundamental parameter method. It is established, that systematic error of simplified FPM is less than a-correction method, if the correction of zero approximation for simplified FPM is used by preliminary established correlation between theoretical and experimental set data. 

The analysis was performed by SRXRF at the XRF beam-line of VEPP-3, Institute of Nuclear Physics, Novosibirsk, Russia. For the accuracy control the different types of the International Certified Reference Materials were used. There were obtained all metrological characteristics, namely precision, accuracy and lower limits of detections. This is the SRXRF method, that allow to analyze the sample mass of 0.5 mg directly without the destruction. The puncture from patient may be picked out more than once. 

The simplest calibration procedure for a gas flow-measuring device is to connect it in series with a reference meter and allow the same flow to pass th tough both instruments. This requires a reference instrument of better metrological quality than the calibrated instrument. One fact to consider when applying this method is that the mass flow rate in the system containing both instruments is constant (assuming no leakage), but the volume flow rate is not. The volume flow rate depends on the fluid density and the density depends on the pressure and the temperature. The correct way to calibrate is to compare either the measured mass... 

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... 

There are some existing texts that cover the material in this book, but I have tried to take a holistic view of quality assurance at a level that interested and competent laboratory scientists might learn from. I am continually surprised that methods to achieve quality, whether they consist of calculating a measurement uncertainty, documenting metrological traceability, or the proper use of a certified reference material, are still the subject of intense academic debate. As such, this book runs the risk of being quickly out of date. To avoid this, I have flagged areas that are in a state of flux, and I believe the principles behind the material presented in this book will stand the test of time. 

RMs Considering the limitations of available primary methods, emphasis is placed by the CCQM on the elaboration of synthetic RMs derived from pure materials. These would then be used for calibration of instruments and hence, could help in the metrological step of analytical procedures. As appears from the cited examples, such RMs are hardly suitable as reference samples in many applications of analytical chemistry. 

A primary method [2] is one that is capable of operation at the highest metrological level, which can be completely described and for which a complete uncertainty statement can be produced in SI units. The amount of substance can be measured either directly, without reference to any other chemical standard, or indirectly, by use of a ratio method which relates the amount of unknown entity X to a chemical standard. Primary direct methods, such as gravimetry and certain electrochemical and thermal methods are the exceptions in chemistry, as the majority of measurements are made indirectly by comparison with other pure substance RMs as discussed above and below. These ratio methods include isotope dilution mass spectrometry and chromatographic and classical methods. Hence the importance of pure substance RMs. 

The Consultative Committee for Amount of Substance (CCQM) has set up a definition of primary methods [1, 2] and has selected some methods with the potential of being primary , from the viewpoint of the end user. From the point of view of metrology, methods used for linking the chemical measurements with the SI system at the highest level should not refer to other amount of substance standards. This requirement excludes methods which are relative in their principle. Some other methods identified as having the potential to be primary yield information expressed as amount fraction. This is essential for evaluation of purity, but in order to convert it to a value useful for transfer of the unit, additional information on the identity (molar mass) and content of the impurities is required. This additional information is needed to convert the result into amount content or similar quantities. 

If the traceability statement for the measurement result refers to a CRM or RM, the certificate of the material is needed to provide information on the method of measurement of the analyte(s), the uncertainty assigned and confidence level. In this respect national legal norms on absorption photometers for medical use require only the use of CRMs in the specific metrological activities. 

Abstract The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to... 

The hierarchical approach to the traceability of pH measurements and pH reference materials is consistent with the agreed approach to traceability for metrology in chemistry [23]. Uncertainties stated for the primary method and for all subsequent measurements permit the uncertainties for all steps to be linked to the primary reference material. 

Non-dispersive infrared analyzers are usually employed to determine carbon dioxide concentration at atmospheric levels, as they are stable, user friendly, and suited to continuous monitoring. At the Istituto di Metrologia G. Colonnetti (IMGC), as in other metrology laboratories, the determination of the C02 concentration in air is carried out for different purposes in mass, length, and environmental measurements. As NDIR spectroscopy is not a primary method of analytical measurement it does not provide direct traceability to the SI it is hence necessary to refer the obtained results to traceable reference materials, namely PRMs of C02 in N2 at appropriate concentrations. 

Most of the primary measurement standards are the realization of the SI units, and are under custody of each country s National Metrology Institute (NMI). The link between the realization of the SI units and primary standards is established through primary methods of measurement. These are methods which do not require any reference of the same quantity. Additionally, through a series of comparisons between NMIs, comparability of measurements among traceable measurement systems at international level are recognized by each country. 

In Mexico this task was initiated by CENAM in 1992. Particularly in the field of metrology in chemistry, the strategy has been developed in a parallel way to the work of the Consultative Committee for the Quantity of Matter (CCQM), by adopting the definitions of primary method of measurement and primary reference material given in the first CCQM meeting in 1995 [1], These definitions... 

In the present paper we would like to discuss primarily the situation when a systematic error in measurement (analytical) results, specific to a measurement method or to a laboratory developing an IHRM, can be overcome. This is possible when a certification of an IHRM is based on a comparative approach providing a characterization of the value carried by the IHRM in comparison to the value carried by the reference material with higher metrological status and sufficiently similar matrix (a CRM). 

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