Accuracy certified reference

The analysis was performed by SRXRF at the XRF beam-line of VEPP-3, Institute of Nuclear Physics, Novosibirsk, Russia. For accuracy control the International Certified Reference Materials were used. There were obtained all metrological characteristics, namely precision, accuracy and lower limits of detections. 

In geochemistry, the introduction of RMs did not take place until 1951 but, once RM usage became a regular part of geochemical analysis, the consequences were not far short of amazing. For many years geochemical analysts had been concerned about the accuracy of their determinations of major elements in rocks, but it was the potential of emission spectrometry for the determination of trace elements which set off the production of the first rock Certified Reference Materials (CRMs),... 

For the high accuracy needed in the quantitative measmement of the species, quality assurance of the analytical procedures is of prime importance. This can only be achieved by using representative RMs, certified for the relevant species. Up to now the number of existing certified reference materials is very limited. This section will give a survey of the main species that are presently determined routinely or for research purposes. 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure-... 

There is an abundance of references defining and describing the role played by QA, Quality Control (QC) and Total Quality Management (TQM) in a modem commercial analytical laboratory. The role played by reference materials (RMs) and certified reference materials (CRMs) in the pursuit of analytical measurement accuracy is also well documented. 

More attention should be devoted to the quantitative determination of analytes in real-life samples. The accuracy of the determinations and the traceability of the overall analytical process are insufficiently ensured [120,121], As no primary methods are available for the purpose, this necessarily implies the use of certified reference materials. 

Quality assurance considerations lead to the need for appropriate reference materials, and their consistent and effective use to monitor the precision and accuracy of laboratory analyses. In this context, certified reference materials (CRMs), now still largely lacking in the polymer/additive area, play an important role. In previous years, some attempts have been undertaken to prepare some inorganic CRMs (VDA and PERM projects), but this is highly insufficient when we consider that some 60 elements are used in polymer/additive formulations. The lack of CRMs for organic compounds in polymeric matrices is an even more serious handicap. Nagoumey and Madan [122] have demonstrated that intermediate or finished in-house materials can be utilised successfully as QA reference materials. Good QC of polymer/additive formulations as yet has not been achieved. 

It is generally difficult to identify developments with high potential where interferences do not preclude general application. To ensure the relevance of a method, its application to real sample analysis must be demonstrated. The accuracy of an analytical method should be confirmed by an independent method, or by the analysis of certified reference materials. Detailed comparative studies of the method developed with other well-established methods for polymer/additive analysis are not frequent in the analytical literature. Nevertheless, some examples may be found in Section 3.6. Improvements in analytical techniques are reasonably sought in sample preparation and in hyphenated chromatographic techniques. However, greatest efficiency is often gained from the use of databases rather than accelerated extraction or hyphenation. 

Determinations of the so-called micronutrients , (e.g., nitrate, phosphate, and dissolved silica), in seawater are among the most commonly performed analyses in oceanographic research and survey work. Surprisingly, there exists no certified reference material that can be used to check the accuracy of such analyses, although intercomparison exercises have been conducted on a regular basis [243 ]. The lack of seawater certified reference material for micronutrients can be attributed both to the difficulty of preparing a suitable material with an adequate shelf life [244,245] and to the dearth of independent methods available for the determinations. 

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. 

A Thermo Finnigan Element 2 Inductively Coupled Plasma Sector-Field Mass Spectrometer (ICP-SF-MS) with guard electrode was employed for trace element analyses. RSD values derived from internal check standard never exceeded 10%. Accuracy was better than 15% for all elements as determined by analyzing the certified reference standard NWRI TM-RAIN 95 trace metal fortified rainwater, every 5 to 8 samples. 

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. 

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. 

Accuracy, uncertainty, and traceability. A certified value is the best approximation of the true concentration of the analyte. During the certification process, a variety of analytical methods may be used to determine this true value. Uncertainty estimates ultimately based on this process, together with information about the material s homogeneity can give a certified reference material traceability, needed for true international comparability. 

Certified Reference Materials. Certified Reference Materials are materials whose properties have been guaranteed or certified by recognized bodies. The certified analyses of these materials can be used as an estimate of the "true" value for assessment of accuracy. The United States National Bureau of Standards (NBS) provides an inventory of various materials whose compositions (and properties) have been measured using definitive and reference methods. These materials, Standard Reference Materials (SRM s), when used in conjunction with reference methods, i.e., one of demonstrated accuracy, make it possible to transfer accuracy between measurement protocols. 

Independent Methods. In the absence of appropriate certified reference materials one may have to rely upon in-house materials that can be analyzed by independent methods (other than the candidate method). These independent methods should include a reference method and other methods that utilize different physical/chemical principles for analyte quantification. Reference methods are generally arrived at by concensus following extensive accuracy testing by a large number of laboratories. The American Society of Testing Materials (ASTM) is one of the largest compilers of reference methods. Additional information on the use of reference methods may be found in a paper by Cali and Reed (.2),... 

Collaborative Testing. A second approach to assessing accuracy, when no certified reference material is available, may be used in conjunction with analysis by independent methods and in-house materials. Sample exhanges with other laboratories can help establish the existence or absence of systematic errors in a method. Collaborative tests are most useful in this regard when some of the participating laboratories use different sample preparation and quantification. The utility of independent analysis methods and comparisons between destructive and non-destructive analysis is again emphasized here. 

In the case of validation, a certified reference material can be used to verify the performance or accuracy of a given measurement method. Questions such as is there any method specific bias or is there any systematic error can easily be answered in this way. Thus this is another useful way of using a certified reference material. In the case shown here certainly there is a significant bias. 

In a farsighted move in 1989, the European Union laboratory IRMM started a series of interlaboratory comparisons to provide objective evidence for the degree of equivalence and the quality of chemical measurements by comparing a participant s measurement results with external certified reference values (IRMM 2006). At the time most proficiency testing schemes used consensus results for the mean and standard deviation to derive z scores. With the IMEP-1 analysis of lithium in serum, the world was alerted to the problem of lack of accuracy in analytical measurements. The data of the first IMEP-1 trial are replotted in figure 5.6 notice that the apparent outlier was the only laboratory to come close to the assigned value. 

Some examples include evaluation of uncertainty components associated with published values (i.e., the analyst did not measure them), uncertainties in a certificate of a certified reference material, manufacturer s statements about the accuracy of an instrument, or perhaps even personal experience. The latter could be viewed as an opportunity for anyone to just make up an uncertainty, but experience does count for something, and it is indeed usually better than nothing. Leaving out a component because of lack of exact knowledge immediately underestimates the uncertainty. 

Standard reference material (SRM) for wavelength accuracy, stray light, resolution check, and photometric accuracy can be purchased from NIST. Certified reference materials (CRMs) which are traceable to NIST and recertification services can be purchased from instrument manufacturers and commercial vendors [12]. The cost of neutral-density filters and prefabricated standard solutions in sealed cuvettes can be substantial. When purchasing performance verification standards from a secondary supplier other than a national standard organizations such as NIST in the United States and National Physical Laboratory (NPL) in the United Kingdom, make sure that the traceability of the standards are available in the certificates. The traceability establishes the relationship of individual results to the national standard through an unbroken chain of comparisons. 

In case there is a need to perform wavelength accuracy and photometric accuracy measurements for the far-UV region below 240 nm, there are new certified reference standards available from Stama Cell [18]. The wavelength standard is a solution of rare earth oxides solvated in dilute sulfuric acid. The standard exhibits well-characterized absorption bands at 210, 211, 222, 240, and 253 nm (Figure 10.13). The photometric accuracy standard consists of a series... 

Certified reference materials (CRMs) are mainly applied to validate the analytical procedure developed for routine analysis in order to determine the accuracy of analytical data, the recovery for selected elements, the uncertainty of trace element determination and the detection limits. Otherwise, in solid-state mass spectrometric techniques, such as SSMS, LA-ICP-MS, GDMS, SNMS or SIMS, one point calibration using CRMs has been established as an important calibration strategy to obtain reliable analytical data. The one point calibration is performed using the experimentally determined relative sensitivity coefficients (RSCs) on a suitable CRM with a similar trace/matrix composition. An RSC of a chemical element is defined as the ratio of the measured element concentration (experimentally determined) divided by the certified element concentration (accepted or recommended value of element concentration) in a given matrix. 

In addition, the residue of the digestion should be quantitatively dissolved in a small volume of high purity acid. The decomposition and accuracy of analytical data should be checked with the aid of certified reference materials. Possible contamination and losses of trace elements by absorption or volatilization should be avoided. 

One alternative is to compare the results of the method with results from an established reference method. This approach assumes that the uncertainty of the reference method is known. Second, accuracy can be assessed by analyzing a sample with known concentrations (e.g., a certified reference material) and comparing the measured value with the true value as supplied with the material. If such certified reference material is not available, a blank sample matrix of interest can be spiked with a known concentration by weight or volume. After extraction of the analyte from the matrix and injection into the analytical instrument, its recovery can be determined by comparing the response of the extract with the response of the reference material dissolved in a pure solvent. Because this accuracy assessment measures the effectiveness of sample preparation, care should be taken to mimic the actual sample preparation as closely as possible. 

Analysis of unknown samples. This step involves the analysis of samples whose concentrations are unknown. Both qualitative and quantitative measurements should be performed. Reliable unknown samples are obtained from commercial sources or governmental agencies as certified reference materials. The accuracy and precision are determined. 

Definitions 2 and 3 allow an evolution in the different techniques and methods as definitive methods for the same analyte (Leijnse, 1982). Indeed, even though systematic errors were investigated during the initial research work, later technical advances may uncover errors that were undetected during the original measurements. The end use and end purposes of the definitive method include the evaluation of the accuracy of reference methods and its application to the quantitation of analytes in certified reference materials present in a biological matrix. 

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