Certified values

Molybdenum is classified among the elements for which the state of art of analysis is still unsatisfactory. The modest number of referenced materials available with certified value for Mo seems to confirm this. 

The X-ray determination of REE in geological samples is normally complicated by the relatively low concentrations of the REE, their complex X-ray spectra, the high concentration of matrix elements and the lack of reference standards with certified values for REE. A rapid and sensitive ion exchange and X-ray fluorescence procedure for the determination of trace quantities of rare earths is described. The REE in two U.S.G.S. standards, two inhouse synthetic mixtures and three new Japanese standards have been determined and corrections for inter-rare earth element interferences are made. 

Material Certified value, % weight Found value, % weight... 

Certified values based on tests of specific disc models in accordance with ASME Section VIII and PTC25. 

Eor NIST PEO standards the agreement in M, between the NIST-certified values and the values determined by SEC with the commercial PEO standards is better for the Shodex SB806M and 806MHQ columns than for the TSK GM-PW and PWxi columns, no matter which set of commercial PEO standards is used. The agreement for TSK, PL, and APSC standards is better than for PSS standards, no matter which column is used. The agreement is about the same in water or in water/methanol for all columns and PEO standards. 

Office of Standard Reference Materials, National Bureau of Standards, Gaithersburg, MD 20899. "Nitrated PAH s do not fluoresce but can be made to fluoresce by passing through a Zn or Pt column to reduce nitro groups to amino groups. Information values for other organics in certificate. Certified values for pesticides and PCB s. 

If the sample and standard have essentially the same matrices (e.g., air particulates or river sediments), one can go through the total measurement process with both the sample and the standard in order to (a) check the accuracy of the measurement process used (compare the concentration values obtained for the standard with the certified values) and (b) obtain some confidence about the accuracy of the concentration measurements on the unknown sample since both have gone through the same chemical measurement process (except sample collection). It is not recommended, however, that pure standards be used to standardize the total chemical measurement process for natural matrix type samples chemical concentrations in the natural matrices could be seriously misread, especially since the pure PAH probably would be totally extracted in a given solvent, whereas the PAH in the matrix material probably would not be. All the parameters and matrix effects. Including extraction efficiencies, are carefully checked in the certification process leading to SRM s. 

The National Institute of Standards and Technology (NIST) released certified standard materials of a baby food composite (SRM 2383) and an infant formula (SRM 1846) containing carotenoids however, the relative uncertainties of certified values are considerably high, ranging Irom 20% for P-carotene (cis + trans) to 28% for lutein (including esters) and to a 47% for free lutein reference value in SRM 2383. ... 

Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes its traceability to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence. 

The most important document, accompanying a CRM is its certificate. ISO Guide 31 (1981) provides guidance for the establishment of certificates, labeling of CRMs, and certification reports. The certificate contains among other information the certified values and their respective uncertainties. As important as this information is the traceability statement, which defines to what references the CRM is traceable. Ideally, a CRM is traceable to a suitable (combination) of SI units. This is not always possible, so other stated references may appear here. Especially when certifying matrix reference materials, making the measurements traceable to SI does not imply that the CRM is traceable to SI as well. The steps necessary to transform the sample into a state that can be measured may have a serious impact on the traceability of the values, and thus on the traceability statement. 

The assessment of the performance of a method is commonly checked by means of a (C)RM. In those cases where there is no RM available, considerable effort is requested from the laboratory to assess the performance of their own methods. The aspect of traceability of the certified value(s) is also of great importance whenever necessary, the laboratory will make modifications in its procedures if the result of a measurement using the RM appears to be imsatisfactory. If the traceabihty of the values to other references is not fully established, then this judgement may be clouded by doubts about the certified value (s). 

The result from a measurement on a RM is commonly a difference between the observed value and the certified value. This difference is called measurement bias, and can, appreciating both the uncertainty on the RM as well as the imcertainty added during the measurement, be tested for (statistical) significance. ISO Guide 33... 

Kucesa J, Sychra V and Koubek J (1998) A set of four soil reference materials with certified values of total element contents and their extractable fractions. Fresenius J Anal Chem 360 402-405. 

In this area the change of water content, Aw.c., as a function of the change of relative equilibrium humidity, Acp, as a function of water activity (a 100 = rp), Aw.c./Acp, is at a minimum. This also minimizes the potential error in a certified value by water taken up from the surrounding area. Based on these findings, it appears absolutely necessary that during the preparation of each material, water activity as well as water content must be determined and adjusted to achieve optimal stability and thus also a long shelf hfe of the final product. 

Independence of analysts and analyses in one organization is a fundamental question. It is important to have, even for the most reliable methods, more than one analyst/laboratory involved to avoid possible analyst/laboratory-specific biases. Certification by a single laboratory, without confirmation by another laboratory or method is risky. Measurement by a single definitive method is usually performed by two or more analysts working independently to minimize possible biases. Frequently, an accurately characterized back-up method is employed to corroborate the data. Some agencies feel that a certification campaign should not be based on a single measirrement procedure and therefore do not normally certify values on the basis of a definitive method applied in one laboratory. 

Evaluation of the results Evaluation of the results consists of (i) technical scrutiny of the consistency and of the quality of the data the acceptance, on technical (not statistical) grounds, of data to be used to calculate the certified value and its uncertainty, (2) the calculation (using the appropriate statistical techniques) of the certified value and its uncertainty. The approach indudes technical discussion of the results among all cooperators, rejection of outliers, statistical evaluation, and calculation of the certified value and uncertainties. 

Tab. 3.5 Certified values for 25 constituent elements in NIST SRM 2704, River sediment...

The sets of technically and statistically acceptable results are represented in the form of bar-charts of which an example is given for Cu in Figure 3.1. The length of a bar corresponds to the 95 % confidence interval of the mean. The certified values... 

Currently available CRMs Presently available are CRM matrices (fish) with certified values for arsenobetaine and DMA, and caUbrants for those compounds. The offer is, however, very limited. Future needs include CRMs certified for arsenosugars and calibrants for those compounds. 

No. Title Certified Values Reference (Noncertified) Values ... 

Certified value A value reported on an SRM Certificate of Analysis for which NIST has the highest confidence in its accuracy in that all known or suspected sources of bias have been fully investigated or accormted for by NIST. 

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-... 

NIST has also used results obtained from inter-laboratory studies as an additional set of results in the two or more methods approach (mode 2 in Table 3.13). For example for the recent value assignment for PCBs and pesticides in SRM 1944, the mean of results from 19 laboratories participating in an inter-laboratory comparison exercise was used as an additional set of data in the determination of the certified values. Similar inter-laboratory study results were also included in the value assignment of PAHs, PCBs, and pesticides for two recently issued mussel tissue materials, SRM 2977 and SRM 2978. 

The analytical procedure is checked by analyses of method blanlcs to assure that secondary contamination by the analytes to be determined is avoided or minimized. Because the water content of the CRM matrix to be analyzed may vary from one laboratory to another (dependent on the local humidity and temperature), the water content has to be determined. Accordingly, at least three independent samples are kept at I05°C for 2 h, then allowed to cool to ambient temperature in a desiccator and the water loss is determined. The certified values are generally reported on a dry mass basis. 

To obtain consensus values for specimens which, when certified values are not known, may be used as surrogate CRMs. 

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