CRMs as calibrants

An important prerequisite for the use of CRMs as calibrants, at least for optical methods and particularly all AAS modes, is that they should match the matrix and level of analyte contents of the materials to be analyzed as closely as possible, so that potential matrix effects will be compensated if calibrant and sample material are affected by the applied method, e.g. the temperature program for furnace techniques, in the same way. Further it is very important for all methods that the CRMs used should not show a nugget effecf, i.e. particles with extremely high analyte content that can lead to a high analyte heterogeneity (Kurfiirst 1991 Kurfiirst et al. 

Currently available CRMs A number of organolead (tetra- and triaUcyllead) compounds are commercially available to be used as calibrants. There are also a number of interesting CRMs an urban dust certified for trimethyUead and more appear to be in development The interest in organolead compotmds is, however, on the decline due to the gradual disappearance of the tetraethyl anti-knock agent from petrol. 

With solid sampling-electrothermal vaporization-inductively coupled atomic emission spectrometry (SS-ETV-ICP-AES), Cu in two environmental CRMs was determined using a third CRM with similar matrix as calibrant. Comparison with a reference solution showed good agreement (Verrept et al. 1993). 

Rossbach M, Ostapczuk P, Emons H (1998) Microhomogeneity of candidate reference materials Comparison of solid sampling Zeeman-AAS with INAA. Fresenius J Anal Chem 360 380-383. Rossbach M, Stoeppler M (1987) Use of CRMs as mutual calibration materials and control of synthetic multielement standards as used in INAA. J Radioanal Nud Chem Artides 113 217-223. Sargent M (1995) Development and application of a protocol for quality assurance of trace analysis. Anal Proc 32 71-76. 

On most occasions CRMs are used as Quality Control materials, rather than as calibrations . As outlined above, this common application adds significantly to the user s uncertainty budget, since at a minimum it is necessary to consider at least two independent measurement events (Um). so increasing the combined uncertainty of the results. Again this process rapidly increases the combined uncertainty with increasing complexity of the analytical system and so the usefulness of a control analysis may be downgraded when a correct uncertainty budget is formulated. 

Systematic effects are estimated by repeated measurements of a CRM, suitably matrix matched. Any difference between the CRM and a routine sample for which the measurement uncertainty is being estimated should be considered and an appropriate uncertainty component added. Suppose a concentration measurement is routinely made in a laboratory that includes measurement of a CRM in the same run as calibration standards and unknowns. The bias (6) is given by... 

Document metrological traceability. This requires identification of all CRMs used as calibrators, calibration certificates for equipment, and a statement of the measurement uncertainty of the measurement result. The metrological traceability chain is thus established. 

According to the definition [1] the traceability chain is the unbroken chain of comparisons or calibrations from the result of a measurement or the value of a measurement standard to the national or international standards, all having stated uncertainties. The uncertainty of each link in this chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time. Therefore, the calibration intervals of measuring equipment used in testing (analytical) laboratories [2, 3] and of measurement standards used for their calibration are very important. The same applies to the shelf-life of a certified reference material (CRM) as a measurement standard. So, taking into account these... 

In the case of chemical measurements, CRMs, as standards of chemical composition, can be effectively introduced into the calibration process. The traceability of the certified value of RMs, as an essential part of the certification process, is as important as that of chemical measurements. In this respect, the metrological approach of titrimetry used for characterization of spec-trometric RMs introduced into the calibration process of spectro(photo)meters is presented here. 

It is important to be clear on this point these positions may appear inconsistent, but in practice generate exactly the same expectations of laboratories. Whether or not we speak of traceability of the result to the value associated with a matrix CRM used in validation studies, we regard matrix CRMs as the most appropriate test of reliability available, and wholeheartedly recommend their use wherever practicable. Since validation is seen as essential in the context of the present guidance, matrix CRM use is as important as ever in this paradigm. It is simply seen as important to validation, rather than important to the calibration chain. 

TMAH was used to solubilize the DORM-1 dogf>sh muscle CRM as a model substance for the determination of As, Cd, Pb, and Se by ET-AAS [8]. The sample was mixed with a small amount of TMAH and heated to 60°C for 10 min in a water bath. The calibration was performed with aqueous solutions in 0.2 percent v/v HN03. Results from the determination of these elements in the DORM-1 CRM were within the 95 percent conbdence interval of the certified values. 

Traceability is one major factor that can be achieved via CRMs as main means in the held of chemical metrology. In general, CRMs are applied for the validation of analytical methods. Standard solutions are then used for instrument calibration. Nonetheless, CRMs should not be understood as the solution for all problems in chemical measurement. It goes without saying that the matrix of a CRM should match the analytical problem as exactly as possible. It is clear that there are not CRMs available for all matrices and analytes. Thus, it is important to have the best matrix match. 

The role and use of reference materials are in principle well known, in particular for Certified Reference Materials (CRMs) used as calibration materials or matrix materials representing - as far as possible - real matrices used for the verification of the measurement process, or (not certified) laboratory reference materials (LRMs also known as quality control (QC) materials) used, for example, in interlaboratory studies or in the maintenance of internal quality control (control charts). Examples of reference materials relevant to WFD monitoring (water, sediment and biota) are described in the literature (Quevauviller, 1994 Quevauviller and Maier, 1999). 

CRMs, at least those which can claim that they are traceable to a recognised reference or standard, e.g. the S.I. unit, can be used to link the measurement to this reference. Such CRMs can be compared to transfer standards, as they are known and used in physics, e.g. mass transfer standards. Unfortunately, except for pure primary substances, such materials hardly exist in chemical measurements for complex materials. Pure materials are often the only real link to the basic S.I. unit of amount of substance, the mole, for many measurement processes. They intervene in fact mainly in the calibration process (see Figure 2.17). Finally, some CRMs, e.g. those used to test material properties or activities (e.g. pH, conductivity, etc.) can be used to realise measurement scales for these properties. The last two structural roles in measurement sciences also represent the primary practical role of CRMs as they intervene directly in the measurement process. 

The essential aspects have been discussed in the introduction on the use of RMs and CRMs. It should be noted that inorganic CRMs, in particular pure metals, are available on the market from several reliable suppliers. They show usually purity values with associated uncertainties that are negligible compared to the uncertainty of the majority of spectrometric methods in which they serve as calibrants. It is usual to find materials of stated (not by definition certified) purity of 99.999% (five nines in analytical jargon) or better. This would mean that any impurity is below 0.001% as a mass fraction. No relative analytical method has precision performances that go down to such levels. Suppliers of ultra pure metals are numerous. NIST sells such metals as certified RMs (SRMs). The certification of the purity is discussed briefly in Chapter 5. It can be mentioned that the measurements are often based on absolute methods. The ultimate detection of impurities can be made with spark source MS. For pure metals the uncertainty linked to the calculated purity is small. Therefore, compared to the intended use and the uncertainty of classical methods applied by the analyst for the determination of elements, it is totally negligible. 

Recently, the synthesis of derivatized standards for ethylation and Grignard derivatization has been carried out at the Free University of Amsterdam " within the framework of an EC-funded certification project (CRM 477) allowing the establishment of optimization and validation studies. The purpose was to prepare highly purified butyltin and phenyltin compounds (in the form of salts) and their ethylated and pentylated derivatives for use as calibration and recovery tests. 

CRMs are products of very high added value. Their production and certification are very costly and, therefore, these materials should not be used for routine quality control or external quality assurance (interlaboratory studies). In order to fulfil the needs related to these uses, noncertified RMs may be prepared, linking them to one or several CRMs as a means of evaluating the accuracy of the values assigned to the so-called secondary materials. This approach is straightforward in the case of calibrants (pure compovmds or calibrating solutions), but is more difficult in the case of matrix CRMs owing to the likely matrix differences. 

Description Certified reference materials (CRMs) may be used as calibrators or measurement trueness control material. When used as calibrator, a CRM permits traceable and thus comparable measurement results. 

The most economic way of using CRMs for calibration purposes is to validate a procedure for routine analysis. The analytical procedure is carried out with the CRMs analysed as samples. "IMth the results achieved, all relevant analytical parameters can be determined, e.g. uncertainty, recovery, reproducibility, selectivity, linearity, etc. The procedure is then well known for the specific sample type and the specific analytes for which it is validated and can be applied routinely for this analytical problem, with a few regular reviews of the analytical performance. CRMs in this case are not used for calibration but rather for validation of the procedure and regular review of the method performance. 

ISO Guide 33 (1998) deals with other uses of RMs. It elaborates on various uses of RMs, excluding calibration, which is the subject of ISO Guide 32. In most cases, RMs are used as a quality control measure, i.e. to assess the performance of a measurement method. Most matrix RMs are produced with this purpose in mind. Other purposes of RMs are the maintenance of conventional scales, such as the octane number and the pH scale. ISO Guide 33 provides guidance on the proper use of RMs, and therefore it is together with ISO Guide 32 the most important document for users of CRMs. 

Currently available CRMs Calibrants are available for TBT (very evident, as TBT is an anthropogenic contaminant) and for a number of other organic tin compounds. There is also an interesting choice of environmental CRMs (sediment and mussel tissue) certified for TBT content. 

BCR Analytical Approach for the Certification of PAHs in Natural Matrix CRMs Prior to the certification analyses for the CRM, each participating laboratory has to prepare standard solutions of the analytes to be determined from certified reference compounds (purity >99.0 %) to calibrate their instruments for response and response linearity (multiple point calibration), detection limit, and reproducibility. In the case of PAH measurements, reference compounds of certified purity are used as internal standards, which are not present at a detectable concentration in the matrix to be analyzed (e.g. indeno[i,2,3-cd]fluoranthene (CRM 267), 5-methylchrysene (CRM 081R), benzo[f ]chry-sene (CRM 046), picene (CRM 168), and/or phenanthrene-dio). 

For relatively simple matrices, such as pure metallic CRMs synthetic reference materials for direct calibration were prepared and used, for example Bi, Cd, Hg, Pb and T1 in high purity gallium (HUtenkamp and Jackwerth 1988), Ag in copper (Pau-wels et al. 1990) and Au and Pd in silver (Hinds 1993). Direct calibration by solid biological materials with added analyte belongs also to these quite successfully applied techniques (Hofmarm et al. 1992). 

For the determination of As, Cd, Zn, Pb, Mn, Cu and Cr with solid sampling GF-AAS in a number of biological samples, calibration curves using one NIES CRM and three NIST SRMs were constructed and successfully used (Atsuya et al. 1987). 

The total content of As, Cd, Cr, Cu, Ni and Pb was determined in contaminated soils and sediments using the slurry technique and Zeeman GF-AAS, either by calibration with aqueous solutions of the analytes or slurries of some suitable CRMs. Except for Cr, where only the calibration with a solid CRM was successful, good agreement was found between both calibration approaches (Klemm and Baumbach 1995)-... 

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