Calibration Solution CRMs

If the CRM is used directly, then is all that is required. If, for example, the material is a pure reference material with certified purity that is to be dissolved in a buffer to provide a calibration solution, then the uncertainties of the dissolution step and the volume presented to the measuring instrument must also be included. 

To prepare a calibration solution of 1 pmol L 1 ofCd+2 from a CRM of pure Cd metal, a mass of Cd is weighed into a flask, dissolved, and made up to the mark. The atomic weight of cadmium is 112.411 g mol 1 with uc = 0.008 g mol 1. Thus, 1.12411 g of pure cadmium metal dissolved in hydrochloric acid and made up to 1.00 L will have an amount concentration of 1.00 x 10 2 mol L 1. An aliquot of 0.100 mL of this solution made up to 1.00 L should create the desired solution of amount concentration 1.00 pmol Lr1. Until a proper estimate of the measurement uncertainty is made the significant fig-... 

If the analyst in a field laboratory can identify a CRM for calibration of working calibration solutions, and its certificate gives sufficient evidence that the CRM embodies a metrologically traceable quantity value, then his or her work is just about done (figure 7.10). The intricacies of what national measurement institutes and calibration laboratories did to ensure that the CRM has metrologically traceable quantity values are all paid for in the certificate. 

Laboratory personnel commonly give one of three common responses to someone who points out the traceability requirements of ISO/fEC 17025 Its ok we have our balances calibrated every year, and when it is further pointed out that calibration extends to the chemical aspects of the measurement, I am sure it is ok to use this analytical-reagent-grade material to make a calibration solution, or we make measurements in such complex matrices, a suitable CRM just does not exist. The first comment stems from the rather physical or engineering flavor of the section in ISO/IEC 17025 (section 5.6) (ISO/IEC 2005), although traceability of calibration materials is mentioned. 

Lagarde, F., Asfari, Z., Leroy, M.J.F., Demesmay, C., Olle, M., Lamotte, A., Leperchec, R and Maier, E.A. (1999b) Preparation of pure calibrants (arsenobetaine and arseno-choline) for arsenic speciation studies and certification of an arsenobetaine solution (CRM 626). Fresenius f. Anal. Chem., 363, 12. 

Forty six field laboratories (mainly from France) have participated in the inter-laboratory comparison. They have analysed the water sample using their regular calibration solution and then have repeated this analysis using the certified standards. They also have analysed the matrix CRM. Laboratories have produced two results (duplicate) per sample. Techniques used were mainly atomic absorption spectroscopy with furnace but also ICP-OES and ICP-MS. 

The groundwater matrix CRM has been analysed in duplicate by laboratories, using certified calibration solutions. Results are presented in Table 5. 

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 main goal of every procedure for analytical quality control is to allow data within assigned values of accuracy and precision to be obtained. Analytical quality control is primarily achieved by the use of Certified Reference Materials (CRMs) and participation in intercomparison exercises, though calibration solutions and spiked samples can also be used (43, 63, 64). 

The selection of reference materials is therefore critical in validating the performance of an analytical method (see Chapter 1). CRMs should be used at least in the initial evaluation studies and in establishing the acceptability of calibrators used in routine service. The specific characteristics of calibrators should be documented, along with the number of different concentrations of calibrating solutions and the frequency of their use. These latter choices depend on the characteristics of tlie analytical method, particularly the stability, reproducibility, and linearity. 

The validation has the objective to identify, during the method development process, all sources of error and eliminate them or to quantify their contribution to the total uncertainty of the determination. For trace organic determinations particular attention must be given to the quantitative extraction and clean-up of all PCBs. Several types of adapted materials must be prepared to test all steps of the process (from simple calibrant solutions or mixtures, spiked extracts, to spiked soil material). CRMs should be used for validating trueness. Laboratory RMs must be prepared for the establishment of control charts when the method is under statistical control. 

All analytical work is to be done with own calibrant solutions prepared — according to the BCR regulations — from pure CRM or (if not possible) from the best verified... 

A set of calibrants containing the seven CB congeners was supplied to each participating laboratory in the form of pure, crystalline, certified materials from BCR (CRMs 291 -298). Each laboratory was requested to prepare separate calibration solutions of the appropriate concentration in iso-octane, to determine the linearity of the electron capture detector and to calibrate the gas chromatograph prior to the analysis of the fish oils. 

Each laboratory was given a free choice of extraction, clean-up and conditions for the final determination. The latter, gas chromatography with high-resolution mass spectrometry, included choice of injection, capillary columns, ion masses monitored etc. [34]. In addition to the CRM, the participants received a calibrant solution containing the PCDDs/PCDFs of interest for this project as a control for their calibration procedure. 

The final solution was injected (2 pL) onto a 50 m fused silica capillary GC column. The GC conditions are described in details elsewhere [21]. The calibrants (BCR CRM Nos 291, 293, 294, 295, 296, 297 and 298) and blanks were treated in exactly the same way as the samples. The recoveries of the CBs varied between 80% and 100%. The determination of each CB was verified to be completed within the linear range of the ECD detector. 

The final determination of the CBs was performed by capillary gas chromatography with electron capture detection mass spectrometry was used as complementary technique to confirm the identity of each of the CBs determined. Each participant had validated its method by performing experiments on recovery, extraction efficiency, procedure blanks and detector linearity. The seven individual CB calibrants were supplied to the participants as pure, crystalline CRMs from BCR (CRMs Nos. 291,293, 294,295, 296. 297 and 298). Each laboratory was requested to prepare separate calibration solutions of the appropriate concentration, in iso-octane, to calibrate the detector and lying within its linear range. The use of at least one internal standard was mandatory the participants, however, were left free to select the internal standard(s) best suited to their methods. They had to verify that the selected compounds did not occur in the candidate reference material or did not interfere with compounds present in the material. A series of pure dichlorobenzylalkyl ethers (DCBEs) was made available to the participants but other internal standards were also accepted of which the list is given in the certification report [21] along with additional details on calibration procedures. 

The solution was again concentrated to 1 mL in a clean, dry stream of air, diluted to 5 mL with iso-octane and concentrated to 2 mL. Approximately 1 mL was weighed out an after addition of the internal standards, dichlorobenzyl, hexyl and hexadecyl ethers (0.39 g of a 1 mg kg solution in iso-octane), transferred to a gas chromatograph (GC) autosample vial prior to the final CB determinations. This solution was injected (0.5 pL) onto a 50 m fused silica capillary GC column of which the conditions are described elsewhere [33]. The calibrants and blanks were treated in exactly the same way as the samples. The recoveries of the CBs varied between 80% and 100%. The linearity of the ECD detector was tested with three calibrant solutions of different concentrations. The CV values for the determination of the CBs taken from separate ampoules were less than 9% for CRM 420 and less than 5% for CRM 449. There was no significant difference in values of within-ampoule and between-ampoule variances. 

The certification of reference materials follows strict rules which are described in special ISO Guide 35 [73] the certified value should be an accurate estimate of the true value with a reliable estimate of the uncertainty compatible with the end use requirements . Depending on the type of property value to be certified and the type of CRM, there may be differences in the approach applied. The certification of primary calibrants such as pure compounds and calibration solutions (see Section 3.3) relies on the identification, purity and stoichiometry assessment and on gravimetric methods. Matrix CRMs cannot be certified on the basis of direct gravimetric methods since samples have to be analysed after a total transformation or removal of the matrix in this case, there are three possible approaches ... 

Numerous sources of okadaic acid exist and some companies offer OA methyl ester. DTX-1 is only available from Wako Chemicals (www.e-reagent.com). The only sonrce of certified reference material and certified calibration solutions is the National Research Conncil (Canada) Certified Reference Materials Programme (CRMP). More information is available online at http //www.imb. nrc.ca/crmp/natural/index e.php. Or email crm.imb nrc-cnrc.gc.ca. 

National Research Council of Canada, Certified Reference Materials Program. Certified Calibration Solution for Yessotoxin, NRC CRM-YTX, 2006. 

Pure substance CRMs provide the basis for many traceability chains in chemistry, and are often considered as primary standards for these measurement systems. The certification of substances for purity is an essential cornerstone of traceability in chemical measurements. These CRMs are used by laboratories to prepare calibration solutions or to certify or prepare other CRMs. The mass or amount fraction of the pure substance within the material is often determined by analyzing for possible impurities (often based on a consideration of the manufacturing process and degradation products) and the purity computed by difference. 

In most analytical procedures, calibration is carried out by means of a calibration curve using com-pound(s) prepared with chemicals of an appropriate purity and verified stoichiometry. Matrix effects must often be taken into account and, consequently, the calibration solutions should be matrix-matched. CRMs of pure compounds may be used for calibration. However, matrix CRMs should in principle not be used for the purpose of calibration unless no other suitable calibrants are available, with the exception of those methods (e.g., spark source mass spectrometry, wavelength-dispersive XRF, etc.) that require calibration with CRMs of a similar, fully characterized matrix (e.g., metal alloys, cements). For such methods, accuracy can only be achieved when certified RMs are used for the calibration. 

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. 

The material to be used for the preparation of RMs or CRMs should be collected in a quantity sufficient to ensure an adequate and sufficiently lasting stock. The amount to be prepared depends on the analytical sample size, stability, shelf-life, and frequency of use. It may vary from 5 to 20 kg of solid material (e.g., for soils, sediments, biological tissues, etc.) or from 5 to 201 (e.g., natural waters, calibrating solutions, etc.) for the preparation of RMs to be used in routine analysis, to up to 100 kg of solid material or several cubic meters of liquids when CRMs are to be prepared. In order to produce RMs and CRMs, laboratories need to be equipped to treat such large amounts of material without substantially changing the analytical methods. The treatment of 3-5 kg of raw material is already the limit for normal laboratory equipment and manual processing. For larger batches,... 

CRMs can be either analytical standards (Section 2.2.1), or calibration solutions that are solutions of an analytical standard at a certified concentration in a clean solvent, or natural material (matrix) CRMs that are prepared from material found in nature, i.e., are surrogates for the real samples to be analyzed. Matrix CRMs are invaluable as tools in quality assurance and quality control (QA/QC) in analytical laboratories, as discussed in Chapter 10. However, CRMs are generally too expensive to be used as everyday QCs and are generally used as method development tools or in occasional checks of the laboratory QCs (Section 2.5.3). 

In many instances, the analytical reference standard will be readily available from one or more commercial sources but this is not always the case. Certified reference materials (Section 2.2.2, not natural matrix CRMs but calibration solutions that are solutions of a reference standard at a certified concentration in a clean solvent) must be prepared and characterized in accordance with the strict guidelines in ISO Guide 34-2000 (ILAC 2000). The reference standard may also be supplied by the chent and this is often the case when working with proprietary compounds in development. If neither of these two options exist, the standard must be synthesized or in some instances extracted from a natural product or commercial formulation and then purified. The requirements for obtaining a certificate of analysis (Section 9.4.4c) or other documentation that establishes the identity and purity of the analyte are discussed in Section 2.2.2 and also below in 9.4.4c, but the amount of standard available is also an important factor in terms of formulating a strategy for the... 

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). 

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). 

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)-... 

Mercury in various cement products was determined with a special mercury oven for solid samples. Calibration was performed with four BCR CRMs and one NIST SRM with different Hg content as well as with a reference solution and excellent agreement found (Bachmann and Rechenberg 1991). 

This chart corresponds to the original Shewhart-chart. For tmeness control, standard solutions, synthetic samples or RM/CRM samples may be analysed. Calibration parameters (slope and intercept) can also be used in a X-chart to check the constancy of the calibration. 

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