Matrix reference materials

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. 

This is not a problem, as long as this common reference already mentioned is available. For the characterization of the reference material, especially in the case of matrix reference materials, it is often desirable to use multiple methods, and often also multiple laboratories. Under these conditions, it is easier to arrive at an uncertainty that represents the state-of-the-arF of the laboratories. 

Fajgelj a and Parkany M, eds.(i999) The Use of Matrix Reference Materials in Environmental Analytical Processes. The Royal Society of Chemistry. 

The use of matrix reference materials in environmental analytical processes, pp 188-195. The Royal Society of Chemistry, Cambridge. 

Okamoto K and Yoshinaga J (1999) Proper use of reference materials for elemental speciation studies. In Fajgelt A and Parkany M, eds. The use of matrix reference materials in environmental analytical processes, pp 46-56. Royal Sodety of Chemistry, Cambridge. 

ScHANTZ MM, Nichols JJ, and Wise SA (1997b) Evaluation of pressurized fluid extraction for the extraction of environmental matrix reference materials. Anal Chem 69 4210-4219. 

Some of the intended categories of use of radioisotopic reference material have been reviewed recently by Fajgelj et al. (1999). They include assignment of property values, establishing the traceability of a measurement result, determining the uncertainty of a measurement result, calibration of an apparatus, assessment of a measurement method, use for recovery studies and use for quality control purposes. It should be noted however that, in general, natural matrix reference materials are not recommended for calibration purposes. This should preferably be done with pure chemical forms of the element labelled with the isotope of interest. Calibrated isotopic sources of this kind are available from a number of commercial suppliers and are not the subject of this review. 

The Use of Matrix Reference Materials in Environmental Analytical Processes, pp 65-80. 

Groning M, Frolich K, De Regge PP, Danesi PR (1999) Intended use of the IAEA reference materials. Part II Examples on reference materials for stable isotope composition. In Fajgelj A, Parkany M, eds. The Use of Matrix Reference Materials in Environmental Analytical Processes, pp 81-92. Royal Society of Chemistry, Cambridge. 

Even though PAHs are commonly determined by GC or HPLC, there are no matrix reference materials for PAHs in urine or serum. A number of reference materials certified for PAHs in animal tissue are available, but they are intended for environmental applications, see Section 3.4. 

Most matrix reference materials are regarded stable for their application within a certain time frame see also Section 2.2. They are usually produced in large batches designed to ensure that the same material is available for a number of years as well as to spread the high cost of production over as many units as is possible. The stability is closely monitored from initial production by the producer lot numbers or even individual unit numbers are allocated and the producers closely monitor, by regular analysis, the condition and quality of their reference materials over time. Because of such careful control, and to minimize waste, the tendency has been for producers to give a usable life from receipt to the customer, commonly 12-24 months. However, the producers can give this shelf life expectancy only for unopened units. 

For example trace elements in milk powder are not consumed as milk, and moisture in transformer oil is not used in transformers, yet matrix reference materials based on milk are imported as food and are subject to health certification requirements and sometimes import quotas. Likewise a matrix based on oil is identified as fuel or lubricating oil and is both classified as a hazardous material and subject to mineral oil tax. These problems arise because RMs are frequently incorrectly classified by specific title of their matrix (as Reference Material of Trace Elements in Rice is classified as rice) and not as reference material which is the intended use. 

For complex sample types, if there is any doubt concerning the ability of the method to unambiguously identify and measure the analyte of interest, check the method using a closely matched matrix reference material, or check the sample using an alternative validated method. 

The committee thus recommends that a limited number of carefully chosen matrix reference materials be prepared (based on seawater, sediments, and algal materials) to respond to a significant portion of the current pressing needs. Such materials, certified for a limited number of key... 

A wide range of high quality, non-certified carotenoid and chlorophyll chemical standards are commercially available (e.g., Sigma-Aldrich, DHI, and Roth). The availability of a mixed pigment reference standard and biological matrix reference materials would improve analytical performance in individual laboratories, facilitate method and laboratory in-... 

To check traeness using a reference material, a series of replicate tests is carried out and the mean value and standard deviation are compared with the characterized value for the reference material. Certified matrix reference materials may be purchased and used directly for validation. If this is not possible reference materials for validation may be prepared by spiking... 

Primary calibrators, indeed all calibrators, must be commutable that is, they must behave during measurement in an identical manner to the native analyte material being measured. Matrix reference materials made by mixing a pure reference material with the components of the matrix are unlikely to be entirely commutable, and for this reason some authorities (EURACHEM for one [EURACHEM and CITAC 2002]) advise against using matrix-matched CRMs for calibration, recommending instead their use to establish recoveries, after calibration by a pure reference standard. 

De Bievre P Are we clear on the function of Matrix Reference Materials on the Measurement Process An ISO/ REMCO - IUPAC - BAM - EURO-LAB-D Workshop on the Proper Use of Environmental Matrix Reference Materials, 23-24 April 1999, BERLIN. Proceedings to be edited by the Royal Society of Chemistry, UK... 

We will now look at the traceability chain of the value carried by a matrix reference material , in order to understand its function in the measurement process. That is needed to produce reference materials which are adequate for the intended use ( fit-for-purpose ). 

To this effect, in analytical chemistry it has been good practice that the analyst obtains a reference material with a matrix Z, similar - but of course not identical - to the one of the material X which needs to be measured. By performing the same operations on a sample of matrix reference material Z as performed on the sample with matrix X, an estimate can be obtained of the overall correction factor X(aE,Z). The value of the amount content 6(aE,Z)RM cert of the reference material as supplied by the reference material producer is known. The value b(aE,Z)sampie Qbs is observed by the analyst Hence, a correction factor can be calculated X(aE,Z) for losses during digestion and recovery etc. (the chemical operations in Fig. 2) as determined with the help of the reference material. It can be applied to the measurement on the unknown sample. In short, this process can be described by Eq 2 ... 

Fig. 2 A matrix reference material can be used to simulate chemical operations and estimate from that simulation a collection factor. The uncertainty of that collection factor must be evaluated and taken into account in the uncertainty ludget...

Practical reduction of uncertainty in the field using a matrix reference material... 

Thus the matrix reference material has fulfilled another function than the AS function for the analyst, a function which is not located in the traceability chain, but outside the chain it enables the analyst to make an independent assessment of the possible magnitude of the conversion factor K, thus assessing - possibly reducing - the uncertainty of the measurement by carrying out a correction . But even this correction carries an uncertainty which must be evaluated. The problem of this correction factor has been treated elsewhere in more technical detail under the name recovery factor ... 

For the value, carried by a reference material, and used to simulate the measurement of an unknown sample (most matrix reference materials) ... 

In Fig. 3a,b (right-hand chain), a full traceability chain of the value of a synthesized matrix reference material is shown. The left-hand chain is the traceability chain of the value resulting from the measurement of an unknown sample. The right-hand chain is the traceability chain of the value carried by the reference material and provided by the producer of the reference material. Both chains consist of successive comparisons from one value to another, all the way down to known values (of base units or of fundamental constants). The chain is constituted by values (in the rectangles) linked by operations called measurements (in the ovals), defined as above. Establishing the sequence value-meas-urement-value-measurement-value, etc, is establishing... 

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