Preparation of the reference material

Commercial sterile evaporated milk was used for the preparation of the reference material, with a dry mass concentration of 200-300 g L and a fat mass concentration of 40 g L . Pink gelatin capsules were used, gamma irradiated with a dose of 10 kGy prior to use. A test strain Enterococcus faecium was cultured in 100 mL THE at 37 C for 48 h, while shaking at 100 rpm. Next the culture was centrifuged for 20 min at 2000... 

The drain tap of each tank was also cleaned with hydrogen peroxide (30 g kg" ), leaving the tap filled for at least 2 h. The PTFE tip which was mounted to the drain tap for filling the ampoules was cleaned in the same way. Just before filling the PTFE tip was heat-sterilised. A test was carried out to estimate the losses by evaporation during the preparation of the reference material. The test consisted of filling three ampoules with ultra pure water. After weighing, the ampoules were left at room temperature for a period of 7 days. The relative loss by evaporation was less than 0.02% /day for both types of ampoules (diameter of 40 and 30 mm for respectively CRMs 408 and 409). 

In those cases where there are any doubts about the feasibility of producing a sufficiently homogeneous and stable reference material, a feasibility study might be needed. For this study, an extra amount of material is needed. Questions regarding the best way of preparing the sample, the stability of the material, or the fitness for purpose might justify the inclusion of a feasibility study in the project. In the BCR projects, it is common practice to have a feasibility study, which usually has as the sole purpose of assessing the performance of the laboratories in the collaborative study in relation to the certification of the reference material. The feasibility study allows the participants to fine-tune their equipment, their methods, and their procedures in view of the characterization measurements. In each of these cases, a considerable extra number of samples is needed. 

In the preparation of many solid state reference materials, reduction of the grain size plays an important role. Usually this reduction is required because of the measurement methods to be used both in the projects and later by the users of the reference material, as well as to come to an acceptable minimum sample intake. The minimum sample intake can be defined as the minimum amount of material needed, so that the heterogeneity of the material does not affect the repeatability of the measurement method. The reduction of the grain size is usually implemented by crushing and/or grinding techniques. The techniques employed and the equipment used must be suitable for the purpose of processing the material. Potential problems of contamination, loss of volatile components, and/or other physical and... 

In cases where test pieces (or items) are prepared, the issue of obtaining a homogeneous batch of items is even more complex. Here the preparation procedure sets limits, in combination with the properties to be certified. The uncertainty of the property values should appreciate this fact, as otherwise the uncertainty of the reference material is only valid for the batch, not for a single item from the batch. This... 

Definition of the reference material, i.e. the matrix, the properties to be certified, and their desired levels Design of a sampling procedure Design of a sample preparation procedure... 

Berman SS, Sturgeon RE (1988) A new approach to the preparation of biological reference materials for trace metals. Fresenius Z Anal Chem 332 546-548. 

DE Boer J (1997) The preparation of biological reference materials for use in interlaboratory studies on the analysis of chlorobiphenyls, organochlorine pesticides and trace metals. Mar Poll Bull 35 84-92. 

European Commission (1993) Guidelines for the production and preparation of BCR reference materials. Document BCR/48/93. DG XII, Brussels, Belgium. 

The preparation of a reference material requires substantial planning prior to undertaking a specific project (see Box 5.1). The process begins with the definition of the material to be produced, for example, preparation of a seawater-based reference material containing the nutrient elements N03, P04, and Si(OH)4 at concentration levels appropriate to oceanic samples and certified for these constituents." Such definitions arise either from internal decisions by reference material producers (such as NIST or NRC-Canada) typically in response to perceived needs, or through external pressure on these producers from potential users. (This report, for example, explicitly identifies a number of pressing needs for reference materials for the ocean sciences.)... 

Approaches to the Preparation of Radionuclide Reference Material Solutions... 

PM Finglas, H van den Berg, I de Froidmont-Gortz. Improvements in the determination of vitamins in foods method intercomparison studies and preparation of certified reference materials (CRMs). Food Chem 57 91-94, 1996. 

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. 

An in vitro bioassay can be designed in several ways, but requires statistical validity. A one point assay is not valid. The bioassay should be designed to consider factors that introduce variability, and the analysis should test such variability. A measurement series of a test sample should be compared to an equivalent series of the reference material, carefully considering the comparisons between the linear portions of the dose-response curves (Mire-Sluis et al., 1996). To test validity of a bioassay inter- and intra-assay variability should be considered in both preparation, and in the case of multiwell plates, the variability between each plate. To reduce the positional effect in plate tests, it is advisable to distribute the points on the curves randomly and also to include a reference standard in each plate (Gaines-Das and Meager, 1995). One of the most widely used techniques to validate a bioassay s performance is to include internal duplicates. The data arising from the comparison can be important in assessing the test s variability. 

The hierarchy established among the different methods and materials suggests an easy organization (Tietz, 1979). However, serious practical problems have hampered fast progress. In the United States and in Europe, the National Committee for Clinical Laboratory Standards (NCCLS) and the European Committee for Clinical Laboratory Standards (ECCLS) have been founded. These standardization organizations promote the development and the ultimate use of reference materials and methods. The National Bureau of Standards (NBS) and the Centers for Disease Control (CDC) in the United States (Bayse, 1982), and the European Community Bureau of Beference (BCR) in Europe, undertake projects oriented more toward the preparation of certified reference materials (BCR, 1982). 

After a thin gas-permeable silica fiber sheet has been placed on the bottom of a draft cell, i.e., the cell with which the sample cell is prepared, about 2 cm of some gas-permeable reference material, such as silica of relatively coarse particles or glass wool, is charged in the cell. The glass jig mentioned in a footnote in Section 7.3 is used at this time to provide a thin vertical hole at the center of the reference material charged in the cell in order to facilitate the insertion of the thermocouple into the material. 

The results obtained within the SWIFT-WFD project, dealing with the preparation of water reference materials, will contribute to the future developments of such materials, especially for the analysis of pesticides and PAHs at low concentration levels. The two different strategies employed, direct fortification in the laboratory by using spiking solutions following detailed protocols, and fortification of the material before dispatch, led to homogeneous and satisfactory stable reference materials for quality control purposes. 

Fig. 4.7. Classical scheme for the preparation of matrix reference materials.

Two 150 L PVC containers (one for each solution) to be used for the preparation and homogenisation of the candidate reference materials were cleaned with a detergent, rinsed with distilled water and further rinsed with ultrapure water. For each of the reference materials, the ampoules were cleaned in a similar way. The ampoules were (air) dried for two days and conditioned for at least 24 h with the solution which they would contain. The preliminary investigations had shown that this procedure was adequate to bring the walls in adsorption equilibrium with the solution. 

When the matrix has been chosen with regards to the feasibility as discussed above and when the parameters to be measured are clearly defined, the collection and the preparation of the test material may start. All requirements of quality, except the certification of the content of certain substances, are valid for all type of reference materials used in interlaboratory studies. These detailed requirements, the preparation and control procedures should be available from the organiser in a production report. They are discussed in more detail in the chapter 4 dealing with (C)RM. 

They are both types of systematic error. Bias usually refers to systematic error in an instrument and is an absolute difference. Recovery is the fraction of an analyte that is presented to the measuring instrument. It is often less than 100% because of losses during preparation of the test material before measurement. Both may be estimated and corrected for. (Sections 1.7, 3.6)... 

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