Reference materials stability testing

Selection of method appropriate for homogeneity and stability testing Design of the characterization of the reference material Sampling... 

Frozen reference materials have been produced by NIST (Wise et al. 1993). These materials do not have the disadvantages of the oils or freeze-dried materials, but are more difficult to transport. Obviously they have to be kept deep-frozen during transport, which makes their use rather expensive. Since the early 1990 s a new approach in this field has been introduced. This concerned the use of wet, sterilized fish and shellfish samples. These samples, packed in glass jars or in tins, were firstly used in the QUASIMEME program as reference materials for inter-laboratory studies (de Boer 1997). Later, when it appeared that the stability was maintained for longer periods, tests for organic contaminants based on this principle were also prepared. 

Stability. Producers must state the length of the reference material s useable life, since they can be sensitive to light, humidity, microbial activity, temperature, time, etc. Long-term testing is required to validate the stability of a material under a variety of storage and transport conditions. 

Since one of the main goals of reference material production is to provide a stable reference material, tests for stability begin early in the production process. Ideally, these should be conducted over the expected lifetime of the reference material prior to its distribution however, these tests can be conducted concurrently if required. 

The useable lifetime of a reference material depends on both its inherent stability and also on its rate of use. It will thus be necessary to establish a timetable for the preparation of further batches of the material so as to ensure that supplies are always available. These subsequent batches (particularly if they are matrix-based) will need to be treated almost like a new material, i.e., they should be tested for stability, and a complete new certification will be needed for each replacement batch. This involves substantial work each time it is done, hence it is desirable to make as large a batch as practical, consistent with the probable shelf-life and expected usage. 

Direct participation of the oceanographic research community will enable collection of matrix-based materials such as natural seawater or sediments in an efficient manner (for example, material collection could be combined with other scheduled research activities). Furthermore, the extensive analytical skills of the oceanographic research community can be harnessed to conduct the necessary testing for homogeneity, stability, and ultimately characterization of the proposed reference materials. At the same time, complete success requires the participation of reference material producers, who bring to the table extensive knowledge and experience in the preparation and stabilization of reference materials, and on occasion access to necessary facilities. 

Because the material will be distributed as a reference material after the study, there must be enough material to satisfy the needs of the test and its future uses. Homogeneity and stability must be demonstrated, and after the first certification round, later rounds can be planned to establish shelf life of the material. Usually one laboratory has the responsibility for performing homogeneity and stability tests. Subsamples are stored at temperatures ranging from —20°C to +50°C and analyzed once a month for a period of 3 or 4 months. 

A number of reference materials for vitamins in foods are under development. Extensive analysis and stability testing have been conducted to assess the potential of vitamin-enriched milk powder, wholemeal flour, and margarine as well as lyophilized brussels sprouts, mixed vegetables, and pork liver for use as reference materials (193-196). The certification study for vitamin C in the brussels sprouts reference material has been completed. However, methodology problems continue to have a significant negative effect on the development of reference materials. 

At the conclusion of the study, surplus test materials are offered for sale as reference materials for quality control and method development purposes. These materials have been studied for homogeneity and stability and have been assigned values for specific analytes. The more recently produced materials (since July 1999) have traceable assigned values with rigorously evaluated uncertainties. Older materials have consensus-based assigned values. 

Precht, D., Molkentin, J. and de Froidmont-Goertz, I. (1998) Anhydrous butterfat reference material CRM 519 certification of triglyceride composition and cholesterol content including homogeneity and stability tests. Fett. Lipid., 100, 546-554. 

In order to cope with the lack of CRMs for endogenous species, the practice of laboratory internal QC materials is developing. A Laboratory Reference Material (LRM) was prepared from Brazil nuts (Bertholletia excelsa) for QC of the selenomethionine determinations. The concentration of selenomethionine was 79.9 p,g g-1. The homogeneity and stability of this candidate CRM passed the relevant tests recommended by the Measurement and Testing Programme [97],... 

Control samples should have a high degree of similarity to the actual samples analyzed otherwise, one cannot draw reliable conclusions on the measurement system s performance. Control samples must be so homogeneous and stable that individual increments measured at various times will have less variability than the measurement process itself. Quality Control samples are prepared by adding known amounts of analytes to blank specimens. They can be purchased as certified reference material (CRM) or may be prepared in-house. In the latter case, sufficient quantities should be prepared to allow the same samples to be used over a longer period of time. Their stability over time should be proven and their accuracy verified, preferably through interlaboratory tests or by other analysis methods. 

At the same time, internal quality control must be carried out to verify the performance stability of the limited-scope performance of the method. Triply redundant verification methods are carried out with regard to control of first, second and/or third line, each set of methods applied to a particular type of test. The first line of verification involves pro forma repetition of all the steps of the test, in order to establish repeatability or reproducibility for quantitative tests, and to verify the range of sensitivity or detection for qualitative tests. This verification is performed by the experimenter himself, as part of the proper performance of the test. A second line of verification is put into operation by administrative decision, and includes testing with blind samples, repetition of samples, internal audit procedures, etc. If necessary, a third line of verification can be set up by the use of certified reference materials (or spiking materials), or through collaborative trials. These procedures are based on external cooperation. External audit procedures and complaints handling procedures are also part of this third line of verification. 

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