Homogeneity testing

Sample preparation Homogeneity testing Stability testing... 

Combination of the results from homogeneity testing, stability testing, and characterization and assembling an uncertainty statement Set-up of a certificate and, if appropriate, a certification report... 

Characterization of the finally-produced material includes final physical characterization along the lines described under interim physical characterization and chemical characterization (homogeneity). The latter includes selection, development, assessment, and validation of methodologies for homogeneity testing includ-... 

Finally, documentation of all collection and processing steps and characterization procedures, material and measurand descriptions and results of homogeneity testing completes the RM collection/preparation phase. 

Examples of the use of INAA in homogeneity testing of RMs are numerous and therefore only a few illustrative examples can be mentioned. More than ten elements can frequently be used for the accurate estimation of homogeneity of several types of biological materials (Tu and Lieser 1984), while a much higher number of elements can usually be precisely assayed in geochemical and environmental RMs. 

Kucera j, Soukal L (1988) Homogeneity tests and certification analyses of coal fly ash reference materials by instrumental neutron activation analysis. J Radioanal Nucl Chem Articles 121 245-259. 

Pauwels J, Kureurst U, Grobecker KH, Quevauviller P (1993) Microhomogeneity study of BCR candidate reference material CRM-422 - cod muscle. Fresenius J Anal Chem 345 478-481. Pauwels J, Lambeety A, Schimmel H 1998a) Homogeneity testing of reference materials. Accred Qual Assur 3 51-55. 

Tu Shu-Db, Lieser KH (1984) Homogeneity test of Chinese biological standard reference materials by means of instrumental neutron activation analysis, f Radioanal Nucl Chem Articles 84 301-306. 

Tab. 4.2 Relative standard deviations measured in homogeneity tests of a candidate CRM PCBs in mussels (BCR 682) (de Boer et al. 2000)...

Biagini R, Dersch R, de Felice P, Jerome SM, Perkin EME, Pona C, de Sanoit J, Woods JM (1995) Homogeneity testing of spiked reference materials. Sci Total Environ 173/174 267-274. 

TABLE 4.12 Homogeneity test for model processed foods"... 

The procedure basically follows the AO AC homogeneity test protocol with some modification. 

Group comparison tests for proportions notoriously lack power. Trend tests, because of their use of prior information (dose levels) are much more powerful. Also, it is generally believed that the nature of true carcinogenicity (or toxicity for that matter), manifests itself as dose-response. Because of the above facts, evaluation of trend takes precedence over group comparisons. In order to achieve optimal test statistics, many people use ordinal dose levels (0,1,2..., etc.) instead of the true arithmetic dose levels to test for trend. However, such a decision should be made a priori. The following example demonstrates the weakness of homogeneity tests. 

The PT provider has to take care that all the distributed samples are comparable with each other. Therefore the provider has to ensme the homogeneity of the testing material before dividing it into sub samples. The provider must have a documented procedure for homogenisation and for homogeneity tests. It has to be assured that the evaluation of the laboratoiy results is not affected by inhomogeneities. 

Another result of the requirements to distribute homogeneous test material is that some pretreatment steps usually performed on routine samples are not done on the interlaboratory sample. For example, in interlaboratory rounds for the determination of metals by inductively coupled plasma, the test material is often a solution of the metals in water. Often a method requires extraction of the metals by acid digestion of the field sample, a procedure that will have significant uncertainty in terms of recovery of analyte. 

The difference between a single observed value and the average value of a large number of observed values (at least 8), obtained by applying the same analytical procedure to the same homogeneous test sample. 

Drying, grinding, sieving, homogenizing Test sample J i I y... 

Sometimes for convenience, and to enable homogeneity testing to be completed in a practical timeframe, one or two representative analytes are selected as markers for homogeneity and undergo full testing as described above. Spiking concentrations of the remaining analytes are checked by duplicate analysis. 

If the above approach creates practical problems, an alternative is to perform single analyses on a minimum of five test portions of the study sample. The standard deviation of replicate analysis results is an indicator of sample homogeneity and method precision. The disadvantage of this approach is that it does not provide a simultaneous measure of the analytical variance under the homogeneity test conditions. Analytical variance must be estimated from historical data (e.g. method validation) or spiked recoveries run with the homogeneity test samples. 

NARL test methods include details of how traceability was established at the time the method was validated. Provided that the documented test method is followed, and all critical control points are addressed, measurements made using the test method will correctly identify and selectively and accurately measure the analyte of interest. Such measurements are traceable to the standards used in method validation and the calibration processes. For the test methods used in homogeneity testing of study samples, traceability of chemical measurements is maintained by ... 

Direct comparison with certified reference materials Homogeneity testing results A primary method of chemical measurement Consensus value from expert laboratories Consensus of participants... 

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