Element- and method-specific test for microhomogeneity of major and trace elements in reference materials


Element- and method-specific test for microhomogeneity of major and trace elements in reference materials

Renno, A.

Homogeneity is a relative property of a sample in relation to the measurement (analytical method), the measurand (analyte), and the intended purpose, like the usage as a reference material (RM). The verification of homogeneity is essential to define a RM as fit for purpose. As a result of the lack of suitable RMs for microanalytical methods bulk RMs are used to calibrate the instruments, to validate methods, to estimate uncertainty, and for internal quality control.
To proof the superiority of synthetic mineral phases over natural ones as RMs for microanalytical methods we started to synthesize, different feldspars, pyrite and columbite/tantalite in the framework of a research project founded by the European Union (ESF) and the Free State of Saxony [1]. We hope that our attempt act as a trigger for forthcoming efforts in the production and certification of such RMs. The usage of these synthetic minerals will not restricted to a single analytical method, like electron probe microanalysis or LA-ICP-MS, non-destructive and destructive.
The assessment of homogeneity is an integral part of the synthesis tests and of the following certification. The test comprises several steps considering the relative character of microhomogeneity. This specific feature requires that all such tests have to be adapted to the specific analytical method, the specific element, and the type of microheterogeneity.
Five types of microheterogeneity were defined, based on the work of Malissa [2], Danzer [3], Kempenaers et al. [4], and Inczédy [5].
•random (stochastic) type
•systematic type
•nugget type
•island type
•periodic type.
A particular sampling strategy was defined for each type of microheterogeneity. The single calculations consider the different element- and method dependent 'information values', by using simulation software like CASINO, PENEPMA, SRIM or PyMCA.
The second step is the assessment of a critical mass in the sense of Danzer [6] and Kempenaers et al. [4] for every element.
The last measure requires the specification of the minimal volume of the whole synthesized mineral and the distance between sub-samples to be checked for microhomogeneity, following the procedures described by Chayes [7] for classical modal analysis of rocks [8].
[1]P.P. Michalak, A.D. Renno, S. Merchel, F. Munnik, M. Wiedenbeck, Microsc. Microanal. 17 (Suppl 2), 2011, 852-853.
[2]H. Malissa, K. Swoboda, Radex, 1963, 494.
[3]K. Danzer, Spectroch. Acta, 1984, 949-954.
[4]L. Kempenaers et al., Anal. Chem., 2002, 5017-5026.
[5]J. Inczédy, Talanta, 1982, 643-645.
[6]K. Danzer, Talanta, 1977, 561-565.
[7]F. Chayes, Petrographic Modal Analysis – An elementary statistical appraisal, John Wiley & Sons, New York, 1956.
[8]This project is supported by the European Union (ESF) and the Free State of Saxony. Many thanks to the custodians of the Geoscientific Collections in Freiberg for saving and providing an invaluable source of material. Motivating discussions on several facets of this study with Jens Gutzmer, Michael Wiedenbeck, Uwe Reinholz, and Slavo Michalak were very helpful.

Keywords: Reference Materials; Homogeneity; Heterogeneity; Homogeneity Test

  • Invited lecture (Conferences)
    Microanalytical Reference Materials - An MAS Topical Conference, 15.-17.05.2012, Golden, USA

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