ISBIS4 Abstract

Contact Author's Name: Dr. Bastiaan Geelhoed
Title of Abstract: Harmonization, representativeness and economics as contradictory partners in standards for the sampling of mixtures of particles
Author(s): Dr. Bastiaan Geelhoed
Affiliation: Sampling consultant

During the analysis and inspection of bulk materials, the measured mass concentration of a compound in a sample is used as an estimate for the corresponding mass concentration in the batch that is investigated. Depending upon the material under study and the analysis technique, sample masses may vary from several grams to up to several kilograms. Theories for the sampling of particulate materials previously developed by Gy (1979), Wilson (1964) and Knott (1967) enabled calculation of the minimum sample mass. The basic equations of these theories, i.e. the equations for the relation between the sample mass, the maximum allowable coefficient of variation of the sample concentration, and the variance of the sample concentration, are often applied in (inter)national standards for sampling of particulate material for chemical, physical, or biological testing. The general aim of standards for the sampling of particulate materials is to achieve comparability by harmonization.

Application of the theories of Gy, Wilson and Knott may yield incorrect estimates for the minimum sample mass when there is aggregation or segregation of the particles in the batch, because the models for the drawing of samples proposed by respectively Gy, Wilson and Knott are not able to incorporate aggregation or segregation in a realistic way. The minimum sample mass may be underestimated, resulting in a relatively high variance of the sample concentration, or overestimated, leading to higher sample processing costs. In this contribution, four general criteria are formulated in order to derive a more realistic model for the drawing of a sample. It is demonstrated that the new model can lead to both a better correspondence with actual sampling processes and a more realistic estimate of the minimum sample mass.

It is discussed how current sampling standards could be improved using a new approach, which is applicable to a wide range of materials including soils, biological products such as coffee beans or soybeans, and minerals such as granite aggregates. As an example, it is discussed how the Dutch standard NEN 5742 for the sampling of sediments and soils and the British standard BS EN 932:1 for the sampling of aggregates could be improved using an iterative sampling design.

References:

Geelhoed, B. 2004, Sampling of particulate materials - New theoretical approach, Delft University Press, Delft, 200 pp. (ISBN 90-407-2517-9).

Gy, P. M. 1979. Sampling of Particulate Materials: Theory and Practice, 1st edition, Elsevier, Amsterdam, 431 pp. (ISBN 0-444-42079-7)

Knott, M. 1967. Sampling Mixtures of Particles. Technometrics, 9, 365-371.

Wilson, A. D. 1964. The sampling of rock powders for chemical analysis. The analyst, 89, 18-30.