Technical Considerations for Sampling and Sample Preparation of Biomedical Samples for Trace Element Analysis

Sampling and sample preparation procedures are to a large extent determined by the analytical method used since different methods vary in the amount of material required for analysis and in how this is pre-treated before being introduced into the measuring instrument. Judging from intercomparison studies conducted by the International Atomic Energy Agency (IAEA), the most widely applicable methods now in use are Neutron Activation Analysis, Atomic Absorption Spectrometry, and Inductively Coupled Plasma Atomic Emission Spectrometry, though the latter still seems to have insufficient sensitivity for many trace elements of biomedical interest. Common to all these methods is the problem of contaminating the sample before or during analysis. For many elements (e.g., As, Cd, Co, Cr, Mn, Mo, Ni, Pb, Sb, and V) sufficient control over contamination can only be achieved by the use of special tools and reagents, and by working in a controlled (dust-free) environment. Several important elements (e.g., As, Hg, Sb, and I) are subject to losses on drying or ashing, but can be recovered reliably if wet-ashed in a closed container such as a PTFE "bomb." For representative sampling it is almost always necessary to start with several grams of material, and to homogenize this, if the effects of sample heterogeneity are to be reduced to an acceptable level. Quality assurance procedures covering all these aspects are difficult both to define and to apply. However, much can be learned from the statistical evaluation of results for duplicate samples, and from a determination of the limit of quantitation of the analytical procedure.