Conclusions

The main advantages of energy-dispersive X-ray fluorescence (EDXRF) analysis are its multielement capability and its nondestructive nature, combined with the need for only simple sample preparation without time-consuming sample destruction, and which only requires personnel with minimum manual skills. This is undoubtedly the cheapest and the simplest analysis technique among the instrumental analytical techniques discussed here.

The main advantages of total-reflection X-ray fluorescence (TXRF) are the possibility of rapidly analyzing large numbers of samples, which are only required in small amounts, and which are prepared according to a simple procedure. However, this is not an appropriate analytical technique for determining low concentrations of light elements (e.g., Na, Mg Al, Si), or for determining Cd in plant material (which contains large amounts of potassium), because potassium K lines strongly interfere with the Cd L lines.

Micro-proton-induced X-ray emission (PIXE) is a highly sophisticated method that is suitable for the spatial analysis of the elemental distributions in biological samples. However, the key point in this analysis is sample preparation, during which the elemental distribution must be preserved. For now, rapid freezing followed by controlled freeze drying is the most appropriate method of sample preparation, although the development of techniques allowing the measurement of samples in a frozen hydrated state is required to reach subcellular levels.

High-resolution X-ray absorption spectroscopy (XAS) has become available with the development of synchrotron radiation sources. This has introduced powerful experimental methods for the investigation of the local structures around selected atoms in biological samples. In X-ray absorption near-edge structure (XANES), the valence state of the selected type of atom in the sample and the local symmetry of its unoccupied orbitals can be deduced from the information hidden in the shape and energy shift of the X-ray absorption edge. In extended X-ray absorption fine structure (EXAFS), the number and species of neighboring atoms, their distances from the selected atom, and the thermal or structural disorder in their positions can be determined from the oscillatory part of the absorption coefficient above the K or L absorption edges.

Acknowledgements The X-ray absorption spectroscopy experiments were performed in various synchrotron laboratories, including ESRF in Grenoble, ELETTRA in Trieste, and HASYLAB, DESY in Hamburg, with the financial support of European Community Contract RII3-CT-2004-506008 (IA-SFS).

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