Nuclear Analytical Techniques for Metallome and Metalloproteome Distribution

2 Nuclear Analytical Techniques for Metallome and Metalloproteome Distribution 

Besides total concentration of multielements, their spatial distribution in samples is also very important in understanding their bioavailability, trophic transfer, and environmental risk. A number of complementary analytical techniques exist for the mapping of elemental distributions in biological tissues including SRXRF (synchrotron radiation X-ray fluorescence) with microbeam (SR-pXRF), microscopic EDX (energy-dispersive X-ray fluorescence), microscopic WDX (wavelength-dispersive X-ray fluorescence), microscopic PIXE (particle-induced X-ray emission), laser ablation ICP-MS, microscopic SIMS (secondary ion mass spectrometry). 

SRXRF tomography can also be used to perform three-dimensional elemental analysis by measuring a series of projected distributions under various angles which are then back-projected using appropriate mathematical algo-rithms. Since this method involves rotation of the sample over 180° or 360° relative to the primary beam, it is limited to the investigation of relatively small objects. The spatial resolutions for XRF tomography are situated at the 1 2 pm level while, routinely, a resolution of 5 pm is employed. More detailed information and appUcations of XRF can be found in Chapter 3 (X-ray Fluorescence) in this book. 

Other XRF-based techniques like EDX have been coupled to microscopes such as a transmission electron microscope (TEM) or scanning electron microscope (SEM) to map elemental in roots and leaves of Arabidopsis thaliana, and mouse liver tissue pyramidal neurons, for example. Although both SEM-EDX and TEM-EDX can provide very good spatial resolution at about 10 nm, the detection limits are at about gkg level, which may hold back their application in the detection of trace elements in biological samples.  

PIXE with a proton microprobe has also been developed and has been applied to elemental distribution in plant and animal tissues, and human blood cells and tumors. Further, 3D micro-PIXE also has been developed to perform depth analysis recently and has a spatial resolution of 4 pm by using characteristic titanium K X rays (4.558 keV) produced by 3 MeV protons with beam spot size of 1 pm.  

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