ANALYTICAL METHODS/Geochemical Analysis (Including X-ray) 57 Other techniques use chromatography, the time taken for one substance to move through another or through a capillary under a given gradient A wide family of geochemical analytical techniques use mass spectrometry to split propelled material, converted into charged particles, using electromagnets Other techniques involve examining the products of heating Earth materials under controlled conditions and studying either the evolved fluids or the changes in the properties of the residual solids X-ray Techniques Origin of X-rays X-ray technologies have proved to be useful in geochemical analysis (Table 2) X-rays are part of the electromagnetic spectrum (Figure 3) and have wavelengths ranging between 0.01 nm and 10 nm (0.1– ˚ ) They are waveforms that are part of a family 100 A that includes light, infrared, and radio waves Since X-rays have no mass and no electrical charge, they are not influenced by electrical or magnetic fields and travel in straight lines X-rays, like all parts of the electromagnetic spectrum, possess a dual character, being both particles and waves The name that has been given to the small packets of energy with these characteristics is photon The simple model of the atom, proposed by Niels Bohr in 1915, is not completely correct, but it has many features that are approximately correct The modern theory of the atom is called quantum mechanics; the Bohr model is an approximation to quantum mechanics that has the virtue of being much simpler than the full theory In the Bohr model neutrons and protons occupy a dense central region (the nucleus), and electrons orbit the nucleus The basic feature of quantum mechanics that is incorporated in the Bohr model is that the energies of the electrons in the Bohr atom are restricted to certain discrete values (the energy is quantized) – only certain electron orbits with certain radii are allowed X-rays are generated when free electrons from an electron gun give up some of their energy when they interact with the orbital electrons or the nucleus of an atom (Figure 4) The energy given up by the electron during this interaction reappears as emitted electromagnetic energy, known as X-radiation Two different atomic processes can produce X-ray photons One is called bremsstrahlung and the other is called electron-shell emission (Figure 5) Bremsstrahlung means ‘braking rays’ When an electron approaches an atom, it is affected by the negative force from the electrons of the atom, and it may be slowed or completely stopped The energy absorbed by the atom during the slowing of the electrons is excessive to the atom and will be radiated as X-radiation of equal energy to that absorbed Bremsstrahlung X-rays tend to have a broad range of energies since the degree of slowing can be variable and materials composed of mixtures have atoms with different properties (Figure 6) Bremsstrahlung tend not to be used for geochemical analysis; that is the preserve of electron-shell emission Analysis of X-rays: Electron-Shell Emission A common geochemical application of X-ray analysis is to direct a focussed electron beam at a polished rock or mineral surface and then collect and quantify the resulting secondary characteristic X-rays (Figure 7) The secondary X-rays help to reveal the elements present in that part of the sample that is directly under the electron beam This technique is known as electron-beam microanalysis, or microprobe analysis, and gives spatially resolved major- and trace-element geochemical data from solid samples, including rocks, minerals, sediments, soils, and glass Many ordinary electron microscopes are fitted with a secondary X-ray detector, making them suitable for geochemical analysis All of these devices rely on electron optics, using electromagnetic lenses to focus and direct a stream of electrons, generated by an electron gun, onto a polished mineral or rock surface (Figure 7) The focused electron beam has a variable radius, but can typically be maintained at slightly greater than about mm The spatial resolution of a microprobe is actually somewhat greater than mm The impinging electron stream interacts with the polished surface and produces a wide range of signals, including secondary and backscattered electron and cathodoluminescence (light) as well as the secondary X-rays of concern here There is an activation volume from which X-rays are generated, below the polished surface, which is several times larger than the primary beam Samples must be highly polished (flat) to avoid scattering When a sample is bombarded by an electron beam, some electrons are knocked out of their quantum shells in a process called inner-shell ionization (Figure 5) Outer-shell electrons fall in to fill a vacancy in a process of self-neutralization The shells are termed K, L, M, and N starting from the innermost most strongly bound shell Electrons moving from one shell to another produce characteristic X-rays K-shell ionizations are commonly filled by electrons from the L shell (Ka radiation) or M shell (Kb radiation) There are two Ka peaks (Ka1 and Kb2) corresponding to two discrete states of the in-falling electron When outer-shell electrons drop into inner shells, they emit a quantized