ANALYTICAL METHODS/Mineral Analysis 109 Table Microanalytical techniques used in mineral analysis Technique Acronym Probe Product Best practical concentration range Electron microprobe analysis Scanning electron microscopy Transmission electron microscopy (analytical electron microscopy) Proton induced X ray emission Micro X ray fluorescence Synchrotron X ray fluorescence Laser ablation inductively coupled plasma mass spectrometry Secondary ion mass spectrometry (sensitive high resolution ion microprobe analysis) EMPA SEM TEM (AEM) Electrons Electrons Electrons X rays X rays X rays ppm 100% 200 ppm 100% 100 ppm 100% 0.5 mm 0.5 mm 20 nm 0.3 mm 0.3 mm 50 100 nm PIXE XRF SXRF LA ICP MS Protons X rays X rays Laser X rays X rays X rays Ions ppm 5% 10 ppm 1% 500 ppb 2% ppb 0.5% mm 10 mm mm mm mm 10 200 mm 40 80 mm 100 mm mm 100 mm mm 10 100 mm SIMS (SHRIMP) Ions Ions 0.1 ppb 1% 50 mm 100 nm mm electron excitation, the resistivity of the film should be less than 10 kO mm and a carbon coat about 25 nm thick is usually sufficient Passing a current of 50 to 150 A through sharpened electrodes in a 10 torr vacuum or carbon thread at 10 torr creates a homogeneous coating at a distance of to 10 cm from the carbon Secondary ion mass spectrometry and PIXE analysts mostly use nm gold films, which are readily produced by heating a weighed quantity of gold in a tungsten wire basket Electron Microprobe Analysis An electron probe is a beam of electrons accelerated through a high voltage and focused at the surface of the sample The beam is commonly produced by passing an electric current through a hairpin tungsten filament that is maintained at up to 30 kV in the scanning electron microscope (SEM), 50 kV in the EMPA, and 400 kV in the AEM The electrons are channeled through a hole in an anode maintained at zero potential and then through a series of magnetic lenses that shape the beam into a circular cross-section and focus it to a size less than the scattering expected in the sample The energy of the electrons at the surface of the sample (depth 0) is denoted E0 and is measured in thousands of electron volts (keV) Electron Scattering When an electron in the beam interacts with an atom in the sample, the most common result is for the electron to be scattered elastically, with a resulting change in direction and insignificant loss of energy Analytical volume Diameter Depth This is a simple Coulomb attraction between the approaching electron and the multiply charged nucleus By comparison, the diffuse electron cloud in the atom, repelling in all directions, has a much smaller effect The probability of elastic scattering increases with atomic number and decreases with higher E0 It is possible for the electron to reverse direction, usually after several scattering events, and to leave the sample; this is known as back scattering and, at high atomic number and low E0, more than half the electrons may be back scattered In the AEM, the very high accelerating voltage ensures that the electrons pass through the thin sample with minimal scattering Electrons are also prone to inelastic scattering, which again involves a Coulomb reaction with the nucleus, but here the force of attraction by the opposite charges is actually translated into a reduction of momentum and the loss of kinetic energy is released in the form of an X-ray These X-rays are known as bremsstrahlung (‘braking radiation’) and may have any energy up to the value of E0 Inelastic scattering eventually causes the electron to come to rest and so defines the size of the interaction volume: the shape of the volume is determined by elastic scattering Figure illustrates such volumes Characteristic X-Ray Generation Electrons may also cause ionization by ejecting an electron from its atomic orbital, provided E0 is greater than the critical excitation energy (Ec) The probability of ionization is much lower than that of inelastic deceleration or inelastic scattering If an inner orbital electron is ejected, an outer electron may fill the