Tools of the Trade Ten Instrumental Techniques

X-ray diffraction (XRD) helps scientists determine the structure of crystalline materials and the stoichiometry of the materials within the crystals. It s based on Bragg s Law of Diffraction (nA = 2d Sin0) that describes how a crystalline material diffracts light waves when they pass through it. Crystalline materials cause diffraction because of the regular and periodic placement of atoms in the crystal. The crystalline and ordered structure breaks up the solid stream of x-ray waves into fragmented streams of x-ray waves according to the crystal order in the material. 

Many pure crystals have been tested by scientists so a library of data now exists that can be used as a comparison against the values obtained by the experimenter. 

Inductively coupled plasma atomic absorption (ICP/AA) spectroscopy is used to determine the concentration of analytes in solution. It can measure concentrations as low as a few parts per billion. This method is commonly used to measure the atomic concentrations of mineralogical samples. The analytes must be in solution to be sprayed into a chamber and excited by a plasma. Upon relaxation energy is released with a specific wavelength. The wavelength corresponds to what atoms are present. The intensity of the energy is related to the concentration of the analjde present. ICP is quantitative, but it must be referenced against a set of standards that are used for comparison. 

Scanning electron microscopy (SEM) is used to visualize materials and give a picture of what they look like. Samples are irradiated with a primary beam of electrons that raster scans the material this excites the atoms in the material so they emit secondary electrons and backscattered electrons. 

Transmission electron microscopy (TEM) is also used to visualize materials using electrons. Samples must be thin enough that the electrons transmit through them and then onto a detector below the sample. TEM can be used to look at the morphology of materials and also probe the crystalline properties. When crystals are present the electrons are diffracted, similar to what occurs with XRD. The diffracted electrons provide a picture with lines crisscrossing each other, and the lines represent the crystal lattice. 

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