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To answer the question “what is this stuff” we have examined our sample with a stereomicroscope and identified the sample’s principal elemental composition using x-ray spectroscopy. We now need to determine how those elements are bound to each other to form distinct chemical compounds. With few exceptions, the most useful techniques for that purpose are infrared and Raman spectroscopy. Most covalent bonds between two elements absorb radiation in the infrared region of the electromagnetic spectrum. This includes virtually all carbon based compounds along with many common inorganic compounds containing oxygen (e.g., oxides, hydroxides and oxy-ions of metals and non-metals). Chemical bonds which are infrared inactive are frequently good Raman scatterers and can be identified using Raman spectroscopy. Both techniques rely upon libraries of reference spectra to compare the chemical bonding present in an unknown sample to that of a pure standard material. In many respects, an infrared or Raman spectrum serves as a “fingerprint” of the chemical bonding present in a compound.
What about compounds with only ionic bonds, like sodium chloride? Neither infrared nor Raman spectroscopy would yield useful information. One might deduce the compound if the only elements present were sodium and chlorine. Alternatively, a technique like x-ray powder diffraction could confirm the composition based upon crystalline structure rather than chemical bonding.
The final installment of this discussion will demonstrate how these techniques were used to identify a particular unknown sample. Stay tuned.
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