Identify The Gems By Using FTIR
Fourier transform infrared spectroscopy (FT-IR) is a common scientific method, which is used to identify heat treatment [1], which was applied on the corundum. Sapphire, for example, a kind of corundum, its characteristics are showed each peaks of the spectrum by FTIR. FTIR is used in the case of identify and determine the heat treatment on a sapphire for the quality and color enhancement.
There are two ways in using a FTIR to identify the sapphire. One way is applying a narrow infrared beam through the sapphire, then, measure the spectrum by a detector on the other side. We can imagine, the absorbed infrared ray can only get through a small part of the gemstone. The other way is applying an constant infrared ray onto sapphire, and then detect the diffuse reflection. This method is so called "DRIFTS, Diffuse Reflectance Infrared Fourier Transform Spectroscopy”.
A natural sapphire will have an obvious peak at the position of 3160 cm-1 in the FTIR absorption spectrum. And to those ones treated by heat treatment process, will have a peak at 3309 cm-1. However, by this result, we can only “assume” the heat treatment applied on this sapphire, but we cannot be 100% sure. It is because the natural sapphire without heat treatment, also has a low peak at 3309 cm-1. If we use the general transmission scan method on a FTIR spectrometer, we can easily make a mistake, as the below spectrum shows:
Transmission Scan Method On A FTIR Spectrometer
According to the further research, scientists found, by using DRIFTS method, one can significantly enhance the accuracy of the result, because the peak at 3309 cm-1 is much clearer. It is because of the structure of crystals. During the crystallization process, the atoms are stuck layer by layer from the beginning to the end. Thus, the constant environmental change results the different characteristics in each layer. If we only applied the infrared bean into a small portion of the sapphire, it would be difficult to see the whole picture of the results and make a mistake in identification.
Therefore, using DRIFTS method, the infrared beam can be applied onto the whole sapphire, and get the full amplitude of peak at 3309 cm-1, and the heat treatment can be easily identified this way.
DRIFTS Method On A FTIR Spectrometer
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[Note 1] About the concept of thermal process of corundum, please refer to the articles from the expert, Mr. Li, Song-Cun.