A series of three articles discussing the use of Fourier-Transform Infrared Spectroscopy (FTIR) in gem identification.
FTIR Intrigue, Part One: Narrow Beam vs. DRIFTS
The Fourier-Transform Infrared Spectrophotometer (FTIR) is used to help determine whether a sapphire has been thermally enhanced. The heights of the various peaks give the gemologist an indication of whether the sapphire being tested has been thermally enhanced. For example, when a strong peak at 3160 wavenumbers is found, this is an excellent indication that the sapphire being tested has not been thermally enhanced.
When a strong peak is found at 3309 in non-basaltic sapphire, this gives an indication that the gem may have been thermally enhanced. Interestingly, a weak-to-medium 3309 peak can be found in both heated and unheated sapphire. Thus great care must be given to analyzing FTIR data.
Two different FTIR methods are commonly used for testing gems. One involves a transmission scan, where a relatively narrow infrared beam is passed through the specimen. As a result of the restricted path, only a portion of the gem is sampled. Another technique, the Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) method, diffuses the beam throughout the gem and thus samples a greater portion of the piece.
We recently tested a 5.72-ct blue sapphire with both techniques. As one can see from the pictured spectra, the narrow-beam method produced a small peak at 3309. However when the DRIFTS method was utilized, a much stronger 3309 peak was found, suggesting that the gem was probably heat treated. This was later confirmed by examination of the inclusions.
Why the two different results? By their very nature crystals possess directional properties and this can impact spectra of all sorts, including infrared. But more important, crystals are composed of layers of atoms stacked one on top of another. As the growth takes place, the conditions constantly change, from start to finish. Thus one layer is never identical to another. We can see this in the arrangement of inclusions within a crystal and it is most obvious in the growth and color zoning that many crystals display.
Differences in composition and structure from one layer to another create growth zoning and are also probably responsible for the different FTIR spectra we obtained in this specimen. In this particular gem, the DRIFTS method was better able to capture the full amplitude of the 3309 peak because it sampled a greater portion of the specimen.
FTIR Intrigue, Part Deux: Acetone Dipping
Acetone is sometimes used to clean specimens prior to taking an FTIR spectrum, the idea being that this will remove surface contaminants. But one must take great care that the acetone itself is not contaminated, as the following two spectra show.
In Tucson I performed a brief experiment that shows the potential perils of acetone cleaning as it relates to the FTIR. First, the FTIR spectrum was run on the specimen without any sample preparation other than wiping with a gem cloth. Then, the gem was dipped into a bottle of acetone that had been previously used to clean many specimens. Following a quick dip, the excess acetone was removed with a paper towel (without touching the gem) and the spectrum re-run. The result was distinctive, with large oil peaks found following the acetone bath.
This simple experiment makes clear that acetone dipping has many perils with the FTIR. As a powerful solvent, it will dissolve oils and other dirt that might be on the specimen, but this then leaves a residue as the acetone evaporates. Even if the gem is carefully wiped clean following this dipping, one can suppose that residue would still remain in pits and fissures that escape the touch of the wiping medium.
The lesson is that acetone "cleaning" may add contamination, rather than remove it. If acetone is to be used at all (and the above suggests it should not), one should use clean acetone for every dipping.
Dr. John Emmett had the following comments on the idea of cleaning a specimen in acetone prior to testing with the FTIR:
First published in 2007, while I was at the AGTA GTC.
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Posted 12 July, 2012; last updated 7 March, 2013