The more convenient technique to identify gemstone is that of the Fourier – FTIR. In the Nicolet avatar 360 E.S.P. model, the spectrometer collects spectra from mid and near infra red region.
- The FTIRspectrometer consists of two parts:
- Michelson interferometer: This combines all the incoming Infra Red radiation into one interferogram.
- A mathematical program that operates on the Fourier transform principle – i.e. converts an interferogram back into a spectrum (first into a transmission spectrum and finally into an absorption spectrum.
The infra red region is a broad region divided into three portions:
- Near Infra Red = 750nm – 2500nm (13300 – 4000cm−1)
- Mid Infra Red = 2500nm – 25000nm (4000 – 400cm−1)
- Far Infra Red = 25000nm – 300000nm (400 – 33cm−1)
- Light is split into two halves by a semi-transparent mirror called a beam splitter.
- These two beams are then reflected back toward one another by two additional mirrors, one fixed, the other moving, so that the two beams interfere when they come back together at the beam splitter, giving rise to an interferogram.
- The interferogram then goes through the sample and some of the wavelengths are absorbed.
- The transmitted wavelengths, still in the form of an interferogram, reach the detector.
- The data is digitized and processed using a Fourier Transform program, which through a sequence of many steps, transforms the final interferogram into an absorption spectrum.
- The Nicolet model is monitored by a computer that not only does the mathematics of the Fourier Transform but also provides considerable flexibility to plot, display, and store and manipulates spectra.
- Absorption in the Infra Red region is due to vibrations; in far Infra Red from rotations of molecular and structural components of the crystal.
- To vibrate, the atoms must get energy from some source, (in this case Infra Red) giving rise to an absorption band.
- Bands are generally sharper for organic materials.
- Carbon in diamond and water in gem materials when present have specific signals in the Infra red region.
- Transmission method
- Diffused Reflectance method
Applications of Infra Red Spectra: Absorption associated with vibrations in the crystal structure, are characteristic of the given combination of atoms constituting the gem.
Identification of gemstones:
- Natural Turquoise = CuAl6(PO4)4 (OH)8 (H2O) from Gilson Synthetic Turquoise – Smoother pattern because of a difference of aggregation.
- Jadeite from Nephrite – sharper peaks in 4000cm−1 region are observed in Nephrite.
- Identification of natural diamond from synthetic moissanite and synthetic cubic zirconia.
Presence of H2O, (molecular) or (OH) group. Various forms of water have characteristic patterns in mid infra red region due to structure, origin or treatment. For H2O molecules, the symmetric and asymmetric vibrations occur in the 3800 – 3600cm−1 region.
- Synthetic amethyst & natural amethyst – different types of water absorption.
- Heat treatment – water leaves the mineral first.
- Natural emerald from synthetic emerald (hydrothermal) – Absence of Type II water molecule and presence of chlorine in the synthetic. Absence of water molecule in synthetic flux emerald.
Types of diamond – la, Ib, lla, llb, nitrogen and boron have different absorption patterns in the mid infra red.
Detection of Treatments through FTIR
- Presence of 4941 and 5165cm−1 means that the diamond has been irradiated and heat treated to produce or enhance yellow to brown coloration.
- Detection of gem impregnations and also the type of agent (polymer, oil, etc.) or material used on the stone, e.g.: polymer – impregnation of opal / turquoise.
- The entire spectrum is recorded at the same time in the form of an interferogram.
- 100 or even 1000 spectra of the same sample can be done in a very short time so that an average can be taken.
- Reduced heating of the sample and consequent spectral perturbations are avoided.
- FTIR concept uses a laser, both to check the moving mirror displacements, as well as an internal reference for wavelengths.
- FTIR is both fast and accurate.
- FTIR spectrometers are available with a microscope which is attached directly to the spectrometer. This provides a fast and non-destructive microanalysis of specific regions such as the filler material within a surface reaching fracture.