Report Verification


FOURIER TRANSFORM INFRA-RED (FTIR) SPECTROMETER

An infrared spectrum presents a fingerprint of a sample with absorption peaks corresponding to the vibrations between the bonds of the atoms, constituting the material. Bonds in the molecules absorb infra red rays at one or more specific frequencies and produce vibrational characteristic of molecules in one of the three major parts of the infra red region, namely, Near Infrared (NIR), Mid Infrared (MIR) and Far Infrared (FIR). Organic compounds, H2O and CO2 may provide characteristic information in the Near to Mid Infrared region, while Far Infrared region display lattice vibrations, allowing identification of the mineral group (such as silicate, carbonate, sulfide, etc). However, this region is used only occasionally, as the molecules in minerals are too dense, and seldom display characteristic peaks, in routine measurement techniques.

FTIR or Fourier Transform Infra Red spectrometer is the instrument which measures all of the infrared frequencies simultaneously with high sensitivity and at a great speed. A typical FTIR spectrometer consists:

  1. optical bench or interferometer, which measures the intensity of a specially encoded IR beam after it has passed through a sample. The result signal called interferogram contains information about all frequencies present in it, and
  2. mathematical program, where a computer reads the interferogram and uses Fourier Transform principle to decode IR information for each frequency and presents spectrum


Schematic principle of a typical FTIR spectrometer


On routine, two common measurement methods are employed - direct transmission and diffused reflectance. When samples are flat or have two parallel surfaces, direct transmission mode is usually preferred, while in case of faceted or opaque samples, diffused reflectance mode is used. Occasionally, 'sandpaper-method' is also applied where very minute particles of the sample from an inconspicuous portion are taken on a piece of sandpaper and mixed with KBr (potassium bromide) powder and using the diffused reflectance accessory for measurements. This, however, is mainly done for ornamental materials, which most of the times are translucent to opaque; this is also useful for large-sized samples. Depending on the type of sample and amount of information required, multiple methods can be used to scan one single specimen. At the time of writing this souvenir, an Agilent imaging FTIR (610+660) has also been installed.

Due to a typical crystal structure with a specific pattern of molecular arrangement, particular gemstones or minerals tend to produce a specific pattern of absorption and transmission. Minerals or gemstones belonging to a particular chemical group such as silicates, oxides, sulfides, carbonates, etc may show a similar pattern, however, their exact pattern will remain distinct. This not only applies to gemstones or minerals but also to other substances, such as plastics, polymers, oils, rubber, etc.

At the Gem Testing Laboratory, FTIR is mainly used:

To identify an unknown gem material, i.e. separating out similar looking gem materials such as jadeite from nephrite or serpentine, emerald from fuchsite or quartz (aventurine / dyed), tanzanite from synthetic forsterite, diamond from synthetic moissanite, aquamarine from topaz, amethyst from scapolite, turquoise from magnesite, ruby from spinel or garnet, sapphire from kyanite, and much more.....each of them having a specific fingerprint pattern of absorptions, helping in their identification.

Here are the few illustrations...


Fuchsite (red trace, natural emerald (blue trace) and aventurine quartz (black trace) can easily be differentiated on the basis of FTIR spectra


Spectra conclusively separate similar looking serpentine (red trace), nephrite (blue trace) and jadeite (black trace).


'Sandpaper' method has proved to be very useful for identification for ornamental gemstones, such as separation of dyed magnesite (red trace) from turquoise (blue trace).

To separate natural and synthetic gems materials, especially in case of emerald, quartz, ruby and sapphire, glass, diamond (in some cases), alexandrite, etc. A synthetic gem material is produced relatively within a very short period than a natural gemstone; to speed up the growth process, various types of chemicals or catalysts are added during the growth, traces of which are left in the grown gem, or they lack some of the ingredients of a natural gem. These features are detected by the FTIR, assisting in their identification and separation

FTIR analysis is also used to differentiate natural emerald (red trace) from synthetic emerald, hydrothermal grown (blue traces) and flux grown (black trace).In case of older hydrothermal synthetics, natural and synthetic counterparts can easily be differentiated on the basis of multiple peaks in the region 3000 - 2000 cm-1, but in recent productions, especially from Russia, the separation is done on minor details (as given in inset).



Currently, FTIR analysis is the most reliable method to differentiate natural and synthetic quartz varieties. The IR spectra of natural (black trace) and synthetic (red trace) rock crystals are quite distinct, but in some cases, there are very close calls.


To identify treatments, such as fracture filling in emeralds, polymer impregnation in turquoise, coral or jadeite, heating or beryllium treatment in rubies and sapphires, irradiation / annealing in fancy coloured diamonds, etc.


FTIR has proved to be most useful for identifying the nature of filler substance, such as oil or resin. The presence of resin (blue trace) or oil (red trace) in emerald (black trace) display characteristic peaks in the infra red region provided the fracture has been oriented properly so that IR rays pass through it.


Unheated yellow sapphire display a characteristic peak at 3160 cm-1 (black trace), heated blue sapphires and rubies display peaks at 3307 and 3230 cm-1 (blue trace), while yellow as well as blue Be-treated sapphires display broad hump centred at around 3050 cm-1 (red trace).

Type classification of diamonds, is done on the basis of presence or absence of nitrogen and /or boron impurities. These impurities not only affect the body colour of a diamond but also assist in determination and separation of natural diamond from synthetic or treated counterparts. For example, if a yellow diamond is identified as Type Ib, it is most likely to be a synthetic HPHT grown. Similarly, if a blue diamond is identified as Type IIb, but does not show strain patterns in cross-polarizer’s, it again is likely to be a synthetic.



Type classification of diamonds, is done on the basis of presence or absence of nitrogen and /or boron impurities. These impurities not only affect the body colour of a diamond but also assist in determination and separation of natural diamond from synthetic or treated counterparts. For example, if a yellow diamond is identified as Type Ib, it is most likely to be a synthetic HPHT grown. Similarly, if a blue diamond is identified as Type IIb, but does not show strain patterns in cross-polarizer’s, it again is likely to be a synthetic.


FTIR MICROSCOPE


Analyses of very minute areas or features in FTIR microscope.

In addition to the standard transmission and diffused reflectance modes of infra red spectrometry, GTL also uses a microscope equipped with liquid nitrogen cooled MCT -A detector and KBr beamsplitter, which is capable to analyse very small areas up to 10 microns in size. The system basically uses a high power microscope attached to FTIR, where sample or area of the sample can be targeted with the infra red rays and signals are captured by the optics of the microscope, which are then sent to the detector of the FTIR for processing. The system helps to characterise a gem in further details at micron levels and assists in the identification of types and treatments of small (stars and melee) sized diamonds, fracture filling in emeralds, impregnations in turquoise or coral, along with separation of natural and synthetic emeralds, quartz, etc. The major advantage of FTIR microscope is the analysis of opaque samples without any preparation or using a sand-paper method. In addition, analyses of stones and diamonds when set in jewellery become much easier and conclusive.

Since, the use of microscope assists in mapping the sample, the results drawn by detailed analyses is very useful in answering the many questions of uneven distribution of colours and associated defects in the structure of a diamond or a coloured stone.

Powered by Gati Technlogies