Characterizing Diamonds with FT-IR Spectroscopy

Diamonds are among the world’s most popular gemstones. Also, many industrial applications use diamonds in large quantities.

The high price for diamond motivates people to purchase cheaper material resembling the appearance of a diamond, for example, synthetic Moissanite or cubic Zirconia. The value of a real diamond however is based on its size, shape, and color. Specific colors yield higher prices, yet special treatments like high pressure high temperature (HPHT) can change the natural color of certain types of diamonds.      

FTIR spectroscopy is a proven method to differentiate between authentic diamonds and counterfeits, to determine the type of a diamond (Ia, Ib, ... IIb), and to verify whether a diamond has been HPHT treated or not.

Instrumentation

Traditional FTIR technology used for gemstone analysis includes sophisticated research grade spectrometers equipped with a delicate beam condenser and a liquid nitrogen cooled MCT*-Detector. A very suitable, cost-efficient and compact alternative is an FTIR solution based on the portable and robust ALPHA II spectrometer equipped for diffuse reflection (DRIFTS).

Figure 1. Bruker Optics ALPHA II with diffuse reflection accessory and sample tray (ALPHA-Drift).

The ALPHA II  features:

• Small footprint (20 x 30 cm)
• Robust instrument, confidence in results
• Universal use (portable)
• Touchscreen operation (no additional PC necessary)
• Full sampling flexibility (solids, powders, liquids, gases)
• Easy operation, no liquid nitrogen needed
• Affordable price

To assess diamonds, the “diffuse reflectance” method has been proven to be a simple and reliable measurement technique. It involves collecting differently scattered light from the sample over a large solid angle and shows significant advantages compared to transmission measurements.

Figure 2. Principle of diffuse reflectance measurement.

In these cases, transmission measurements are particularly cumbersome because the beam condenser and sample must be precisely aligned or positioned. Also, the lower light flux here often necessitates the use of a more sensitive liquid nitrogen cooled detector. This is no the case for DRIFT measurements.

Measurement Procedure

The FTIR analysis of gemstones using the ALPHA II involves a very easy sampling procedure. Once the background measurement is done with the included reference gold sample, the gemstone is positioned in the sampling cup. Brilliants are typically placed on their flat side in the central position of the sample holder. Stones with lower symmetry (e.g. oval) are orientated with their long axis along the sample holder bar. Irregularly shaped rough diamonds might have to be turned a few times until a high-quality spectrum is obtained. The sampling cup height including the stone is adjusted using a turning knob to focus the IR beam on the surface of the sample. The maximal signal intensity reflects the best sample position.

Subsequently, the spectrum of the sample is acquired for about 30 seconds. The spectrum collected is then compared with reference spectra of certain diamond types and/or counterfeits for data evaluation in order to confirm whether the diamond sample is authentic and to identify its class, respectively.

FT-IR Spectra of Diamond

Some typical FTIR spectra of diamonds are shown in Figure 3. The broad absorption between the wavelength range of 1800-2700 cm^-1 is because of the carbon itself. The sharp absorption band observed at 3100 cm^-1 represents the hydrogen content, whereas the spectral region between the 1500 and 1000 cm^-1 includes the absorption from the nitrogen.

Figure 3. Mid-infrared spectra of diamonds.

Diamond or Fake?

Synthetic Moissanite (SiC) and cubic Zirconia (ZrO₂) are the most commonly known diamond counterfeits. Zirconia can be easily detected by the commonly used “thermal pen test” but Moissanite could pass as a diamond because its thermal characteristics are too similar to that of the diamond. Other identification methods are based on  several consecutive tests that can only be performed by experienced gemologists.

FTIR allows even inexperienced operators to distinguish the mentioned materials. The IR spectra of diamond, Zirconia and Moissanite are illustrated in Figure 4 and the differences between these specific spectral patterns are very clear.

Figure 4. FT-IR spectra of a diamond and different imitations.

Which Type of Diamond?

Diamonds, in general, are classified into type I and II where type II does not contain any measurable traces of nitrogen (N). The subtypes include:

• Type Ia --> Diamond with aggregated N
• Type Ib --> Diamond with isolated N (often synthetic)
• Type IaA --> Diamond with groups of 2 N’s
• Type IaB --> Diamond with groups of 4 N’s
• Type IIa --> Diamond without N or Boron
• Type IIb --> Diamond with Boron (blue or gray)

Type IIa and type IaB, which are often brown or gray color, are of special interest as they can be converted to pink or colorless by appropriate HPHT treatment. The value of these diamonds increases significantly due to these color changes. The differentiation between type I and II is clear because only the spectra of type I diamonds show the absorption features of nitrogen between 1450 cm^-1 and 1000 cm^-1.

Figure 5 shows the characteristic differences of type Ib and IaB. The peaks at 1136 cm^-1and 1344 cm^-1 are typical for disperse monoatomar nitrogen and therefore type Ib. The absorbance at 1172 cm^-1 is typical for the “B-centers” of 4 nitrogen atoms and therefore type IaB ^[1].

Figure 5. FTIR spectra of different diamond types.

Since synthetic diamonds are typically of type Ib, the nitrogen peak at 1344 cm^-1 is assigned to be indicative for synthetic diamond.

Summary

Very compact and robust FT-IR spectrometer like Bruker Optics ALPHA can be a valuable instrument in the gemological laboratory. The “diffuse reflectance” technique seems to be the most universal and easiest method used for gemstone analysis. FTIR spectroscopy also holds potential for use in other fields in gemology, for example, differentiating synthetic emeralds or lead glass treated rubies from their natural counterparts.

* FTIR Fourier Transformation Infra-Red
* MCT Mercury Cadmium Telluride; infrared sensitive detector
^[1] P. Thongnopkun, S. Ekgasit; FT-IR Spectra of faceted diamonds and diamond simulants; Diamond & Related Materials 14 (2005) 1592 - 1599

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.