Reliable Diamond Analysis Performed By FTIR Spectroscopy

It has become harder and harder to assess the real value of a gemstone since the emergence of synthetic diamonds, treated diamonds and nearly perfect imitates. Infrared spectroscopy is an elegant way to classify diamonds and detect imitates. It can provide valuable information on whether the stone is synthetic, natural or can be treated using a high pressure high temperature technique (HPHT).

Bruker Optics offers the right tools for all typical applications. Their tools assist with the identification and type determination of rough and polished diamonds as well as melee-sized diamonds and mounted stones. Anything can be tested – from large rough diamonds with dozens of carats to small fragments in the sub millimeter range.

• Detection of imitates
• Reliable type determination
• Automated screening
• Polished and rough diamonds
• Diamonds mounted in jewelry

Type Classification: Requirement of a Changing Market

Since the 1950s when the first reproducible process for diamond synthesis was reported, technology has continuously improved. Diamonds can be synthesized with properties that are suitable for industrial use as well as with gem quality. Treatments have also been developed that allow quality enhancement of both synthetics and natural stones.

Consequently, analytical techniques which provide information about the genesis and history of the stone are sought after. Although there is no single indicator that will give a definite answer as to whether the stone is synthetic, natural, or has undergone certain quality enhancement treatments, knowledge of the type of diamond allows extensive conclusions to be drawn.

How to Determine the Diamond Type

Diamond is not simply a modification of carbon: in most instances, foreign atoms such as hydrogen, boron or nitrogen are present inside the crystal lattice. The color of the diamond can be affected by these foreign atoms and their arrangement as isolated atoms, larger aggregates or pairs. The most important impurity is nitrogen and so its absence or presence is used as the foundation of the type classification system.

When diamonds contain sufficient nitrogen to be detectable by IR spectroscopy, they are classified as type I. Diamonds in which no nitrogen can be identified are classed as type II. The two main types are further subdivided, for example into Ia (aggregated nitrogen) and Ib (isolated nitrogen). Moreover, it is possible to identify candidates that can potentially be subjected to high pressure temperature (HPHT) treatment.

As previously mentioned, the method of choice for the classification of diamonds is IR spectroscopy. The IR spectroscopic analysis of diamonds used to be a complex task that had to be performed by educated specialists. Traditional IR spectrometers are bulky and difficult to operate. The interpretation of the resulting IR-spectra is particularly complicated because there are many different mixtures and spectra types.

In collaboration with HRD Antwerp, Bruker is offering the ALPHA II Diamond Analyzer (see Figure 1) which enables users who are not trained in spectroscopy to measure and classify a diamond in less than a minute.

High throughput screening accessory (HTS-XT) is the tool of choice for very large sample batches like melee-sized stones because it enables automatic measurement and classification of a large number of samples within a very short period of time. The FTIR microscope LUMOS II can be used to measure and visually inspect the smallest diamonds and complex arrangements on jewelry.

Figure 1: ALPHA II Diamond Analyzer.

Polished and Rough Diamonds

Bruker offers the right solution to specific measurement demands, depending on sample type and number. The ALPHA II Diamond Analyzer, based on the very compact and robust Fourier-Transform-IR (FTIR) spectrometer ALPHA II, is best for measuring single loose diamonds.

It has the footprint of a laboratory book and only weighs 7 kg. The process of measurement itself is very easy: the diamond is placed on the gold coated sample plate and moved inside the DRIFT-unit (DRIFT = Diffuse Reflectance Infrared Fourier Transform).

The user interface changes its appearance dynamically during the measurement and evaluation process. This provides the functionality to perform the next step. The ALPHA II Diamond Analyzer provides an easy to use, automated and dedicated solution for diamond detection and type analysis based on the FTIR method. It distinguishes diamonds from imitations or other precious gemstones using the FTIR diffuse reflectance method.

Read Next: Analyzing Gemstones and Diamonds in Jewelry

Rough Diamonds

Rough diamonds require a different measurement approach than polished diamonds due to their irregular shape. It is possible to measure rough diamonds using the ALPHA II transmission module (see Figure 2) – from small diamonds to those up to a weight of a few tens of carats.

Different measurement approaches can be combined for rough, mounted and polished diamonds in one ALPHA II system because the measurement modules are easily interchangeable within seconds.

Figure 2: ALPHA II with transmission module.

Principle of IR Spectroscopy

IR spectroscopy uses thermal radiation, which is invisible to the human eye, and interacts with matter by triggering lattice or molecular based vibrations. Each type of diamond has characteristic wavelength-regions where these vibrations occur.

The light is being absorbed and the diamond is more or less non-transparent in these regions because the energy of the infrared light is converted into vibrational motion at these specific wavelengths. With the appropriate measurement set-up, the IR-spectrometer system can measure these absorptions.

The IR spectrum is the plot of the intensity of these absorptions in the infrared spectral range against the wavelength. This contains enough information to classify a diamond.

Figure 3. IR-spectrum of diamond.

Measurement Procedure for Polished Diamonds

Any diamond can be classified within a minute using the ALPHA II Diamond Analyzer. Firstly, the background spectrum of the empty gold-coated sample holder is measured and then the diamond is placed in the center holder (figure 4). After moving into the instrument, the measurement is ready to start. The software gives detailed user instructions (see figure 5).

Figure 4: Diamond placed on gold-coated sample holder of the ALPHA II Diamond Analyzer.

Figure 5: Entering sample information.

It takes less than 30 seconds for measurement and evaluation using this infrared spectrometer. The result is shown in combination with the spectrum of the diamond (see figure 6).

Figure 6: Display of the analysis result.

Figure 7 shows the automatically generated PDF-report. The report can be opened in a single mouse click and contains all the required information including the identification result (diamond or not a diamond), the type classification and the spectrum.

Figure 7: PDF report of a diamond analysis.

Mounted Diamonds

The ALPHA II can be used to measure mounted stones from jewelry, using the forward looking reflection module (shown in Figure 8). The stone has to be placed in front of the aperture hole for the analysis.

A built-in camera helps to correctly position the stone and so enables the user to selectively measure even smaller stones in a narrow arrangement. A picture of a 0.25-carat diamond that was taken with the internal camera of the ALPHA II reflection module is shown in Figure 8.

Figure 8: ALPHA II with front reflection module. Inset: Picture of a diamond taken with the integrated camera.

Very Small Mounted Stones

The LUMOS II FTIR microscope can be used to selectively measure very small mounted stones in jewelry in close contact with each other (Figure 9). The LUMOS II stands out because of its ease-of-use and full automation combined with infrared spectroscopic performance and sample visualization of the highest quality. It has a large working distance of 30 mm and an excellent accessibility of the sample stage.

Figure 9: LUMOS II FTIR microscope.

Samples with a height up to 40 mm can be investigated without any change to the hardware. With the aid of a special sample holder vice that enables the fixation of all kinds of jewelry, mounted stones can be inspected.

Figure 10 demonstrates the sample holder vice with a diamond ring, with the whole ring shown in the picture inset. In Figure 11, the measurement of one of the stones together with the according visual image is shown as an example.

Figure 10: Diamond ring fixed in the sample holder vice on the LUMOS II stage.

Figure 11: Diamond spectrum of one of the stones from the ring.

The spectrum clearly indicates a type IaA diamond. The spectrum shows additional signatures from organic concentrations, for instance from skin fat, which is typical for diamonds in jewelry. The stretching vibrations of C-H result in a double peal below 3000 cm^-1, and this is typical for organic substances.

Automated Screening

The high-throughput-screening accessory (HTS-XT, see figure 12) is capable of the automatic measurement and classification of large numbers of diamonds.

Diamonds are placed in specially designed sample plates, available with 96, 384 and 1536 measurement positions. This measurement set-up allows for the measurement of even very small melee-sized diamonds, from the sub-milimeter range up to diamonds of several milimeters.

Figure 12: HTS-XT high throughput accessory with INVENIO S spectrometer.

For one position, the measurement time is about five seconds. The spectrum is automatically evaluated and classified after the measurement. As with the ALPHA II Diamond Analyzer, the HTS-XT based analyzer is able to distinguish between over twelve different diamond types and is also able to identify diamond imitates.

FTIR Spectra and Diamond Types – The Basics

Diamonds are generally divided into type I and II, where type II contains no measurable levels of nitrogen (N). The subtypes are as follows:

• 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 (potentially CVD synthesized)
• Type IIb: Diamond with Boron (grey or blue)

There are also many mixture types. The analysis software that is used in the HTS-XT Diamond Analyzer and the ALPHA II Diamond Analyzer can discriminate between more than twelve different types of diamond.

An example of the spectra of the most important types is shown in figure 13. The x-axis shows the reciprocal value of the wavelength in cm^-1 (i.e. how many waves exist over one minute) which is a commonly used unit in IR-spectroscopy, and the y-axis the absorption.

Figure 13: Example spectra of different diamond types with characteristic peak positions.

Typically, boron impurities who peaks around 2800 and 2460 cm^-1 and nitrogen impurities show absorptions between ca. 500 and 1500 cm^-1. The strong, broad bands between ca 1600 and 2700 cm^-1 are known as phonon bands and are characteristic for all diamonds. These are an excellent marker and can be used to differentiate between imitates and diamonds.

Interpreting the Type Information

In many cases, knowledge about the type of diamond is critical because it can be used to detect synthetic stones and possible candidates for HPHT-treatment. Of special interest are type IIa and type IaB diamonds because they can often be HPHT treated to change them from grey or brown to colorless or pink. These changes in color significantly increase the value of those diamonds.

Conversely, because type Ib diamonds very rare in nature, they are almost exclusively HPHT-grown synthetic stones. The reason they are rare in nature is because natural diamonds were subjected to very high pressures and temperatures over very long time periods. In these conditions, the isolated nitrogen atoms of Ib diamonds can move around within the lattice and aggregate into groups.

An A-aggregate forms when two N atoms combine, and the combination of two A-aggregates (with a vacancy between them) forms a B-aggregate. As synthetic CVD-diamonds are also available in gem-quality, type IIa diamonds can also be synthetic and may therefore be subjected to further evaluation.

Table 1: Occurrence of colorless to near colorless diamonds in relation to the diamond type.

Table 2: Diamond type classification is based on the presence or absence of nitrogen and boron impurities and their specific configurations in the diamond lattice. This diagram shows only "pure" diamond types, most diamond types are "mixed" types.

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

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