In the Laboratory
Determining the Authenticity of Gemstones Using Raman Spectroscopy Aaron Aponick, Emedio Marchozzi, Cynthia Johnston, and Carl T. Wigal* Department of Chemistry, Lebanon Valley College, Annville, PA 17003
The benefits of laser spectroscopy in the undergraduate curriculum have been the focus of several recent articles in this Journal (1–3) with Raman spectroscopy (4 –8) being of particular interest. The similarities of Raman to conventional infrared spectroscopy make the interpretation of spectral data well within undergraduate comprehension. In addition, the accessibility to this technology is now within the reach of most undergraduate institutions (9). At Lebanon Valley College, we have developed an experiment using Raman spectroscopy that determines the authenticity of both diamonds and pearls. The samples in question need no preparation other than mounting in a vial holder. The resulting spectra provide an introduction to vibrational spectroscopy and can be used in a variety of laboratory courses ranging from introductory chemistry to instrumental analysis. Rationale Raman scattering is the result of radiation emitted from an oscillating dipole induced by the electromagnetic field of a laser (10). The Raman spectrum consists of frequency shifts of visible light inelastically scattered by the sample. The scattered photons have lost energy to the sample’s vibrational levels. This loss in energy corresponds to the observed frequency shifts of the Raman effect, and is identical to the infrared absorption frequencies. Thus, the Raman and IR spectra contain the same structural information, with variations in intensity due to selections rules. Diamonds and pearls are the two most frequently imitated gemstones. The word “diamond” comes from the Greek word “adamas” which means “unconquerable”, an early recognition of the hardest of all natural minerals. The supreme hardness, combined with exceptional luster and light dispersion, gives the diamond the fiery brilliance for which it is prized. Pearls are the oldest prized gemstone, dating back more than 6000 years. Natural production of pearls occurs in several species of mollusk, most commonly the oyster. When a foreign body is introduced inside the mollusk shell, layers of nacre, a mixture of calcium carbonate and an organic binding agent called conchiolin, are deposited around the irritant, forming a pearl. Each layer of nacre is translucent, allowing light to pass through to lower layers. Light reflecting from various layers creates the iridescence and luster characteristic of pearls. The authenticity of gemstones is an intriguing problem to most undergraduates. To the untrained eye, pearls and diamonds are indistinguishable from their simulants. Diamond is an allotrope of carbon containing only carbon–carbon σ bonds. The most common diamond simulant is cubic zirconia (CZ), which is composed of zirconium silicate (11). Diamonds and cubic zirconia both are clear, colorless stones that scratch glass (12). Pearls are primarily calcium carbonate, whereas their synthetic counterparts are usually organic polymers or *Corresponding author.
glass beads covered with an iridescent varnish. The authenticity of both gemstones can be investigated using vibrational spectroscopy. Raman is particularly useful in this experiment because of the ease of obtaining spectra from solid materials. Results and Discussion As a prelude to the experiment, students are reminded of safety issues concerning lasers (13) and introduced to the principles of vibrational spectroscopy including: • • •
Introducing different types of molecular vibration and the 3N – 5 and 3N – 6 rules for linear and nonlinear systems. Quantization of energy as it relates to vibrational energy levels. Similarities and differences of IR and Raman spectroscopy.
The application of the harmonic oscillator approximation is introduced in lecture. The effects of varying atomic mass and bond force constant on the vibrational frequency are examined by using a spreadsheet to calculate the frequency of a molecular stretching vibration from the atomic masses using the harmonic oscillator approximation. A range of bond force constants (4.75–6.40 N/m)1 was used to predict the frequency range of C–H, C–O, C–Cl, and Si–O σ-bonds. Students are then asked to simulate the vibrational spectrum of chloroform using the molecular modeling software CAChe, which illustrates the various vibrational modes and predicts the corresponding vibrational frequencies. Using the CAChe model and the harmonic oscillator approximation, students identify the frequency corresponding to the C–H stretch of chloroform. An alternative to CAChe is the program MOLVIB (14), which animates the normal vibrational modes of simple molecules. Students obtain the Raman spectrum of chloroform and compare their predictions (3010 cm᎑1 using CAChe) with the experimental value (3020 cm᎑1). Since Raman spectra can be obtained in glass, student exposure to chloroform, a potential mutagen, is minimized by using sample vials prefilled with the halogenated solvent. Using Hooke’s law, students then predict the frequency of a C–D stretch. They obtain the Raman spectrum of CDCl3 and compare their prediction (2173–2503 cm᎑1 using the harmonic oscillator approximation) to the experimental value (2254 cm᎑1). Raman spectra of CHCl3 and CDCl3 are shown in Figure 1. This exercise demonstrates experimentally the relationship between atomic mass and frequency and validates the mathematical models used to predict vibrational frequency. The authenticity of gemstones serves as a practical application of vibrational spectroscopy. Since pearls are 90% calcium carbonate, comparison of reference and gemstone spectra can be used to determine authenticity. Students first obtain a reference Raman spectrum of calcium carbonate. To make the experiment more intriguing, we used an eggshell to obtain the standard CaCO3 Raman spectrum. Students
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In the Laboratory
shows an intense absorbance at 1331 cm᎑1 characteristic of the carbon–carbon stretch (15). Cubic zirconia shows an absorbance at 619 cm᎑1 corresponding to the Si–O stretch of silicate. Comparing the predicted stretching frequencies for carbon–carbon (1162–1338 cm᎑1 using the harmonic oscillator approximation) and silicon–oxygen bond of silicate (660 cm᎑1 using CAChe) to the obtained Raman spectra, students readily identify the real diamond. Conclusions
Figure 1. Raman spectra of CHCl3 and CDCl3.
At Lebanon Valley College, we use this experiment to introduce vibrational spectroscopy to first-year students. Students are encouraged to bring in their own gemstones for analysis. The opportunity to work with gemstones, molecular modeling, and the Raman spectrometer heightens student interest in the experiment. Students work in pairs employing a rotating schedule of molecular modeling and experimental data acquisition. This approach allows a laboratory of 16 students to easily complete the experiment in a 3-h lab period. Experimental Details The Raman spectra for this experiment were obtained using a Perkin-Elmer FT-Raman spectrometer equipped with a Nd:YAG pulse NIR laser and InGaAs detector at ambient temperature. Laser power level was set at 200 mW. Samples were mounted in a conventional vial holder. The mounting of earrings was facilitated by placing a cork over the post. Loose stones or rings were mounted directly in the vial holder.
Figure 2. Raman spectra of CaCO3, pearl, and faux pearl.
Acknowledgments We wish to thank the National Science Foundation Division of Undergraduate Education for support of this project through grants DUE -9551199 (Molecular Modeling Laboratory) and DUE-9451379 (Raman-IR spectrometer).
Diamond
Note 1. As suggested by a reviewer, students in upper-level courses can calculate an effective force constant by using the change in frequency upon deuteration of chloroform in place of the range given in this paper.
Cubic Zirconia
Figure 3. Raman spectra of diamond and cubic zirconia.
then obtain the Raman spectrum of two alleged pearls, one real and one fake. Raman spectra for pearl, faux pearl, and calcium carbonate are shown in Figure 2. Comparison to the standard CaCO3 spectrum allows students to readily identify the real pearl. Students determine the authenticity of diamond by comparing the actual spectral frequencies to the predictions made by the harmonic oscillator approximation and molecular modeling. They obtain the Raman spectrum of two clear, colorless stones, one diamond and one CZ. The Raman spectra for diamond and cubic zirconia are shown in Figure 3. Diamond 466
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Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
