Analysis of Impurity Effects on the Coloration of Corundum by Laser-Induced Breakdown Spectroscopy (LIBS) Sut Kam Ho Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, FST Building E11, Room 4028, Avenida da Universidade, Taipa, Macau, China
Corundum was analyzed using laser-induced breakdown spectroscopy (LIBS) for impurity effects in their multiple colorings. Qualitative measurements were attained for impurities of chromium, magnesium, iron, and titanium in red, yellow, and blue samples. Moreover, treatment with a beryllium diffusion, which can modify corundum to obtain an attractive color, was tested in the yellow sample. In this work, most of the measurements were acquired using a laser pulse energy of 5 mJ and impurity emissions were appreciable. The signal-to-noise ratios were 11, 6.5, 10, and 4 for the Cr 425.44 nm, Fe 404.58 nm, Be 313.04 nm, and Mg 285.21 nm lines, respectively, for five laser shots. The amount of damage to the corundum samples was also monitored by measuring the craters after laser analysis. It was found that the crater size was about 30 lm after 10 laser shots. As such, the damage to corundum sample is almost imperceptible after the LIBS analysis. Index Headings: Laser-induced breakdown spectroscopy; Corundum; Beryllium diffusion; Destructiveness.
INTRODUCTION Accurate gemstone identification is significant in jewelry trade to retain the commercial value of natural gemstones. Therefore, there has been an urgent need for powerful analytical instruments for distinguishing natural gemstones from synthetic and treated ones.1–3 For decades, refractometers, polariscopes, microscopes, and hand spectroscopes were used for the identification of most gem materials. Later, more sophisticated tools, such as infrared and ultraviolet visible spectrometers, X-ray fluorescence (XRF), and Raman instruments were developed to meet the analytical needs of gemological laboratories.4–8 The coloration of a natural gemstone is a prime factor determining its quality and market value. There are multiple origins of color in gem minerals that have been explored in recent years.9–11 Basically, five mechanisms have been recognized as causes of color in gemstones. First, one of the causes is dispersed metal ions in gem materials. Light illumination of dispersed metal ions can cause absorption in certain wavelength ranges of the light source and cause electrons jump to an unstable higher state; luminescence is emitted as energy released from the electrons and causes the color of stone. In fact, the identity of the ion is not the only factor that can affect the color; other factors such as the valence state, nature of neighboring atoms, and ion coordination Received 25 September 2013; accepted 5 August 2014. E-mail: [email protected]. DOI: 10.1366/13-07304
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also play roles in stone color. Second, absorption occurs when electrons jump to another atom, and this charge transfer process leads to coloration in stone. This happens when there is a charge transfer from oxygen to the metal and an intervalence charge transfer between two metal ions. Third, irradiation can affect the metal ions and change their oxidation state, inducing color centers. A color center is a defect, such as missing atoms or additional atoms, and causes absorption. Fourth, apart from transitions between energy levels, absorption can also be occur due to transitions between energy bands, and they can be responsible for color. Last, phenomena such as interference, diffraction, scattering, and the presence of colored inclusions can affect the color of gem minerals. The treatment of natural gemstones to improve their color or appearance traces back thousands of years.12 The most common treatment methods that involve a change of color are exposure to heat or radiation. Other, less common treatments include surface coatings, dyeing, and diffusion. In 2002, the diffusion of beryllium into corundum to modify the color was developed in Thailand.13–14 This treatment changes corundum to more graceful hues, usually yellow or orange to orangey red; this color is rare and valuable in untreated corundum. Therefore, the ability to detect beryllium diffusion has become indispensable in corundum with an unusual color. However, the detection of beryllium diffusion in corundum has proved to be difficult using the traditional nondestructive analytical methods, such as XRF, because they cannot detect light elements such as beryllium.15 Detection requires highly sophisticated methods such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). Instrumentation for LA-ICP-MS includes a laser ablation unit for removing a small amount sample of the stone. The vaporized materials are then sent to a mass spectrometer for analysis with a carrier gas. This method can detect almost all chemical elements within detection limits in the range of parts per million to even parts per billion.2,16–18 In SIMS, a beam of oxygen ions is focused on the sample in an ultra-high vacuum environment. Ions are knocked off the surface of sample and transferred to a mass spectrometer. This is a powerful method for the analysis of most elements with a sensitivity in the range of parts per billion to parts per trillion.2,19,20 Both mass spectrometer techniques are slightly destructive for analysis. Also, the instrumentation for these two analytical methods is quite expensive. The costs for LA-ICP-MS range from US$20 000 to $500 000, and the costs for SIMS may run over US$2
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FIG. 1. Schematic setup.
million. Furthermore, special sample preparation and a vacuum environment are required, so gemological laboratories are rarely equipped to use these techniques. As a result, more affordable techniques have to be considered. Since 2003, laser-induced breakdown spectroscopy (LIBS) has been explored for its potential application to gemstone analysis.13,21–28 This technique is also slightly destructive because it uses laser ablation. Still, it is a welcome alternative for corundum analysis because of its simplicity, low cost, lack of required sample preparation, and quick analysis. As a result, earlier studies were devoted to the application of LIBS to the analysis of gemstones. Garcia-Ayuso et al. used it to characterize the most common noble metals as well as other metals present in jewelry pieces.24 JuradoLopex and Luque de Castro identified alloys used in the manufacture of jewelry pieces.25 McMillan et al. developed LIBS for the analysis of gem beryl21 and analyzed 52 mineral samples using LIBS and identified them in a database of LIBS composite spectra.22 De Giacomo et al. presented the analysis of gemstone alexandrite.23 Alvey et al. indicated that LIBS analysis can be used to discriminate garnets of different composition and has the potential to discern geographical origin.26 Yetter and McMillan applied LIBS to the analysis of rubies and sapphires from three general tectonic regions for comparison.27 Krzemnicki and coworkers detected boron, lithium, and beryllium in corundum.13,28 Different aspects of the application of LIBS in gemology have been investigated. As we have seen, earlier studies analyzed some light elements in corundum, and the emphases of these studies were in the detection of treated gemstones and the determination of gemstone origins.28 Nonetheless, studies of the effect of impurities on the color of corundum are still needed. The coloration of gemstones is a considerable index for setting of their market price. As explained, the coloration in gem materials can have different origins. In this study, a qualitative analysis was
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conducted for the effects of dispersed metal ions and ion pairs for change transfer between atoms in corundum. For example, a trace amount of chromium in corundum is the chromophore that is responsible for red color, and its color saturation depends on concentration. In this study, corundum samples of different colors were analyzed and chromium, magnesium, iron, and titanium were detected. As already mentioned, the detection of color modification in corundum contributed by beryllium diffusion is of much concern in the gemological market recently. In this study, we demonstrate using LIBS for the detection of beryllium and treatment with beryllium diffusion in one of the samples to provide evidence of the practical use of LIBS in gemology. We performed the analysis to detect various impurities using different numbers of laser shots. In addition, some previous studies of gemstones reported that visible craters were created on the sample targets as a result of the analyses.13,21,22 Because the destruction of the sample during analysis is critically important for precious stones, in this study the sample surface was examined after the LIBS analysis after different numbers of laser shots to demonstrate the minimal destructiveness of LIBS. The study shows that optimal experimental conditions can be identified to obtain noticeable emissions with indiscernible damage to the samples.
EXPERIMENTAL The experimental setup is shown schematically in Fig. 1. The neodymium-doped yttrium aluminum garnet (Nd : YAG) laser (Spectra-Physics) can provide energy per pulse as high as 450 mJ at 1064 nm and with a repetition rate of 10 Hz. The Q-switch operating mode of the laser produced pulses of approximately 6 ns. Before the ablation of the sample using the laser, we focused the beam to a small laser spot to provide a high laser energy fluence. After ablation, we imaged the luminous plume onto the entrance slit of a 0.5 m spectrograph (Dongwoo 500i) using collection optics of fused silica with a focal
FIG. 2. Spectra of the Cr 425.44, 427.48, and 428.97 nm lines using LIBS analysis with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the red corundum sample analyzed.
length of 100 mm. The slit width was set at 200 lm, giving an overall spectral resolution of 0.2 nm. The spectrograph was equipped with three gratings: 150 l/mm, 600 l/mm, and 2400 l/mm. In all the measurements in this study, only grating of 2400 l/mm was used for its highest resolution of 0.2 nm. The detection of the emission signal was accomplished using an intensified charge-coupled device (ICCD) camera (Andor iStar 734D) mounted on the exit plane of spectrograph. The laser and ICCD were triggered using a delay generator (Stanford Research Systems DG645) with a resolution of 1 ps. With this time resolution, the firing time of the laser and gating time of the ICCD could be set precisely. The ICCD was connected to a computer for automatic control. The settings of the ICCD (e.g., number of accumulations, integration time, and intensifier gain) were set using Andor software. The central wavelength and grating of spectrograph were also configured using this software. In this study, a gate width of 1 ls was used to obtain the integrated signal. Moreover, the highest gain of 255 was used for a maximum signal. The emission spectrum was obtained by accumulating 5, 10, 20, 60, and 100 events for comparison. All the displayed spectra were smoothed using a nine-pixel sliding average, which was equivalent of 0.07 nm window. The instrumental resolution of 0.2 nm was well preserved. The damage to the corundum sample was monitored by measuring the craters in the samples after the laser shots. An optical microscope (Olympus) was used for this purpose, and digital micrographs were captured by the computer. An electronic shutter was used to block the laser beam energy so the pulses were stable. The electronic shutter was set to turn on for 100 ms to pass only one laser pulse to ablate the sample while the laser was running at 10 Hz. In this way, the craters could be produced consistently using a predetermined number of shots.
In this study, three synthetic samples of corundum (red, blue, and yellow; Asian Gemological Institute and Laboratory Ltd., Hong Kong) from Thailand were analyzed using the described experimental arrangement. The yellow corundum sample was treated with beryllium. The variation in the concentrations of trace impurities for minerals is large, even for the same color. The concentrations of impurities are difficult to estimate; nevertheless, the literature provides rough concentrations for saturated colors.14 Therefore, our results for analytical performance are discussed here using those references.
RESULTS AND DISCUSSION The corundum is composed of aluminum oxide (Al2O3). One of the origins for color in corundum relies on trace impurities that replace the aluminum in the crystal lattice. For example, the chromium ion Cr3þ is a basic cause of corundum’s color; this ion is called isovalent because it has the same chemical valence as the aluminum ion (Al3þ). It produces a pale pink to deep red color as the concentration increases. We first analyzed a ruby sample with a weight of 0.78 ct for Cr impurities using different numbers of laser shots at an energy of 5 mJ/pulse ( 2.5 J/cm2). For the analysis of a precious sample such as this, the requirement for nondestructiveness is extremely demanding and the number of laser shots is a limiting parameter in addition to the laser energy used. As such, the spectra were captured using several numbers of accumulations, and a suitable number of laser shots could be determined to limit the amount of damage. Figure 2 shows the spectra of the Cr 425.44, 427.48, and 428.97 nm lines obtained using 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots. To show analytical performance, the signal-to-noise ratio (SNR) was defined as the ratio of the average signal intensity after subtracting the background and noise as the fluctuation of the continuum
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FIG. 3. Spectra of the Fe 404.58, 406.36, and 407.17 nm lines using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the blue corundum sample analyzed.
background in the neighboring spectral region. The SNRs of the Cr 425.44 nm line for 5, 10, and 20 accumulations are 11, 15, and 17, respectively. As can be seen, five laser shots are adequate for a visible signal. Earlier research reported that about 2550 parts per million (ppm) Cr gives a saturated red color in corundum.14 Commonly, emission signal with an SNR of 3 is apparent; thus, we can roughly estimate about 695 and 450 ppm Cr can be detected using 5 and 20 laser shots, respectively. Other impurities have a different number of valence electrons than Al3þ. In these cases, electrons have to be transferred from one atom to another atom to maintain an electrically neutral crystal. One common impurity in corundum is iron(II) ion (Fe2þ), which produces no color
by itself. However, after a charge transfer occurs between Fe 2þ and the titanium ion (Ti4þ), strong absorptions at 580 and 700 nm result in a blue color. In this study, a blue sample with a weight of 0.56 ct was analyzed using LIBS, and the Fe and Ti emissions were monitored. Figure 3 shows the spectra of the Fe emissions at 404.58, 406.36, and 407.17 nm using a laser energy of 5 mJ/pulse ( 2.5 J/cm2) with different accumulations of laser shots. Under identical experimental conditions, the SNRs for accumulations of 5, 10, and 20 were determined to be 6.2, 11, and 16, respectively. Subsequently, Ti emissions at the 499.11, 499.95, 500.72, 501.42, and 502.00 nm lines were also acquired (shown in Fig. 4). However, a higher laser energy at 6 mJ/pulse ( 3 J/cm2) and more laser shots
FIG. 4. Spectra of the Ti 499.11, 499.95, 500.72, 501.42, and 502.00 nm lines using LIBS with 60 (lower trace) and 100 (upper trace) laser shots at a pulse energy of 6 mJ. Inset shows a photograph of the blue corundum sample analyzed.
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FIG. 5. Spectra of the Fe 404.58, 406.36, and 407.17 nm lines using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
had to be used for the detection of the Ti emission lines. The SNRs for the Ti 499.11 nm line for 60 and 100 accumulations were calculated to be 5.0 and 9.2, respectively. A concentration of 50 parts per million atomic (ppma) Fe2þ-Ti4þ pairs will produce a saturated deep blue color,14 and this is equivalent to [Fe] of 137 ppm and [Ti] of 117 ppm. As such, we determined that concentrations of Fe of 66 and 26 ppm can be detected using 5 and 20 laser shots, respectively. For Ti, concentrations of 70 and 38 ppm can be found using 60 and 100 laser shots, respectively. We can see that a higher pulse energy and more laser shots were required for the analysis of Ti. The sensitivity of LIBS for detecting Ti is lower because the detected Ti lines are hard transitions compared to Fe lines. Other impurities, magnesium ion (Mg2þ) and iron(III) ion (Fe3þ) are speculated to be in yellow corundum.14 The charge compensation of Mg2þ appears to be in holes when grown or heated under oxidizing conditions. The Mg2þ-induced trapped hole absorbs light very strongly in the blue region of the spectrum, giving a yellow to orange–yellow color. Simultaneously, a high concentration of Fe3þ can cause a weak yellow coloration. With this in mind, we analyzed a yellow corundum sample of weight 0.80 ct for Mg and Fe. Figure 5 shows the spectra of the Fe emissions at the 404.58, 406.36, and 407.17 nm lines from LIBS analysis of the yellow sample using 5 (lower trace), 10 (middle trace), and 20 (upper trace) accumulations at a laser pulse energy of 5 mJ. The SNRs for the 404.58 nm line were calculated to be 6.5, 8.9, and 11, respectively. Earlier studies reported that about 6850 parts per million Fe3þ produces a saturated yellow color.14 Our results imply that Fe can be detected in corundum at a level of 3160 ppm using 5 laser shots and at a level of 1834 ppm using 20 shots. This LIBS sensitivity for detecting Fe is lower than in the analysis of the blue corundum sample. We can attribute this discrepancy to many causes. For example, Fe3þ is not the unique cause of yellow coloration in corundum, and
the [Fe3þ] is only a rough estimate of the saturated yellow color. As such, there is no direct relation between the analyses of iron in the yellow and blue corundum samples. Subsequently, we analyzed the same yellow corundum for Mg emissions. Figure 6 shows the spectra of the Mg 285.21 nm line at a pulse energy of 6 mJ for 5 (lower trace), 10 (middle trace), and 20 (upper trace) acclamations of laser shots; their SNRs are 4.0, 5.1, and 7.0, respectively. Some previous analyses showed that 17.9 ppm Mg can cause a saturated yellow color.14 Thus, a [Mg] of 13 and 4.7 ppm can be assessed using 5 and 20 laser shots, respectively. From these results, we can see the relationship between corundum coloration and trace elements. Thus, color modifications can be accomplished by forcing a particular element into the sample. In this study, the yellow corundum sample was treated with a beryllium diffusion, and the presence of beryllium in corundum can cause a color modification. As shown in earlier studies, the beryllium ion (Be2þ) positions itself in the corundum lattice by replacing an Al3þ. It then acts much like Mg2þ and produces a strong yellow color. Therefore, we also tested for beryllium in this yellow sample target. Figure 7 shows the emission spectra of the Be 313.04 nm line. At a laser energy of 5 mJ and 5, 10, and 20 laser shots, the SNRs obtained are 10, 14 and 19, respectively. The analysis of the yellow corundum sample shows that the coloration may also be ascribed to Be2þ. Some previous analyses showed that 15.5 ppm Be can result in a wide variety of colors in corundum.14 Therefore, the [Be] of 4.7 and 2.5 ppm can be determined using 5 and 20 laser shots, respectively. The results for the SNRs for the different impurities found when analyzing red, yellow, and blue corundum are shown in Table I. We can see that all the SNRs are higher than 3 and that the emission signals are visible when sampling using only five laser shots, except for Ti. Moreover, all the analytes were analyzed using a laser pulse energy of 5 or 6 mJ. Although this study is
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FIG. 6. Spectra of the Mg 285.21 nm line using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 6 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
qualitative, our analytical results can be considered along with the effective impurity concentrations for color saturation in earlier research. We found that in corundum levels of a few parts per million of Be; tens of parts per million of Mg, Fe, and Ti;, and hundreds of parts per million of Cr can be detected using LIBS. These results reveal the usefulness of LIBS in gemology. In addition to our analytical results, we studied the damage to the sample due to the laser analysis using an optical microscope; this is important in the study of precious samples. To align the part of the study with the previous measurements, we used a laser pulse energy of 5 mJ and took photographs of the damage to the sample after 10–300 laser shots. The images are
displayed in Fig. 8. We took repeated measurements to determine the amount of damage to the sample , and they were found to be reproducible. The sizes of the crater in the sample are summarized in Table II. Acute damage to the sample can be clearly seen after 300 laser shots. There is evidence of melting and mixing of the surface layer. Moreover, the redistribution of impurities occurred. After 150 laser shots, damage to the sample is still noticeable, but the melting of the surface is not as obvious as after 300 shots. Images were also taken after fewer laser shots. In this study, the diameter of smallest measurable crater was 30 lm, produced using 10 laser shots. This result is smaller than the size of a human hair, which normally ranges from 40 to 120 lm. Recall
FIG. 7. Spectra of the Be 313.11 nm line using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
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TABLE I. SNRs of emission signals for different elements from red, blue, and yellow corundum samples using various numbers of laser shots.
TABLE II. Size of the crater on the corundum sample after LIBS analysis using various numbers of laser shots. Number of shots
Element
Energy per pulse (mJ)
Number of shots
SNR
Red
Cr
5
Yellow
Fe
5
5 10 20 5 10 20 5 10 20 5 10 20 5 10 20 60 100
11 15 17 6.5 8.9 11 10 14 19 4.0 5.1 7.0 6.2 11 16 5.0 9.2
Corundum sample
Be
Blue
Mg
6
Fe
5
Ti
6
that our findings indicate that most of the analytes were detectable using a pulse energy of 5 mJ and only five laser shots. This suggests that the damage to corundum due to LIBS analysis is extremely minimal and almost invisible. Damage to sample gem minerals was also monitored in previous studies using LIBS. In these studies, however, a higher laser energy of 100 mJ/pulse and 20 laser shots were adopted for the analyses of corundum, carbonates, silicates, and beryl, resulting in a crater diameter of about 100 lm.18,21,22 This size crater would be obvious to the human eye, and therefore using LIBS with these parameters is not beneficial for the analysis of costly gem material. In contrast to that previous work, we have demonstrated that it is possible to obtain an exceptional LIBS analysis of corundum with negligible sample damage.
FIG. 8. Photographs of sample damage after LIBS analysis at laser energy of 5 mJ with 10, 20, 50, 100, 150, and 300 laser shots.
10 20 50 100 150 300
Diameter of crater (lm) 30 40 50 60 80 100
CONCLUSION In this study, impurities that cause coloration in corundum were analyzed using LIBS. Emission signals from various analytes were found in red, blue, and yellow corundum samples. Of these analytes, Be can lead to a modification from the natural color, and thus it has attracted a great deal of interest for analysis. We detected Cr, Fe, Ti, Mg, and Be using different numbers of laser shots. Then, using an optical microscope, we captured the images of the craters made at a laser pulse energy of 5 mJ to determine the amount of damage to a sample. Even when such a weak pulse energy is used, impurities produce noticeable emissions and can be detected. We found that, after laser analysis at this energy, the damage to the sample is slight and almost imperceptible. We conclude that LIBS is a practical and sensitive technique for the analysis of impurities in corundum that cause multiple colors. ACKNOWLEDGMENTS The author acknowledges the Asian Gemmological Institute and Laboratory Limited, Hong Kong, for providing the corundum samples and Mr. Leung Chung Hong for his technical assistance in this study. This work was supported by the Science and Technology Development Fund of Macao SAR under grant number 045/2014/A1. 1. J.E. Shigley. ‘‘A Review of Current Challenges for the Identification of Gemstones’’. Geologija. 2008. 50(4): 227-236. 2. C.M. Breeding, A.H. Shen, S. Eaton-Magana, G.R. Rossman, J.E. Shigley, A. Gilbertson. ‘‘Developments in Gemstone Analysis Techniques and Instrumentation During the 2000s’’. Gems Gemol. 2010. 46(3): 241-257. 3. B. Devouard, F. Notari. ‘‘The Identification of Faceted Gemstones: From Naked Eye to Laboratory Techniques’’. Elements. 2009. 5(3): 163-168. 4. T. Hamschwang. ‘‘The Uses, Potential, and Risks of Analytical Equipment in Gemology’’. InColor. 2010. (13): 34-42. 5. C. Zhou, A. Homkrajae, J. Wing Yan Ho, A. Hyatt, N. Sturman. ‘‘Update on the Identification of Dye Treatment in Yellow or ‘Golden’ Cultured Pearls’’. Gems. Gemol. 2012. 48(4): 284-291. 6. Y. Kim, H. Choi, B. Lee, A. Abduriyim. ‘‘Identification of Irradiated South Sea Cultured Pearls Using Electron Spin Resonance Spectroscopy’’. Gems. Gemol. 2012. 48(4): 292-299. 7. J.E. Shigley. ‘‘High-Pressure-High-Temperature Treatment of Gem Diamonds’’. Elements. 2005. 1(2): 101-104. 8. C.P. Smith, G. Bosshart, S. Graeser. ‘‘Poudretteite: A Rare Gem Species from the Mogok Valley’’. Gems. Gemol. 2003. 39(1): 24-31. 9. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions’’. Gems Gemol. 1987. 23(3): 126-139. 10. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers’’. Gems Gemol. 1988. 24(1): 3-15. 11. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 3: Colors Caused by Band Gaps and Physical Phenomena’’. Gems Gemol. 1988. 24(2): 81-102.
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12. T.W. Overton, J.E. Shigley. ‘‘A History of Diamond Treatments’’. Gems Gemol. 2008. 44(1): 32-55. 13. M.S. Krzemnicki, H.A. Hanni, R.A. Walters. ‘‘A New Method for Detecting Be Diffusion–Treated Sapphires: Laser-Induced Breakdown Spectroscopy (LIBS)’’. Gems Gemol. 2004. 40(4): 314-322. 14. J.L. Emmett, K. Scarratt, S.F. McClure, T. Moses, T.R. Douthit, R. Hughes, S. Novak, J.E. Shigley, W. Wang, O. Bordelon, R.E. Kane. ‘‘Beryllium Diffusion of Ruby and Sapphire’’. Gems Gemol. 2003. 39(2): 84-135. 15. C. Streli, P. Kregsamer, P. Wobrauschek. ‘‘Low Z Total Reflection Xray Fluorescence Analysis—Challenges and Answers’’. Spectrochim. Acta, Part B. 1999. 54(10): 1433-1441. 16. A. Abduriyim, H. Kitawaki. ‘‘Applications of Laser AblationInductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to Gemology’’. Gems Gemol. 2006. 42(2): 98-118. 17. A. Ulianov, O. Muentener, U. Schaltegger. . ‘‘The Data Treatment Dependent Variability of U-Pb Zircon Ages Obtained Using MonoCollector, Sector Field, Laser Ablation ICPMS’’. J. Anal. Atom. Spectrom. 2012. 27(4): 663-676. 18. C.J. Garrido, J.L. Bodinier, O. Alard. ‘‘Incompatible Trace Element Partitioning and Residence in Anhydrous Spinel Peridotites and Websterites from the Ronda Orogenic Peridotite’’. Earth Planet. Sc. Lett. 2000. 181(3): 341-358. 19. P. Hoppe, S. Cohen, A. Meibom. . ‘‘NanoSIMS: Technical Aspects and Applications in Cosmochemistry and Biological Geochemistry’’. Geostand. Geoanal. Res. 2013. 37(2): 111-154. 20. M. Koch-Muller, S.S. Matsyuk, D. Rhede, R. Wirth, N. Khisina. ‘‘Hydroxyl in Mantle Olivine Xenocrysts from the Udachnaya Kimberlite Pipe’’. Phys. Chem. Miner. 2006. 33(4): 276-287. 21. N.J. McMillan, C.E. McManus, R.S. Harmon, F.C. De Lucia Jr., W. Andrzej. ‘‘Miziolek. ‘‘Laser-Induced Breakdown Spectroscopy Analysis of Complex Silicate Minerals—Beryl’’. Anal. Bioanal. Chem. 2006. 385(2): 263-271.
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22. N.J. McMillan, R.S. Harmon, F.C. De Lucia, A.M. Miziolek. ‘‘LaserInduced Breakdown Spectroscopy Analysis of Minerals: Carbonates and Silicates’’. Spectrochim. Acta, Part B. 2007. 62(12): 15281536. 23. A. De Giacomo, M. Dell’Aglio, R. Gaudiuso, A. Santagata, G.S. Senesi, M. Rossi, M.R. Ghiara, F. Capitelli, O. De Pascale. . ‘‘A Laser Induced Breakdown Spectroscopy Application Based on Local Thermodynamic Equilibrium Assumption for the Elemental Analysis of Alexandrite Gemstone and Copper-Based Alloys’’. Chem. Phys. 2012. 398: 233-238. 24. L.E. Garcı´ a-Ayuso, J. Amador-Herna´ndez, J.M. Ferna´ndez-Romero, M.D. Luque de Castro. ‘‘Characterization of Jewellery Products by Laser-Induced Breakdown Spectroscopy’’. Anal. Chim. Acta. 2002. 457(2): 247-256. 25. A. Jurado-Lopex, M.D. Luque de Castro. ‘‘Rank Correlation of Laser-Induced Breakdown Spectroscopic Data for the Identification of Alloys Used in Jewelry Manufacture’’. Spectrochim. Acta, Part B. 2003. 58(7): 1291-1299. 26. D.C. Alvey, K. Morton, R.S. Harmon, J.L. Gottfried, J.J. Remus, L.M. Collins, M.A. Wise. ‘‘Laser-Induced Breakdown SpectroscopyBased Geochemical Fingerprinting for the Rapid Analysis and Discrimination of Minerals: The Example of Garnet’’. Appl. Optics. 2010. 49(13): C168-C180. 27. K.A. Yetter, N.J. McMillan. ‘‘Provenance of Gem Corundum: A Global LIBS Study’’. Geochim. Cosmochim. Acta. 2010. 74(12 Suppl 1): A1185. 28. M.S. Krzemnicki, T. Pettke, H.A. Ha¨nni. ‘‘Perspectives of LIBS in Gemstone Testing. Analysis of Light Elements Such As Beryllium, Boron, Lithium’’. Poster presented at: 1st GIT International Gem & Jewelry Conference 2006. Chanthaburi, Thailand; December 5-9, 2006.
Corundum was analyzed using laser-induced breakdown spectroscopy (LIBS) for impurity effects in their multiple colorings. Qualitative measurements were attained for impurities of chromium, magnesium, iron, and titanium in red, yellow, and blue samples. Moreover, treatment with a beryllium diffusion, which can modify corundum to obtain an attractive color, was tested in the yellow sample. In this work, most of the measurements were acquired using a laser pulse energy of 5 mJ and impurity emissions were appreciable. The signal-to-noise ratios were 11, 6.5, 10, and 4 for the Cr 425.44 nm, Fe 404.58 nm, Be 313.04 nm, and Mg 285.21 nm lines, respectively, for five laser shots. The amount of damage to the corundum samples was also monitored by measuring the craters after laser analysis. It was found that the crater size was about 30 lm after 10 laser shots. As such, the damage to corundum sample is almost imperceptible after the LIBS analysis. Index Headings: Laser-induced breakdown spectroscopy; Corundum; Beryllium diffusion; Destructiveness.
INTRODUCTION Accurate gemstone identification is significant in jewelry trade to retain the commercial value of natural gemstones. Therefore, there has been an urgent need for powerful analytical instruments for distinguishing natural gemstones from synthetic and treated ones.1–3 For decades, refractometers, polariscopes, microscopes, and hand spectroscopes were used for the identification of most gem materials. Later, more sophisticated tools, such as infrared and ultraviolet visible spectrometers, X-ray fluorescence (XRF), and Raman instruments were developed to meet the analytical needs of gemological laboratories.4–8 The coloration of a natural gemstone is a prime factor determining its quality and market value. There are multiple origins of color in gem minerals that have been explored in recent years.9–11 Basically, five mechanisms have been recognized as causes of color in gemstones. First, one of the causes is dispersed metal ions in gem materials. Light illumination of dispersed metal ions can cause absorption in certain wavelength ranges of the light source and cause electrons jump to an unstable higher state; luminescence is emitted as energy released from the electrons and causes the color of stone. In fact, the identity of the ion is not the only factor that can affect the color; other factors such as the valence state, nature of neighboring atoms, and ion coordination Received 25 September 2013; accepted 5 August 2014. E-mail: [email protected]. DOI: 10.1366/13-07304
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also play roles in stone color. Second, absorption occurs when electrons jump to another atom, and this charge transfer process leads to coloration in stone. This happens when there is a charge transfer from oxygen to the metal and an intervalence charge transfer between two metal ions. Third, irradiation can affect the metal ions and change their oxidation state, inducing color centers. A color center is a defect, such as missing atoms or additional atoms, and causes absorption. Fourth, apart from transitions between energy levels, absorption can also be occur due to transitions between energy bands, and they can be responsible for color. Last, phenomena such as interference, diffraction, scattering, and the presence of colored inclusions can affect the color of gem minerals. The treatment of natural gemstones to improve their color or appearance traces back thousands of years.12 The most common treatment methods that involve a change of color are exposure to heat or radiation. Other, less common treatments include surface coatings, dyeing, and diffusion. In 2002, the diffusion of beryllium into corundum to modify the color was developed in Thailand.13–14 This treatment changes corundum to more graceful hues, usually yellow or orange to orangey red; this color is rare and valuable in untreated corundum. Therefore, the ability to detect beryllium diffusion has become indispensable in corundum with an unusual color. However, the detection of beryllium diffusion in corundum has proved to be difficult using the traditional nondestructive analytical methods, such as XRF, because they cannot detect light elements such as beryllium.15 Detection requires highly sophisticated methods such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). Instrumentation for LA-ICP-MS includes a laser ablation unit for removing a small amount sample of the stone. The vaporized materials are then sent to a mass spectrometer for analysis with a carrier gas. This method can detect almost all chemical elements within detection limits in the range of parts per million to even parts per billion.2,16–18 In SIMS, a beam of oxygen ions is focused on the sample in an ultra-high vacuum environment. Ions are knocked off the surface of sample and transferred to a mass spectrometer. This is a powerful method for the analysis of most elements with a sensitivity in the range of parts per billion to parts per trillion.2,19,20 Both mass spectrometer techniques are slightly destructive for analysis. Also, the instrumentation for these two analytical methods is quite expensive. The costs for LA-ICP-MS range from US$20 000 to $500 000, and the costs for SIMS may run over US$2
0003-7028/15/6902-0269/0 Q 2015 Society for Applied Spectroscopy
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FIG. 1. Schematic setup.
million. Furthermore, special sample preparation and a vacuum environment are required, so gemological laboratories are rarely equipped to use these techniques. As a result, more affordable techniques have to be considered. Since 2003, laser-induced breakdown spectroscopy (LIBS) has been explored for its potential application to gemstone analysis.13,21–28 This technique is also slightly destructive because it uses laser ablation. Still, it is a welcome alternative for corundum analysis because of its simplicity, low cost, lack of required sample preparation, and quick analysis. As a result, earlier studies were devoted to the application of LIBS to the analysis of gemstones. Garcia-Ayuso et al. used it to characterize the most common noble metals as well as other metals present in jewelry pieces.24 JuradoLopex and Luque de Castro identified alloys used in the manufacture of jewelry pieces.25 McMillan et al. developed LIBS for the analysis of gem beryl21 and analyzed 52 mineral samples using LIBS and identified them in a database of LIBS composite spectra.22 De Giacomo et al. presented the analysis of gemstone alexandrite.23 Alvey et al. indicated that LIBS analysis can be used to discriminate garnets of different composition and has the potential to discern geographical origin.26 Yetter and McMillan applied LIBS to the analysis of rubies and sapphires from three general tectonic regions for comparison.27 Krzemnicki and coworkers detected boron, lithium, and beryllium in corundum.13,28 Different aspects of the application of LIBS in gemology have been investigated. As we have seen, earlier studies analyzed some light elements in corundum, and the emphases of these studies were in the detection of treated gemstones and the determination of gemstone origins.28 Nonetheless, studies of the effect of impurities on the color of corundum are still needed. The coloration of gemstones is a considerable index for setting of their market price. As explained, the coloration in gem materials can have different origins. In this study, a qualitative analysis was
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conducted for the effects of dispersed metal ions and ion pairs for change transfer between atoms in corundum. For example, a trace amount of chromium in corundum is the chromophore that is responsible for red color, and its color saturation depends on concentration. In this study, corundum samples of different colors were analyzed and chromium, magnesium, iron, and titanium were detected. As already mentioned, the detection of color modification in corundum contributed by beryllium diffusion is of much concern in the gemological market recently. In this study, we demonstrate using LIBS for the detection of beryllium and treatment with beryllium diffusion in one of the samples to provide evidence of the practical use of LIBS in gemology. We performed the analysis to detect various impurities using different numbers of laser shots. In addition, some previous studies of gemstones reported that visible craters were created on the sample targets as a result of the analyses.13,21,22 Because the destruction of the sample during analysis is critically important for precious stones, in this study the sample surface was examined after the LIBS analysis after different numbers of laser shots to demonstrate the minimal destructiveness of LIBS. The study shows that optimal experimental conditions can be identified to obtain noticeable emissions with indiscernible damage to the samples.
EXPERIMENTAL The experimental setup is shown schematically in Fig. 1. The neodymium-doped yttrium aluminum garnet (Nd : YAG) laser (Spectra-Physics) can provide energy per pulse as high as 450 mJ at 1064 nm and with a repetition rate of 10 Hz. The Q-switch operating mode of the laser produced pulses of approximately 6 ns. Before the ablation of the sample using the laser, we focused the beam to a small laser spot to provide a high laser energy fluence. After ablation, we imaged the luminous plume onto the entrance slit of a 0.5 m spectrograph (Dongwoo 500i) using collection optics of fused silica with a focal
FIG. 2. Spectra of the Cr 425.44, 427.48, and 428.97 nm lines using LIBS analysis with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the red corundum sample analyzed.
length of 100 mm. The slit width was set at 200 lm, giving an overall spectral resolution of 0.2 nm. The spectrograph was equipped with three gratings: 150 l/mm, 600 l/mm, and 2400 l/mm. In all the measurements in this study, only grating of 2400 l/mm was used for its highest resolution of 0.2 nm. The detection of the emission signal was accomplished using an intensified charge-coupled device (ICCD) camera (Andor iStar 734D) mounted on the exit plane of spectrograph. The laser and ICCD were triggered using a delay generator (Stanford Research Systems DG645) with a resolution of 1 ps. With this time resolution, the firing time of the laser and gating time of the ICCD could be set precisely. The ICCD was connected to a computer for automatic control. The settings of the ICCD (e.g., number of accumulations, integration time, and intensifier gain) were set using Andor software. The central wavelength and grating of spectrograph were also configured using this software. In this study, a gate width of 1 ls was used to obtain the integrated signal. Moreover, the highest gain of 255 was used for a maximum signal. The emission spectrum was obtained by accumulating 5, 10, 20, 60, and 100 events for comparison. All the displayed spectra were smoothed using a nine-pixel sliding average, which was equivalent of 0.07 nm window. The instrumental resolution of 0.2 nm was well preserved. The damage to the corundum sample was monitored by measuring the craters in the samples after the laser shots. An optical microscope (Olympus) was used for this purpose, and digital micrographs were captured by the computer. An electronic shutter was used to block the laser beam energy so the pulses were stable. The electronic shutter was set to turn on for 100 ms to pass only one laser pulse to ablate the sample while the laser was running at 10 Hz. In this way, the craters could be produced consistently using a predetermined number of shots.
In this study, three synthetic samples of corundum (red, blue, and yellow; Asian Gemological Institute and Laboratory Ltd., Hong Kong) from Thailand were analyzed using the described experimental arrangement. The yellow corundum sample was treated with beryllium. The variation in the concentrations of trace impurities for minerals is large, even for the same color. The concentrations of impurities are difficult to estimate; nevertheless, the literature provides rough concentrations for saturated colors.14 Therefore, our results for analytical performance are discussed here using those references.
RESULTS AND DISCUSSION The corundum is composed of aluminum oxide (Al2O3). One of the origins for color in corundum relies on trace impurities that replace the aluminum in the crystal lattice. For example, the chromium ion Cr3þ is a basic cause of corundum’s color; this ion is called isovalent because it has the same chemical valence as the aluminum ion (Al3þ). It produces a pale pink to deep red color as the concentration increases. We first analyzed a ruby sample with a weight of 0.78 ct for Cr impurities using different numbers of laser shots at an energy of 5 mJ/pulse ( 2.5 J/cm2). For the analysis of a precious sample such as this, the requirement for nondestructiveness is extremely demanding and the number of laser shots is a limiting parameter in addition to the laser energy used. As such, the spectra were captured using several numbers of accumulations, and a suitable number of laser shots could be determined to limit the amount of damage. Figure 2 shows the spectra of the Cr 425.44, 427.48, and 428.97 nm lines obtained using 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots. To show analytical performance, the signal-to-noise ratio (SNR) was defined as the ratio of the average signal intensity after subtracting the background and noise as the fluctuation of the continuum
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FIG. 3. Spectra of the Fe 404.58, 406.36, and 407.17 nm lines using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the blue corundum sample analyzed.
background in the neighboring spectral region. The SNRs of the Cr 425.44 nm line for 5, 10, and 20 accumulations are 11, 15, and 17, respectively. As can be seen, five laser shots are adequate for a visible signal. Earlier research reported that about 2550 parts per million (ppm) Cr gives a saturated red color in corundum.14 Commonly, emission signal with an SNR of 3 is apparent; thus, we can roughly estimate about 695 and 450 ppm Cr can be detected using 5 and 20 laser shots, respectively. Other impurities have a different number of valence electrons than Al3þ. In these cases, electrons have to be transferred from one atom to another atom to maintain an electrically neutral crystal. One common impurity in corundum is iron(II) ion (Fe2þ), which produces no color
by itself. However, after a charge transfer occurs between Fe 2þ and the titanium ion (Ti4þ), strong absorptions at 580 and 700 nm result in a blue color. In this study, a blue sample with a weight of 0.56 ct was analyzed using LIBS, and the Fe and Ti emissions were monitored. Figure 3 shows the spectra of the Fe emissions at 404.58, 406.36, and 407.17 nm using a laser energy of 5 mJ/pulse ( 2.5 J/cm2) with different accumulations of laser shots. Under identical experimental conditions, the SNRs for accumulations of 5, 10, and 20 were determined to be 6.2, 11, and 16, respectively. Subsequently, Ti emissions at the 499.11, 499.95, 500.72, 501.42, and 502.00 nm lines were also acquired (shown in Fig. 4). However, a higher laser energy at 6 mJ/pulse ( 3 J/cm2) and more laser shots
FIG. 4. Spectra of the Ti 499.11, 499.95, 500.72, 501.42, and 502.00 nm lines using LIBS with 60 (lower trace) and 100 (upper trace) laser shots at a pulse energy of 6 mJ. Inset shows a photograph of the blue corundum sample analyzed.
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FIG. 5. Spectra of the Fe 404.58, 406.36, and 407.17 nm lines using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
had to be used for the detection of the Ti emission lines. The SNRs for the Ti 499.11 nm line for 60 and 100 accumulations were calculated to be 5.0 and 9.2, respectively. A concentration of 50 parts per million atomic (ppma) Fe2þ-Ti4þ pairs will produce a saturated deep blue color,14 and this is equivalent to [Fe] of 137 ppm and [Ti] of 117 ppm. As such, we determined that concentrations of Fe of 66 and 26 ppm can be detected using 5 and 20 laser shots, respectively. For Ti, concentrations of 70 and 38 ppm can be found using 60 and 100 laser shots, respectively. We can see that a higher pulse energy and more laser shots were required for the analysis of Ti. The sensitivity of LIBS for detecting Ti is lower because the detected Ti lines are hard transitions compared to Fe lines. Other impurities, magnesium ion (Mg2þ) and iron(III) ion (Fe3þ) are speculated to be in yellow corundum.14 The charge compensation of Mg2þ appears to be in holes when grown or heated under oxidizing conditions. The Mg2þ-induced trapped hole absorbs light very strongly in the blue region of the spectrum, giving a yellow to orange–yellow color. Simultaneously, a high concentration of Fe3þ can cause a weak yellow coloration. With this in mind, we analyzed a yellow corundum sample of weight 0.80 ct for Mg and Fe. Figure 5 shows the spectra of the Fe emissions at the 404.58, 406.36, and 407.17 nm lines from LIBS analysis of the yellow sample using 5 (lower trace), 10 (middle trace), and 20 (upper trace) accumulations at a laser pulse energy of 5 mJ. The SNRs for the 404.58 nm line were calculated to be 6.5, 8.9, and 11, respectively. Earlier studies reported that about 6850 parts per million Fe3þ produces a saturated yellow color.14 Our results imply that Fe can be detected in corundum at a level of 3160 ppm using 5 laser shots and at a level of 1834 ppm using 20 shots. This LIBS sensitivity for detecting Fe is lower than in the analysis of the blue corundum sample. We can attribute this discrepancy to many causes. For example, Fe3þ is not the unique cause of yellow coloration in corundum, and
the [Fe3þ] is only a rough estimate of the saturated yellow color. As such, there is no direct relation between the analyses of iron in the yellow and blue corundum samples. Subsequently, we analyzed the same yellow corundum for Mg emissions. Figure 6 shows the spectra of the Mg 285.21 nm line at a pulse energy of 6 mJ for 5 (lower trace), 10 (middle trace), and 20 (upper trace) acclamations of laser shots; their SNRs are 4.0, 5.1, and 7.0, respectively. Some previous analyses showed that 17.9 ppm Mg can cause a saturated yellow color.14 Thus, a [Mg] of 13 and 4.7 ppm can be assessed using 5 and 20 laser shots, respectively. From these results, we can see the relationship between corundum coloration and trace elements. Thus, color modifications can be accomplished by forcing a particular element into the sample. In this study, the yellow corundum sample was treated with a beryllium diffusion, and the presence of beryllium in corundum can cause a color modification. As shown in earlier studies, the beryllium ion (Be2þ) positions itself in the corundum lattice by replacing an Al3þ. It then acts much like Mg2þ and produces a strong yellow color. Therefore, we also tested for beryllium in this yellow sample target. Figure 7 shows the emission spectra of the Be 313.04 nm line. At a laser energy of 5 mJ and 5, 10, and 20 laser shots, the SNRs obtained are 10, 14 and 19, respectively. The analysis of the yellow corundum sample shows that the coloration may also be ascribed to Be2þ. Some previous analyses showed that 15.5 ppm Be can result in a wide variety of colors in corundum.14 Therefore, the [Be] of 4.7 and 2.5 ppm can be determined using 5 and 20 laser shots, respectively. The results for the SNRs for the different impurities found when analyzing red, yellow, and blue corundum are shown in Table I. We can see that all the SNRs are higher than 3 and that the emission signals are visible when sampling using only five laser shots, except for Ti. Moreover, all the analytes were analyzed using a laser pulse energy of 5 or 6 mJ. Although this study is
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FIG. 6. Spectra of the Mg 285.21 nm line using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 6 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
qualitative, our analytical results can be considered along with the effective impurity concentrations for color saturation in earlier research. We found that in corundum levels of a few parts per million of Be; tens of parts per million of Mg, Fe, and Ti;, and hundreds of parts per million of Cr can be detected using LIBS. These results reveal the usefulness of LIBS in gemology. In addition to our analytical results, we studied the damage to the sample due to the laser analysis using an optical microscope; this is important in the study of precious samples. To align the part of the study with the previous measurements, we used a laser pulse energy of 5 mJ and took photographs of the damage to the sample after 10–300 laser shots. The images are
displayed in Fig. 8. We took repeated measurements to determine the amount of damage to the sample , and they were found to be reproducible. The sizes of the crater in the sample are summarized in Table II. Acute damage to the sample can be clearly seen after 300 laser shots. There is evidence of melting and mixing of the surface layer. Moreover, the redistribution of impurities occurred. After 150 laser shots, damage to the sample is still noticeable, but the melting of the surface is not as obvious as after 300 shots. Images were also taken after fewer laser shots. In this study, the diameter of smallest measurable crater was 30 lm, produced using 10 laser shots. This result is smaller than the size of a human hair, which normally ranges from 40 to 120 lm. Recall
FIG. 7. Spectra of the Be 313.11 nm line using LIBS with 5 (lower trace), 10 (middle trace), and 20 (upper trace) laser shots at a pulse energy of 5 mJ. Inset shows a photograph of the yellow corundum sample analyzed.
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TABLE I. SNRs of emission signals for different elements from red, blue, and yellow corundum samples using various numbers of laser shots.
TABLE II. Size of the crater on the corundum sample after LIBS analysis using various numbers of laser shots. Number of shots
Element
Energy per pulse (mJ)
Number of shots
SNR
Red
Cr
5
Yellow
Fe
5
5 10 20 5 10 20 5 10 20 5 10 20 5 10 20 60 100
11 15 17 6.5 8.9 11 10 14 19 4.0 5.1 7.0 6.2 11 16 5.0 9.2
Corundum sample
Be
Blue
Mg
6
Fe
5
Ti
6
that our findings indicate that most of the analytes were detectable using a pulse energy of 5 mJ and only five laser shots. This suggests that the damage to corundum due to LIBS analysis is extremely minimal and almost invisible. Damage to sample gem minerals was also monitored in previous studies using LIBS. In these studies, however, a higher laser energy of 100 mJ/pulse and 20 laser shots were adopted for the analyses of corundum, carbonates, silicates, and beryl, resulting in a crater diameter of about 100 lm.18,21,22 This size crater would be obvious to the human eye, and therefore using LIBS with these parameters is not beneficial for the analysis of costly gem material. In contrast to that previous work, we have demonstrated that it is possible to obtain an exceptional LIBS analysis of corundum with negligible sample damage.
FIG. 8. Photographs of sample damage after LIBS analysis at laser energy of 5 mJ with 10, 20, 50, 100, 150, and 300 laser shots.
10 20 50 100 150 300
Diameter of crater (lm) 30 40 50 60 80 100
CONCLUSION In this study, impurities that cause coloration in corundum were analyzed using LIBS. Emission signals from various analytes were found in red, blue, and yellow corundum samples. Of these analytes, Be can lead to a modification from the natural color, and thus it has attracted a great deal of interest for analysis. We detected Cr, Fe, Ti, Mg, and Be using different numbers of laser shots. Then, using an optical microscope, we captured the images of the craters made at a laser pulse energy of 5 mJ to determine the amount of damage to a sample. Even when such a weak pulse energy is used, impurities produce noticeable emissions and can be detected. We found that, after laser analysis at this energy, the damage to the sample is slight and almost imperceptible. We conclude that LIBS is a practical and sensitive technique for the analysis of impurities in corundum that cause multiple colors. ACKNOWLEDGMENTS The author acknowledges the Asian Gemmological Institute and Laboratory Limited, Hong Kong, for providing the corundum samples and Mr. Leung Chung Hong for his technical assistance in this study. This work was supported by the Science and Technology Development Fund of Macao SAR under grant number 045/2014/A1. 1. J.E. Shigley. ‘‘A Review of Current Challenges for the Identification of Gemstones’’. Geologija. 2008. 50(4): 227-236. 2. C.M. Breeding, A.H. Shen, S. Eaton-Magana, G.R. Rossman, J.E. Shigley, A. Gilbertson. ‘‘Developments in Gemstone Analysis Techniques and Instrumentation During the 2000s’’. Gems Gemol. 2010. 46(3): 241-257. 3. B. Devouard, F. Notari. ‘‘The Identification of Faceted Gemstones: From Naked Eye to Laboratory Techniques’’. Elements. 2009. 5(3): 163-168. 4. T. Hamschwang. ‘‘The Uses, Potential, and Risks of Analytical Equipment in Gemology’’. InColor. 2010. (13): 34-42. 5. C. Zhou, A. Homkrajae, J. Wing Yan Ho, A. Hyatt, N. Sturman. ‘‘Update on the Identification of Dye Treatment in Yellow or ‘Golden’ Cultured Pearls’’. Gems. Gemol. 2012. 48(4): 284-291. 6. Y. Kim, H. Choi, B. Lee, A. Abduriyim. ‘‘Identification of Irradiated South Sea Cultured Pearls Using Electron Spin Resonance Spectroscopy’’. Gems. Gemol. 2012. 48(4): 292-299. 7. J.E. Shigley. ‘‘High-Pressure-High-Temperature Treatment of Gem Diamonds’’. Elements. 2005. 1(2): 101-104. 8. C.P. Smith, G. Bosshart, S. Graeser. ‘‘Poudretteite: A Rare Gem Species from the Mogok Valley’’. Gems. Gemol. 2003. 39(1): 24-31. 9. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions’’. Gems Gemol. 1987. 23(3): 126-139. 10. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers’’. Gems Gemol. 1988. 24(1): 3-15. 11. E. Fritsch, G.R. Rossman. ‘‘An Update on Color in Gems. Part 3: Colors Caused by Band Gaps and Physical Phenomena’’. Gems Gemol. 1988. 24(2): 81-102.
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12. T.W. Overton, J.E. Shigley. ‘‘A History of Diamond Treatments’’. Gems Gemol. 2008. 44(1): 32-55. 13. M.S. Krzemnicki, H.A. Hanni, R.A. Walters. ‘‘A New Method for Detecting Be Diffusion–Treated Sapphires: Laser-Induced Breakdown Spectroscopy (LIBS)’’. Gems Gemol. 2004. 40(4): 314-322. 14. J.L. Emmett, K. Scarratt, S.F. McClure, T. Moses, T.R. Douthit, R. Hughes, S. Novak, J.E. Shigley, W. Wang, O. Bordelon, R.E. Kane. ‘‘Beryllium Diffusion of Ruby and Sapphire’’. Gems Gemol. 2003. 39(2): 84-135. 15. C. Streli, P. Kregsamer, P. Wobrauschek. ‘‘Low Z Total Reflection Xray Fluorescence Analysis—Challenges and Answers’’. Spectrochim. Acta, Part B. 1999. 54(10): 1433-1441. 16. A. Abduriyim, H. Kitawaki. ‘‘Applications of Laser AblationInductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to Gemology’’. Gems Gemol. 2006. 42(2): 98-118. 17. A. Ulianov, O. Muentener, U. Schaltegger. . ‘‘The Data Treatment Dependent Variability of U-Pb Zircon Ages Obtained Using MonoCollector, Sector Field, Laser Ablation ICPMS’’. J. Anal. Atom. Spectrom. 2012. 27(4): 663-676. 18. C.J. Garrido, J.L. Bodinier, O. Alard. ‘‘Incompatible Trace Element Partitioning and Residence in Anhydrous Spinel Peridotites and Websterites from the Ronda Orogenic Peridotite’’. Earth Planet. Sc. Lett. 2000. 181(3): 341-358. 19. P. Hoppe, S. Cohen, A. Meibom. . ‘‘NanoSIMS: Technical Aspects and Applications in Cosmochemistry and Biological Geochemistry’’. Geostand. Geoanal. Res. 2013. 37(2): 111-154. 20. M. Koch-Muller, S.S. Matsyuk, D. Rhede, R. Wirth, N. Khisina. ‘‘Hydroxyl in Mantle Olivine Xenocrysts from the Udachnaya Kimberlite Pipe’’. Phys. Chem. Miner. 2006. 33(4): 276-287. 21. N.J. McMillan, C.E. McManus, R.S. Harmon, F.C. De Lucia Jr., W. Andrzej. ‘‘Miziolek. ‘‘Laser-Induced Breakdown Spectroscopy Analysis of Complex Silicate Minerals—Beryl’’. Anal. Bioanal. Chem. 2006. 385(2): 263-271.
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22. N.J. McMillan, R.S. Harmon, F.C. De Lucia, A.M. Miziolek. ‘‘LaserInduced Breakdown Spectroscopy Analysis of Minerals: Carbonates and Silicates’’. Spectrochim. Acta, Part B. 2007. 62(12): 15281536. 23. A. De Giacomo, M. Dell’Aglio, R. Gaudiuso, A. Santagata, G.S. Senesi, M. Rossi, M.R. Ghiara, F. Capitelli, O. De Pascale. . ‘‘A Laser Induced Breakdown Spectroscopy Application Based on Local Thermodynamic Equilibrium Assumption for the Elemental Analysis of Alexandrite Gemstone and Copper-Based Alloys’’. Chem. Phys. 2012. 398: 233-238. 24. L.E. Garcı´ a-Ayuso, J. Amador-Herna´ndez, J.M. Ferna´ndez-Romero, M.D. Luque de Castro. ‘‘Characterization of Jewellery Products by Laser-Induced Breakdown Spectroscopy’’. Anal. Chim. Acta. 2002. 457(2): 247-256. 25. A. Jurado-Lopex, M.D. Luque de Castro. ‘‘Rank Correlation of Laser-Induced Breakdown Spectroscopic Data for the Identification of Alloys Used in Jewelry Manufacture’’. Spectrochim. Acta, Part B. 2003. 58(7): 1291-1299. 26. D.C. Alvey, K. Morton, R.S. Harmon, J.L. Gottfried, J.J. Remus, L.M. Collins, M.A. Wise. ‘‘Laser-Induced Breakdown SpectroscopyBased Geochemical Fingerprinting for the Rapid Analysis and Discrimination of Minerals: The Example of Garnet’’. Appl. Optics. 2010. 49(13): C168-C180. 27. K.A. Yetter, N.J. McMillan. ‘‘Provenance of Gem Corundum: A Global LIBS Study’’. Geochim. Cosmochim. Acta. 2010. 74(12 Suppl 1): A1185. 28. M.S. Krzemnicki, T. Pettke, H.A. Ha¨nni. ‘‘Perspectives of LIBS in Gemstone Testing. Analysis of Light Elements Such As Beryllium, Boron, Lithium’’. Poster presented at: 1st GIT International Gem & Jewelry Conference 2006. Chanthaburi, Thailand; December 5-9, 2006.
