IDENTIFICATION OF IRRADIATED SOUTH SEA CULTURED PEARLS USING ELECTRON SPIN RESONANCE SPECTROSCOPY
Irradiated South Sea cultured pearls (SSCPs) from the Pinctada maxima mollusk typically show colors from light gray to silver. It is difficult to identify gamma-ray irradiation of SSCPs using standard gemological methods because of their thick nacre. Therefore, an advanced analytical technique such as electron spin resonance (ESR) spectroscopy is needed to detect the treatment.
ESR measurements of minute amounts of SSCP powders revealed the formation of CO2 – radicals, and the parameter known as the g-factor was
measured at 2.0015 ± 0.0005. Higher levels of CO2 – radicals were detected in the pearl nacre than in the nucleus. Therefore, the existence of CO2 – radicals is an indicator of irradiated SSCPs.
Irradiation, dyeing, bleaching, and heat treatment are widely used methods to alter pearl color. Although most artificial colors are easily recognized, some resemble attractive colors that occur in nature (Elen, 2001; Li and Chen, 2001; Zachovay, 2005; Wang et al., 2006; “Better techniques improve brown pearls,” 2006; McClure et al., 2010).
Lower-quality freshwater and saltwater cultured pearls are regularly exposed to 60Co gamma-ray radiation in an attempt to simulate black pearls or enhance orient (Crowningshield, 1988; Li and Chen, 2002; O’Donoghue, 2006). In recent years, the irradiation process has been applied to not only Akoya cultured pearls and freshwater cultured pearls (FWCPs), but also to South Sea cultured pearls (SSCPs) (Choi et al., 2012). The irradiation-induced color change results from the darkening of the nucleus, caused by MnCO3 oxidation, as well as denatured damage to the
pearl’s conchiolin (Matsuda and Miyoshi, 1988). FWCPs have a higher abundance of proteinous components and manganese than saltwater pearls (Hatano and Ganno, 1962).
Gamma-ray irradiated SSCPs (figure 1) were first discovered in the Korean market in April 2011. At the March 2011 Hong Kong Jewelry Show, a Japanese trader reportedly sold a Korean counterpart irradiated SSCPs without disclosing the treatment. They were light gray or silver loose cultured pearls and beads 10–16 mm in size. While a cream, yellow, or black color is produced by a protein pigment in the nacre, a blue or silver color is caused by organic material between the nacre and nucleus (Komatsu, 1999; O’- Donoghue, 2006). Korean consumers typically prefer SSCPs with a silver color created by organic material. According to the research of Choi et al. (2012), gamma-ray irradiated SSCPs with colors ranging from white to cream turned light gray to silver, with the depth of color correlating with increasing irradiation dose. A dose of 0.5–1 kGy caused a light gray color, while a dose above 5 kGy produced a silver color.
For Akoya cultured pearls, with a typical nacre thickness of 0.2–0.6 mm, irradiation can be identified through standard gemological tests (Komatsu, 1999; O’Donoghue, 2006). But for SSCPs, which have a nacre thickness of roughly 1.5–3.0 mm, detecting irradiation is difficult with methods such as transmitted light, magnification, fluorescence reaction, and UV-Vis spectrometry (Choi et al., 2012). This study attempted to identify irradiated SSCPs using electron spin resonance (ESR) spectroscopy. This method, also known as electron paramagnetic resonance (EPR) spectroscopy, identifies the presence of unpaired electrons. Moreover, the study sought to minimize damage during examination by obtaining a minimal sample of powder from each cultured pearl.
MATERIALS AND METHODS
For the study, some 300 SSCPs weighing 6.55–18.05 ct (8.0–16.6 mm in diameter) with white to cream color were exposed to gamma-ray irradiation at room temperature. The irradiation was conducted at the 60Co facility of the KAERI (Korea Atomic Energy Research Institute) in Jeongeup, South Korea. The absorbed doses were set at 0.2, 0.4, 0.6, 0.8, 1, 5, and 100 kGy.
Inductively Coupled Plasma-Atomic Emission Spectrometer. Chemical composition analyses of the SSCPs were performed with an inductively coupled plasma–atomic emission spectrometer (ICP-AES, Varian Vista-PRO). The nacre, nucleus (bead), and conchiolin were separated and powdered, and 0.2 g of each powder was dissolved in a solution of 37%HCl (6 ml) and 65% HNO3 (2 ml). We tested the samples after 20 minutes at 200°C and after 10 minutes at the same temperature to obtain an average value. Electron Spin Resonance Spectroscopy. This study relied on electron spin resonance analysis to observe radicals produced by the irradiation process. The ESR spectrometer gauges the absorbed dose corresponding to the splitting energy of unpaired electrons in a magnetic
field. The technique can rapidly identify an irradiation- related signal from a small amount of sample in a few minutes. For this study, we collected at least 10 mg of SSCP powder from both the nacre and the nuclei of each cultured pearl. To determine if the ESR signals correlated with Mn2+, solid samples of FWCP, which contain more manganese than SSCPs, were irradiated with a 100 kGy dose.
Room-temperature ESR spectra were recorded using a JEOL FA-300 spectrometer with a manganese marker (MgO: Mn2+), using 9.8 GHz microwave frequency, 1 mW microwave power, a 1–2 G modulation amplitude, a 2 min sweep time, and a 0.03 s response time (figure 2).
Mn marker for ESR analysis. The g-factors of free radicals created by irradiation are approximately 2.00. For comparison, the “free electron” g-factor is 2.0023. Standard reference samples can be used to correct for any systematic errors in the measured magnetic field values and to verify the sensitivity of the system. Standard samples include DPPH (2.2-diphenyl-1-picryl-hydrazyl), TCNQ-Li (tetracyanoquino-dimethane Li saly), CaO:Mn2+, and MgO:Mn2+. The choice of standard sample used depends on what the user wants to determine. For example, DPPH is used to calculate gfactors, to monitor the sensitivity of the equipment, and to quantify spin concentrations. TCNQ-Li is used to find the g-factor. CaO:Mn2+, MgO:Mn2+, and Mn2+ are used to measure the g-factor and to correct magnetic field variations.
The g-factor of most standard samples is also located around 2.00. The Mn marker is shown with six Mn2+ signals; the third (2.034) and fourth (1.981) signals are used to correct magnetic field variations. Each signal has a regular interval from 2.00. From this property, the MgO:Mn2+ marker could be more suitable to measuring the g-factor than the alternative standard samples. The MgO:Mn2+ marker was supplied with the Jeol X-band spectrometer in the shape of a small rod that can be electromechanically inserted externally into the microwave cavity. When a sample and a Mn marker are measured simultaneously, the resulting ESR spectrum will contain signal contributions from both. It is easy to distinguish the ESR spectra of one from the other, since the Mn2+ signals have the opposite phase to that of the sample’s signal (i.e., the signal’s lineshape will appear to have been flipped across the baseline).
RESULTS AND DISCUSSION
The major element of a pearl is calcium. Chemical composition analysis of bead-cultured pearls using ICP-AES demonstrates that the nacre and the freshwater nucleus contain similar trace elements but vary in their composition. The nacre contains more Na, Mg, and Sr, while the nucleus has higher Mn and P contents (table 1).
After 60Co gamma-ray irradiation at a dose of 5 kGy, the SSCPs exhibited gray to silver coloration (figure 3). The interior of one of the irradiated pearls revealed a grayish brown to dark gray nucleus, along with an altered nacre color (figure 4). The irradiationinduced color change is chiefly attributed to the darkening of the nucleus (bead), which in turn darkens the nacre—especially in the thinner-skinned Akoya cultured pearls (Komatsu, 1999). As shown in this experiment, color change took place in the nacre as well.
Figure 5 shows that the concentration of radicals produced by irradiation exposure increases with the absorbed dose. Formerly undetected free radicals were observed after a low-dose radiation of 0.2 kGy. The g-factor was 2.0015 ± 0.0005, which agrees with that of CO2 – radicals (Wieser et al., 1985; Ikeya, 1993; Seletchi and Duliu, 2007). With higher absorbed doses, the CO2 – radical signal intensity further intensified. The identification of CO2 – radicals through ESR analysis thus serves as a way to distinguish irradiated cultured pearls.
Matsuda and Miyoshi (1988) reported that the irradiation – induced change of color is caused by manganese (Mn). They noted that MnCO3 in the nucleus (bead) turned into oxidations such as Mn3O4, Mn2O3, and Mn2O after irradiation. Their results are still cited in literature related to color change in irradiated pearls (e.g., Komatsu, 1999; Wada, 1999; McClure, 2010).
Yet existing mechanisms are insufficient to explain the alteration of pearl color by irradiation (Li and Chen, 2002). Based on the results of gamma-ray irradiation tests in this study, the authors believe that post-irradiation color change cannot solely be attributed to MnCO3 oxidation. Two factors support this hypothesis:
- After irradiation, the pearl nacre blackened to a similar extent as the nucleus (bead), even though it contains approximately 20 times less Mn (see figure 4 and table 1). Figure 6 is an ESR spectrum comparing untreated FWCP, irradiated (100 kGy) FWCP, and a Mn marker (MgO: Mn2+) attached to the JEOL equipment. The Mn marker consists of Mn2+ and shows six sharp peaks in the ESR spectrum (figure 6b). Before (figure 6a) and after (figure 6c) irradiation spectra of FWCPs (typical in the carbonate spectrum) do not match the positions of the Mn2+ signals. Nevertheless, a change was observed in the spectra before and after irradiation: the formation of CO2 – radicals between the third and fourth Mn2+ peaks (highlighted by the green circle in figure 6c). Because these results were the same among all SSCPs investigated in this study, peaks in the ESR spectrum are unrelated to Mn.2.CO2 – radicals appeared as irradiation doses increased and multiplied in proportion to the dose (figure 7). The intensity of CO2 – radicals was also proportional to the blackening of the pearl nucleus (bead). The CO3
- molecular ion in CaCO3 is easily ionized by radiation. Elementary defects induced by ionizing radiation are an electron center ( CO3- 3–) and a hole center (CO3–). While the CO3- 3– and CO3 – centers are stable at low temperatures, the electron center CO2 –, formed by irradiation, is an electron center similar but more stable than CO3- 3– (Ikeya, 1993). Additionally, we found that the color of nacre and nucleus had been bleached
under incandescent light (approximately 50°C) for 30 days. The color changed by irradiation and heat (by light) is related to the color center. Therefore, the color change of the nacre and the blackening of the nucleus (bead) are believed to be related to color centers formed by CO2 – radicals. Choi et al. (2012) found that after irradiation, glutamic acid decreased 11.43% (from 3.5% to 3.1%), alanin 21.8% (from 22.5% to 21.8%), and histidine 43.75% (from 1.6% to 0.9%), according to amino acid analysis to examine the change of protein between aragonite platelets in pearl nacre. Hatano and Ganno (1962) found that gamma-ray irradiation destroyed 32% of the histidine, 16.6% of the methionine, 11% of the glutamic acid, and 9.3% of the proline in the
protein of the FWCPs. The destruction of protein caused by irradiation can also alter the color of SSCPs.
CO2 – radicals at the absorbed irradiation dose of 0.2 kGy are barely visible in the nucleus sample but far more intense at doses above 0.4 kGy (figure 5, right). In particular, CO2 – radicals emerging after irradiation were better observed in the nacre than in the nucleus at the same absorbed dose (figure 5, left).
After normalizing the results of figure 5 to a nonirradiated spectrum (0 kGy, black line), the increased intensity of radicals was calculated by peak-to-peak height. The intensity of the CO2 – radical is stronger in the nacre than in the nucleus when irradiated with a dose above 0.4 kGy (figure 7).
Ikeya (1993) reported that Mg2+ ions might be accompanied by H2O molecules, leading to a rapid reduction in hydrated radicals. The saturation level of isotropic CO2 – also increases with the Mg/Ca ratio. Barabas et al. (1989) studied synthetic carbonate crystals doped with Mg2+ and observed the following:
- ESR spectra that displayed signals at the same spectroscopic properties as natural carbonates; and
- an increase of the g-factor signal with Mg concentration in the carbonate crystals. Mg also plays an important role in the formation of the crystal lattice of carbonates (Katz, 1973) and may enhance the formation of specific defects (Barabas et al., 1992). Lattice distortions caused by the incorporation of Mg2+ ions (Goldsmith and Graf,1958) may lead to CO2 – by creating larger interatomic distances (Barabas et al., 1992). In this context, thehigher abundance of CO2 – radicals in the nacre is thought to be related to the Mg/Ca ratio.
Considering the combined published observations on Mg2+ and CO2 – (Ikeya, 1993; Barabas et al., 1989, 1992; and Katz, 1973) it is likely that the saturation level of CO2 – rises proportionally with the Mg/Ca ratio in pearls of this study. As shown in table 1, the nacre and the nucleus (bead) contain 100 and 26 ppm of Mg, respectively. The nacre’s Mg/Ca ratio is approximately four times greater than that of the nucleus (bead). Mg, which is more abundant in the
nacre, therefore results in the preferential formation of CO2 – in the nacre rather than in the nucleus when exposed to the same absorbed radiation dose. This is consistent with the higher CO2 – ESR signal intensity observed in the nacre than in the nucleus (again, see figure 5). This suggests it is possible to identify an irradiated SSCP using ESR spectroscopy.
Identifying irradiated SSCPs through traditional gemological methods has been difficult, as their nacre is usually quite thick. But as this ESR study demonstrates, the separation of untreated pearls from irradiated pearls is possible. In doing so, an infinitesimal amount of sample was taken from the nacre in the form of powder. After irradiation, CO2
– radicals were formed, and their presence was confirmed using ESR spectroscopy. The amount of CO2 – radicals increased in proportion to the irradiation dose, and they were more observable in the nacre than in the nucleus
(bead). Until now, irradiation-induced color changes in pearls were thought to be due to the change of the MnCO3 oxidation number. But as this study notes, such color alteration is apparently related to an alteration caused by protein destruction rather than Mn, as well as color centers created by CO2 – radicals.
- Gamma-ray irradiation is routinely applied to South Sea cultured pearls (SSCPs), typically producing a light gray to silver color.
- For SSCPs, which have a particularly thick nacre, detecting irradiation is difficult using methods such as transmitted light, magnification, fluorescence reaction, and UV-Vis spectrometry.
- Electron spin resonance (ESR) spectroscopy rapidly identifies the presence of CO2 – radicals, whose concentration is proportional to the absorbed irradiation dose.
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