Diamond fingerprinting for source discrimination using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and Fourier transform infrared spectrometry (FTIR)

Abstract

The Kimberley Process Certification Scheme (KPCS) was established in 2000 as a means of controlling the flow of conflict diamonds, mostly, from the African continent. In 2013, the KPCS imposed an embargo on diamonds from the Central African Republic (CAR). Since then the embargo has been lifted in certain prefectures of the country, however, smuggling is suspected from non-compliant areas. Three parcels of diamonds suspected to have mining origins in the CAR, were analysed. These diamonds were investigated for their morphological and chemical characteristics, to establish a diamond fingerprint and to determine if these diamonds had the same fingerprint as previously analysed diamonds from CAR or the Democratic Republic of the Congo (DRC). The analyses of these diamonds were included in the already established diamond database of rough diamonds from the African continent. The morphological characteristics identified included the mass (ct), colour, surface coatings, dominant, secondary and tertiary form, shape, breakage, inclusions, abrasion and surface features that are specific to octahedral, dodecahedral and cubic shapes. The morphological characteristics determined from the diamonds revealed that morphology alone cannot be used as a discriminatory method for diamond fingerprinting. Fourier transform infrared spectroscopy (FTIR) identified the nitrogen concentration and aggregation state of that N. This allowed for the typing of the diamonds as Type I (containing N) and Type II (containing no measureable N). The concentration of N in the three parcels is less than 600 ppm. Further classification of Type I diamonds was performed according to the N aggregation state as single, double or four-fold. The vast majority of diamonds show a combination of nitrogen aggregation states while few were classified as Type II. Fourier transform IR showed no discernible trends between the current study and the established database. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used as a means of determining the trace element concentrations of 69 Ga, 88 Sr, 89 Y, 90 Zr, 93 Nb, 133 Cs, 137 Ba, 139 La, 140 Ce, 141 Pr, 146 Nd, 147 Sm, 153 Eu, 157 Gd, 159 Tb, 163 Dy, 165 Ho, 166 Er, 169 Tm, 172 Yb, 175 Lu, 178 Hf, 181 Ta, 232 Th and 238 U. Laser ablation ICP-MS determined that not all elements produce statistically viable data, however, the data could still be used to discern trace element d...

Methods and Challenges of Establishing the Geographic Origin of Diamonds

“…Data must exceed the LOQ (the minimum accurate quantifiable value) to be considered truly quantitative. A number of studies have reported conventional online laser ablation results from gem-quality diamonds but with such low signal-to-noise ratios that LOQ criteria could not be met (Rege et al, 2005(Rege et al, , 2010Coney et al, 2012;Brill et al, 2020). If measurements do not exceed the LOQ, they may carry qualitative meaning, but large uncertainties will obscure comparison between samples, restricting their utility.…”

“…ing global sustainability efforts across all industries, consumers are interested in knowing the origin and impact of the goods they purchase. These social pressures have prompted direct efforts, by both privately and publicly funded research groups, to search for distinct geographic signatures among diamonds (e.g., Watling et al, 1995;McNeill et al, 2009;Dalpé et al, 2010;Rege et al, 2010;Coney et al, 2012;Melton et al, 2012;Brill et al, 2020;McManus et al, 2020).…”

“…Trace element characteristics, in conjunction with other observations, have proven use-ful for origin determination of other gem materials (see box A). Similarly, trace element analysis using mass spectrometry has been regarded as the most promising approach for diamonds (Watling et al, 1995;McNeill et al, 2009;Dalpé et al, 2010;Rege et al, 2010;Coney et al, 2012;Melton et al, 2012;Brill et al, 2020). One study has also used laser-induced breakdown spectroscopy (LIBS) for this application (McManus et al, 2020).…”

Analytical Proof of Origin for Raw Materials

“…For diamonds, the heterogeneity of compositions encountered in the sources, and the difficulties in receiving material that can be used to establish a large database are both regarded as major challenges. Brill et al [46] reported a case where diamond sets that were suspect to have origins in the Central African Republic (CAR) were compared to diamonds previously analyzed as originating from CAR and the Democratic Republic of the Congo (DRC). Sample morphology, FTIR, and LA-ICP-MS combined with descriptive statistics were applied.…”