Using Elemental Analyzers for Gemstones and Precious Metals

If Saturday matinees and dime novels have taught us anything about discerning genuine from imitation products in regards to diamonds and gold, it is that you drag the former on a piece of glass, and attempt to set your teeth into the latter. However, even when besotted nineteenth century Skagway prospectors were still busy biting coins and nuggets, scientists were already dissecting the contributions of carbon and nitrogen to gemstones and precious metals. Precise measurement of organic elements, and of trace amounts of constituent metals, ores, and oxides, can confirm or invalidate authenticity. Moreover, it can further subdivide the genuine articles into a hierarchy of quality and rarity that exerts a colossal impact on market value. The annual world trade in diamond jewelry is more than $75 billion, and it is therefore crucial to know whether a stone is of the most desirable quality, is a rather middling one, or is perhaps an industrial replica. Pure gold, on the other hand, is currently valued at around $1,800 per ounce, but the per carat price drops off quite dramatically.

Enter the elemental analyzers

Innovations using platinum and high-grade quartz glass, and implementing combustion to atomize samples under high heat and quantify the constituent organic elements, led in part to the awarding of the Nobel Prize in Chemistry to Fritz Pregl in 1923. The first commercial elemental analyzers became available in the 1930s, and the need for the petrochemical industry to assess diesel fuels for trace metals drove additional innovations from the 1940s onwards. In more recent years, analyzers have become appreciably user- and space-friendly, with numerous desktop and handheld models facilitating their entry into smaller scale, individual laboratories.

Analysis of trace elements in gems and precious metals can do more than simply verify or nullify their authenticity. For market and retail purposes, knowledge of the constituent trace elements can distinguish between natural and industrial stones; place gems in different classes of quality and rarity that can greatly influence value; and, just as importantly, identify geographical origins, which can also impact rarity and thus value, but may have prohibitive geopolitical ramifications. Although the standard organic elements (hydrogen, carbon, oxygen, nitrogen, sulfur) can provide a wealth of information about gemstone quality, various elemental analyzers can also identify transition metals including iron, vanadium, chromium, nickel, and ores or oxides. For example, a natural emerald may have a much greater concentration of sodium, chromium, and iron oxides than a synthetic one; a natural ruby or sapphire may have a significant concentration of vanadium oxide, while synthetic ones will not. Natural yellow diamonds are rarer than clear ones, and their value goes up drastically with the intensity and brilliance of the color. However, synthetic diamonds often take on a yellow hue because of the use of nitrogen in the production process. Finally, geographical origin can do more than just inform buy/no-buy decisions. The balance of trace elements unique to geographical origins of gemstones can help formulate the basis of geological studies of lithic stratification and time- and pressure-dependent processes of their formation.

Combustion and sublimation of the sample: the standard analyzer

The baseline principle of function for an elemental analyzer is to partition off a small amount of sample into a combustion chamber, and quickly mineralize it using excess oxygen under high temperature (usually >900 degrees Celsius). The resulting by-products contain measurable elements in preserved stoichiometric ratios, which can be identified and quantified by one of several methods, including: colorimetry, spectroscopy, chromatography, infrared or fluorescent detection, and the Kjeldahl method. Extending this principle, a commercially available combustion analyzer can employ a vertical quartz furnace with a tin capsule that holds a sample and transfers it to a sealed purge chamber, where atmospheric gas is removed before depositing it into a ceramic crucible. The sample undergoes combustion in pure oxygen, and a thermoelectric cooler moves the gaseous products to ballast, where they are preserved and equilibrated. Finally, their content is measured in an aliquot within an inert gas stream, with detection of carbon and hydrogen accomplished by infrared, and/or nitrogen by thermal conductivity cell. Carousel attachments on this type of device can handle between 30 and 120 samples, resulting in high-throughput capability for elemental analysis.

Preserve the sample: energy dispersive X-ray fluorescence

Although these types of apparatuses are powerful and efficient, one drawback is the partitioning of the gemstone to be measured, and the destruction of the resulting sample. Energy dispersive X-ray fluorescence (EDXRF) is another elemental analysis platform that avoids this problem. Using a 50-watt X-ray tube with a wide variety of excitation voltages and beam filters, an EDXRF instrument can generate consistent and reproducible peaks for trace elements in gemstones. After calibration of the read-out spectrometer with up to 20 elements and compounds, by varying the filter and voltage used, the EDXRF analyzer can variously identify and quantify miniscule concentrations of transition metals that can distinguish genuine from fake, natural from industrial, and use the information gleaned to pinpoint geographical origin. 

Analyzers are available in desktop or even handheld format. Conveniently, a handheld EDXRF analyzer can assess precious metal content, discerning 24- from 18- and 14-karat gold, and can then analyze silver, platinum, and palladium, all without re-calibration between measurements. It can also identify dangerous heavy elements, including lead, cadmium, and uranium. 

Additionally, some models come with optional CCD cameras with spot capture features to image and analyze specific regions of interest on samples that may be heterogenous in nature. Regardless of sample hetero- or homogeneity, or of origin, elemental analyzers can be an indispensable tool in the authentication and study of gemstones and precious metals.

For additional resources on elemental analyzers, including useful articles and a list of manufacturers, visit www.labmanager.com/elemental-analyzers