Laser-Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the state-of-the-art technique for the chemical micro-analysis of solid materials. It has a wide range of applications in many different fields of science and industry. In the past couple of years the importance of LA-ICP-MS to gemmology has grown as standard analytical techniques come against limits.
Our LA-ICP-MS system consists of the CETAC LSX-213 solid state Nd:YAG UV laser and the Perkin Elmer ELAN DRCe quadruple ICP mass spectrometer. The laser has a wavelength of 213 nanometres which, combined with a laser pulse repetition rate of 10 Hz, allows for precise ablation without cracking or splintering of the sample. During the laser ablation process, the laser pulses release small amounts of the sample from the surface in the form of particles, ions, atoms and molecules. Ablated material is caught by a He gas current and transported into the Argon plasma of the mass spectrometer. In the plasma the 60-150 nanometre-sized particles are disintegrated at ~7000°C and ions are created, which can be selectively detected. The mass spectrometer is capable of quasi simultaneous detection of almost all natural elements, including very light elements such as Li, Be, B and Na, which are difficult or impossible to analyse by more traditional chemical analyses such as the X-Ray Fluorescence method. Most elements can be detected at very low concentrations down to sub-ppb (parts per billion) levels, which make this method unchallenged by any other standard analytical instrumentation.
Standard testing of gemstones by LA-ICP-MS generally comprises four individual spot analyses placed on the girdle of faceted stones or anywhere according to the client’s wish, e.g. on a facet due to be re-polished. The laser beam diameter, and therefore the size of the sample pit, is ideally 50 microns – half the diameter of a human hair. If made on a reflective (polished) surface, the sample pits are barely visible with the naked eye. The weight loss for each set of analyses (four laser pits) is between 0.000004 ct and 0.000008 ct and therefore negligible. The appearance, quality and value of the stone remain unaffected.
We apply LA-ICP-MS to determine the beryllium treatment of corundum and to support the origin determination of emeralds. We are currently working on the development of methods for the origin determination of rubies, sapphires, spinels, alexandrites and Paraiba tourmalines.
Origin & species determination of tourmaline
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Origin determination of spinel
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