MATRIX-MATCHED CORUNDUM STANDARDS FOR KEY TRACE ELEMENTS IN RUBY AND SAPPHIRE

The accurate determination of corundum's key trace elements is essential in the deduction of geographic origin, formation history, and color mechanism. The trace element detection techniques currently used for analyzing mineral species (such as Electron Microprobe Analysis (EMPA), Secondary Ion Mass Spectrometry (SIMS), Laser Induced Breakdown Spectroscopy (LIBS), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP MS)) require calibrated standards to perform quantitative analyses for each trace element of interest. Essentially, these methods are limited by the adequacy of the standards used.

Commonly used standards such as those produced by the National Institutes of Standards and Technology (NIST) are generally not available in matrix-specific forms, so silicate glasses such as NIST Standard Reference Material (SRM) 610 and 612 are frequently used. As the silicate glass matrix differs significantly both in composition and structure from the crystalline alumina matrix of sapphires and rubies, what is known as a “matrix effect” will result in inaccuracy for many of the key trace elements characterized. Furthermore, the NIST 610 and 612 glasses are not certified for many of the trace elements they are used for: for example, out of the seven key trace elements looked at in corundum with LA-ICP-MS, only iron and chromium are certified.

To address this, we created matrix-matched corundum standards containing targeted levels of beryllium, magnesium, silicon, titanium, vanadium, chromium, iron, and gallium along with an ultra-high purity “true matrix zero” to perform highly accurate trace elemental analysis in ruby and sapphire. To our knowledge, these sets contain the most accurate standards for the key trace elements in corundum today. Additionally, we have expanded our calibration methods to cover nickel and calcium in corundum.

Significantly, we were able to accurately quantify silicon in corundum using the high mass resolving power of Secondary Ion Mass Spectrometry, historically a trouble spot for LA-ICP-MS, where the ²⁷Al¹H⁺ interference to ²⁸Si⁺ was not resolvable. Our ability to accurately quantify silicon in sapphire is allowing us to see the role silicon plays in the color of sapphire.