Role of Ti Content on the Occurrence of the 3309-cm^–1 Peak in FTIR Absorption Spectra of Ruby Samples

The study is based on the determination of trace elements in ruby samples, Ti in particular, using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS). The samples were then divided into three groups: high (≥ 200 ppma), moderate (100–200 ppma), and low (≤ 50 ppma) Ti content. In addition, X-ray absorption spectroscopy (XAS) was employed to confirm that the Ti oxidation state was Ti⁴⁺. The heat experiment conditions were set at 800, 1200, and 1650°C to investigate the transformation of the –OH vibration in the FTIR spectra of ruby samples. The FTIR spectra showed that samples containing a low Ti concentration did not show the 3309-cm^–1 absorption peak either before or after heating at any of the designed temperatures. An obvious 3309-cm^–1 peak appeared only in samples with a moderate to high Ti content after heating at ≥ 1200°C. The results also suggest that, in addition of Ti⁴⁺, the occurrence of the 3309-cm^–1 peak in the FTIR spectra is strongly affected by the charge compensation of –OH in Al sites, i.e., the structural –OH; thus, the structural –Ti–OH stretching in the ruby samples. Hence, for gemological identification, the 3309-cm^–1 peak can be applied to indicate whether a ruby has undergone heat treatment if those samples contain enough Ti ions in their structure, i.e., Ti ≥ 100 ppma. This condition is usually found in ruby samples from major deposits such as Mong Hsu and Mogok, Myanmar; Luc Yen, Vietnam; and Montopuez, Mozambique.