New applications for laser ablation high resolution ICPMS (LA-HR-ICP-MS): Quantitative measurements of gallium diffusion across human root dentin

Laser-ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) has been used for in situ determination of gallium concentration in treated human teeth. Gallium salts have an inhibitory effect on osteoclastic activity. Established as a therapeutic agent in regulating serum calcium and bone resorption, gallium has recently been proposed as a treatment for external root resorption. This study examines the diffusion of gallium introduced as the nitrate, through root dentin from the prepared root canal toward the root surface. Root canals from extracted human teeth were biomechanically prepared, sealed apically, and closed off coronally with polyethylene tubing. A 1.0 M solution of gallium nitrate, chelated by 1.0 M citrate buffer (pH 7.4), was introduced into canals, and roots were then suspended in test tubes of distilled water. The gallium-treated teeth were analyzed for ⁴³Ca, ⁶⁹Ga, and ⁷¹Ga isotopes. Elemental data were obtained by ablation with Nd:YAG laser operating in the UV region (266 nm) by Merchantek and isotopic analysis with a Sector Field ICP-MS by Finnigan MAT. A 50 ppm glass standard (NIST-612) was also analyzed for the same isotopes. Quantification was performed by assuming a uniform nominal calcium concentration value of 27 g per 100 g of dry dentin across the root and using the ⁴³Ca signal for internal calibration. The precision (RSD%) of measurements for ⁴³Ca for the NIST-612 glass standard was found to be within 3.2% to 4.6% and for the tooth samples within 15.4% to 19.1%. The precision of determination for ⁶⁹Ga for the NIST-612 glass standard was between 3.1% and 5.7%. The analysis revealed a significant uptake of gallium by the root dentin, ranging from ≈20 000 ppm for the upper portion of the root to ≈8000 ppm for the areas in the lower sections. The analysis also revealed that a significant concentration gradient was established for gallium nitrate across the root dentin. The results from this study suggest that, in an effort to inhibit root resorption, therapeutic concentrations of gallium can be achieved throughout the root dentin when the prepared root canal is treated with 1.0 M gallium nitrate.