Robust determination of low Ti contents in zircon using LA–ICP–MS/MS with NH3 reaction gas

Abstract

The Ti-in-zircon thermometer is widely used in geoscience for constraining the temperatures of geological settings. Precise and accurate determination of Ti contents (∼1–10 μg g⁻¹) in zircon is a prerequisite for the application of this thermometer; however, traditional laser ablation–single quadrupole–inductively coupled plasma–mass spectrometry (LA–SQ–ICP–MS) analysis of Ti contents in zircon is challenging due to the low abundance of the interference-free isotope of ⁴⁹Ti (isotope abundance: 5.51%), resulting in poor precision (>30%, 2RSD), in particular for the high spatial resolution (laser spot size <20 μm) analysis. We have developed a robust technique for determining low Ti contents in zircon using LA–ICP–MS/MS with NH₃ as the reaction gas. Using an iCAP TQ ICP–MS/MS instrument (Thermo Scientific, USA), high-purity NH₃ was a more effective reaction gas than the commonly used 1 : 9 NH₃ : He mixture gas, and a three-fold improvement in sensitivity was achieved using an N₂ flow rate of 5.0 mL min⁻¹. The reaction products of Ti, Ca, and Zr with NH₃ were identified over a mass range of 40 to 160 amu. The reaction product ⁴⁸Ti(¹⁴N¹H)(¹⁴N¹H₃)₄ (a mass shift of +83, expressed as ^(48+83)Ti) was measured to separate ⁴⁸Ti⁺ from ⁴⁸Ca⁺ and ⁹⁶Zr²⁺. Interfering ions ⁴⁸Ca⁺ and ⁹⁶Zr²⁺ have low reaction rates (∼0.0046% and ∼0.07%) at a mass shift of +83, and corrections are not required for glass reference materials and zircon samples. Five zircon reference materials with Ti contents of 2.5–5.0 μg g⁻¹ were analysed, and the analytical precision was better than 10%. Compared with the single quadrupole (SQ) mode, our method is more robust for determining low Ti contents in zircon at high spatial resolutions (laser spot size <20 μm), making it useful for the analysis of complex zircon grains, e.g., metamorphic zircon.