BACKGROUND Critical metal elements are a group of metal elements including rare metal elements (e.g., Li, Be, Rb, Cs, Nb, Ta, Zr, Hf, W), rare earth elements (REEs), rare disperse elements (e.g., Ga, Ge, Se, Cd, In, Te, Re, Tl) and rare precious elements (e.g., PGE, Cr and Co), which are important for the development of emerging industries. In recent years, the critical metal elements have shown great economic characteristics in emerging industries such as advanced materials, new energy resources and national defense and military industry uses, which is important strategic significance for the development of the national economy and technology. Therefore, it is necessary to investigate the geochemical properties and metallogenic mechanism of critical metal elements. How to accurately determine trace elements in geological materials is a prerequisite for these investigations. Critical metal elements in geological materials can be determined by conventional chemical wet digestion methods. However, chemical wet digestion methods can only obtain an average chemical composition without spatial distribution information of critical metal elements. Compared to digestion methods, in situ microanalysis technology can obtain micrometer scale elemental distribution in silicate minerals, omit tedious chemical processing processes and avoid the use of a large amount of chemical reagents. However, the abundance of critical metal elements in the crust is low (μg/g level) and the carrier minerals containing critical metal elements are at the micrometer scale. Therefore, it is necessary to establish a high spatial resolution in situ analysis technique to determine trace elements in geological materials.

OBJECTIVES To improve sensitivity of LA-ICP-MS for the determination of critical metal elements (ng/g-μg/g level) in silicate minerals by high-frequency ablation mode combined with Ar-N₂ mixed plasma technique.

METHODS Experiments were carried out using a single collector ICP-MS (Element XR Thermo Fisher Scientific, Bremen, Germany) in combination with a 193nm excimer laser ablation system (GeoLas 2005, Lambda Physik, Gttingen, Germany) at the Ministry of Natural Resources Key Laboratory of Gold Mineralization Processes and Resources Utilization. The X skimmer cone was used to improve sensitivity of ICP-MS. To obtain high sensitivity and reduce oxide interference, a small amount (0-10mL/min) of nitrogen was added into the carried gas, downstream from the ablation cell by a T junction. The ablation frequency was 5Hz or 20Hz. The ablation spot size was 10-24μm. Each measurement consisted of 18s of acquisition of the background signal, followed by 10s ablation signal acquisition. The washing time was 20s between each measurement. The standard reference materials NIST 610, NIST612 and NIST614 were used as calibration standards. The comparison of signal intensity, sensitivity, oxide yield and U/Th ratio in LA-ICP-MS were investigated at low and high frequency ablation modes in Ar plasma or Ar-N₂ mixed plasma. Before testing, the signal of ²³²Th and ²³⁸U were higher than 1×10⁶cps when ablating NIST 612 at 24μm. Moreover, U/Th was close to 1 and ThO⁺/Th⁺ was lower than 0.5%. At optimum condition, an in situ elemental quantitative method with high spatial resolution (10-24μm) was established to determine 42 trace elements in MPI-DING and USGS silicate glass reference materials.

RESULTS Sensitivity in LA-ICP-MS is the primary factor for the elemental quantitative analysis with high spatial resolution. Compared to Ar plasma, sensitivities of most elements were improved by a factor of 1.5-9 when using Ar-N₂ mixed plasma at high-frequency (20Hz) ablation mode. In LA-ICP-MS analysis, analytical results can be influenced by oxide yield and elemental fractionation. When using X skimmer cone in SF-ICP-MS, the oxide yield and elemental fractionation was significantly reduced in Ar-N₂ mixed plasma at high-frequency (20Hz) ablation mode. There was a wide range of carrier flow rate (0.9-1.075L/min) for obtaining good analysis conditions (ThO⁺/Th⁺<0.5% and U/Th=1). The limits of detection for 30 trace elements were lower than 0.02μg/g when ablation spot and ablation frequency were at 24μm and 20Hz, respectively. At optimum conditions (ablation spot 10-24μm and ablation frequency 20Hz), 42 trace elements in MPI-DING and USGS silicate glass reference materials were analyzed by LA-ICP-MS in Ar-N₂ mixed plasma. The accuracy of analytical results for 34 trace elements was better than 10% and the precision was better than 15%, which suggested high-frequency ablation mode combined with Ar-N₂ mixed plasma technique can be used to achieve the determination of critical metal elements (ng/g-μg/g level) in silicate minerals with high spatial resolution.

CONCLUSIONS Compared to low-frequency ablation mode, high-frequency ablation mode combined with Ar-N₂ mixed plasma technique can improve sensitivity and reduce oxide yield and elemental fractionation. Moreover, the analysis time of high-frequency ablation mode is very short, which can improve the analysis efficiency of LA-ICP-MS. Due to the high sensitivity and high spatial resolution of LA-ICP-MS at high-frequency ablation mode, LA-ICP-MS can be applied to quantify trace elements and complex internal chemical compositions in micrometer level minerals, such as distribution of trace elements in mineral growth zones. Furthermore, this high-frequency ablation mode may also be applied to the development of in situ accessory mineral dating and isotope ratio analysis.