Sensitive and accurate determination of REEs using a high-efficiency miniaturized ultrasonic nebulization sampling system coupled with the inductively coupled plasma mass spectrometer

Abstract

The determination of rare-earth elements (REEs) mass fractions is of great significance in various fields including materials science, geology, ecotoxicology, environmental safety and human health. However, the conventional pneumatic nebulization (PN) sampling system commonly used with the inductively coupled plasma mass spectrometer (ICP-MS) suffers from low sample introduction efficiency and a high yield of oxide interferences. In this work, the feasibility of a high-efficiency micro-ultrasonic nebulization (MUN) sampling system coupled with an ICP-MS for the sensitive determination of REEs was evaluated. It was observed that the use of MUN could enhance the sensitivity by more than one order of magnitude. In addition, we also found that operating the MUN with a square waveform and a duty ratio of 50% could significantly improve the signal intensities and signal-to-noise ratios of REEs compared to our previous sinusoidal waveform. This improvement can be attributed to the lower aerosol size distribution (d₅₀ = 6.0 μm) generated with the square-waveform MUN. Moreover, MUN-ICP-MS exhibited lower oxide ratios (¹³⁹La¹⁶O⁺/¹³⁹La⁺ = 0.79%, ¹⁴⁰Ce¹⁶O⁺/¹⁴⁰Ce⁺ = 0.73%, ¹⁵⁰Sm¹⁶O⁺/¹⁵⁰Sm⁺ = 0.06% and ¹⁵⁹Tb¹⁶O⁺/¹⁵⁹Tb⁺ = 0.10%). It is worth noting that achieving similar levels of low oxide ratios with the PN-ICP-MS system requires the use of a reaction/collision cell, resulting in nearly a 50% loss in sensitivity. Under optimized conditions, the LODs for 16 REEs ranged from 0.03 (Tm and Lu) to 1.07 (Sc) ng L⁻¹, and the repeatability (RSD, n = 8) was between 0.3% (Eu, 10 ng mL⁻¹) and 3.4% (Lu, 1 ng mL⁻¹). The accuracy of the proposed method was validated using certified reference materials (CRMs) of sediments, soils and basalts. The proposed MUN provides a novel low-cost high-efficiency sample introduction strategy for sensitive and accurate REEs analysis using ICP-MS.