Determination of Major and Trace Elements in Geological Samples by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry with Ammonium Fluoride Digestion

The accurate determination of concentrations of key metal elements in geological samples (such as rocks, soils or ores) is an important foundation for the basic theory research of critical metal ores, supernormal enrichment mechanism of critical metal elements, mineral resource exploration and green utilization of mineral resources. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has advantages of providing adequate sensitivity, low interferences of polyatomic ions, small quantities of sample consumption and simple sample preparation. It is perceived as a green geological analytical technique with great potential in the whole elemental determination. However, the pretreatment of geological samples is the key factor limiting its pervasive application. The current bulk analysis of LA-ICP-MS is limited by the problem of sample homogenization. Most of the traditional pretreatment techniques are obtained by physical grinding, which has low efficiency and the risk of cross contamination. Therefore, new pre-processing techniques for geological samples need to be proposed to meet the requirements of LA-ICP-MS analysis.

To develop a new pre-treatment technology for geological samples and establish a rapid, efficient, green, and environment-friendly LA-ICP-MS elemental quantitative analysis technology.

The ammonium fluoride (NH₄F) was used to digest five rock reference materials (BCR-2, BHVO-2, AGV-2, RGM-2, GSP-2). Ultrafine particulate powders obtained after evaporation and stirrer processes were directly pressed into tablets. The rapid whole elemental quantitative analysis of LA-ICP-MS can be achieved by adding the indium (In) internal standard and external standard correction of geological rock reference material. The digestion time of NH₄F, the morphology, element composition, element distribution and other characteristics were also investigated. The mass fraction of SiO₂ was evaluated to the total normalization 100% and gravimetric method.

The results show that 2h digestion time can completely decompose geological rock samples. Moreover, the digestion products have the characteristics of ultrafine powder with a typical grain size of smaller than 5μm. Various silicate rocks after NH₄F digestion have a consistent grain morphology and size, allowing the production of pressed powder pellets that have excellent cohesion and homogeneity suitable for laser ablation micro-analysis without the addition of a binder. The mass fraction of SiO₂ in rock samples is obtained accurately using the gravimetric method. The analytical results of five reference materials generally agreed with the recommended values, with the analytical precision within 10% for 45 major and trace elements.

A NH₄F digestion method as sample preparation for the rapid determination of major and trace elements in silicate rock powders by LA-ICP-MS was introduced. The analytical results obtained for five rock reference materials generally agree with the recommended values within a relative deviation of <10%, confirming the usefulness of this method for quantitative elemental analysis of silicate rock samples. The applicability field of this method includes most common silicate rock samples. Furthermore, sediment, solid and ore can also be analyzed by the NH₄F digestion method. There are many innovations for the new technique, including reducing matrix effects between reference materials and samples, spiking the internal standard simply and feasibly and sample batch processing. The optimized method can be used to quickly prepare compact tablet samples with high uniformity and without elements loss, which are suitable for LA-ICP-MS. This method only needs the use of NH₄F solid reagent, which greatly reduces the consumption of other inorganics acids. Moreover, NH₄F is a neutral reagent, which decreases the potential risk of acid reagent harmful in the pretreatment process to laboratory personnel, and has the added advantage of greener environmental protection, safety, and high efficiency.