Effect of Aluminum Fluoride Formation on ICP-MS Determination of Critical Elements in Alumina-Rich Coal Fly Ash

Coal fly ash is a potential source of critical metals, and accurate trace element analysis is essential for evaluating its resource value and environmental impact. Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) is widely used due to its low detection limits and its capability of multi-element determination. In this study, a mixture of acids (5 mL HF + 2 mL HNO 3 for each 50 mg sample) was used for microwave-assisted digestion of fly ashes from eight Chinese coal-fired power plants and NIST Standard Reference Materials (SRMs) 1633b and 1633c.For low-alumina samples and SRM 1633c, ICP-MS results were consistent with those from X-ray fluorescence spectroscopy (XRF). However, for alumina-rich fly ashes, ICP-MS significantly underestimated most elements. In the worst case, the ICP-MS/XRF ratios for Sc, Ga, Rb, Sr, Y, Ba, La, and Ce in a single sample ranged from 0.37 to 0.68. XRD and EPMA analyses revealed that the formation of AlF 3 during digestion caused co-precipitation and trace element loss. To address this issue, a digestion method using ultrafine grinding (<23 μm) and lower test portion mass (<20 mg) was developed. This modification effectively suppressed AlF 3 formation by lowering the aluminum concentration in solution, resulting in improved ICP-MS/XRF ratios (>0.84) for alumina-rich samples. The method was further validated with additional high-alumina samples and SRMs 1633b and 1633c. This study proposes a simple yet effective pretreatment method to enhance the reliability of ICP-MS results for critical metal determination, which can serve as a valuable reference for elemental analysis of alumina-rich geological and environmental samples.