Determination of Trace Rare Earth Elements in Coal Ash by Inductively Coupled Plasma Tandem Mass Spectrometry

Coal ash is one of the most important alternative rare earth elements (REEs) sources to conventional REEs ores. Standard methods for determining REEs in coal ash have still not been established. Compared with other traditional ores, the chemistry and mineralogy of coal ash are more complicated. The reported analytical methods have severe problems in the pretreatment process (low efficiency) and are difficult to reduce the mass interferences when using inductively coupled plasma mass spectrometry (ICP-MS) as an analytical tool to detect REEs in the liquid digested sample. This study proposed a new efficient and accurate REEs determination method based on graphite digestion-(dynamic reaction cell) inductively coupled plasma tandem mass spectrometry (ICP-MS/MS). By comparing the effects of different digestion reagents (nitric acid, nitric acid-hydrochloric acid, nitric acid-hydrofluoric acid, nitric acid-hydrochloric acid-hydrofluoric acid) and different digestion temperature (100, 140, and 180 degrees C) on the test results, it was found that the HNO3-HCLHF digestion at 140 degrees C was the best pretreatment method. The addition of HFcould significantly improve the extraction efficiency of REEs, and the secondary addition of water to eliminate HF avoided the formation of insoluble fluoride and hydrofluoride, which decreased adverse effects of F-1 on mass spectrometry. ICP-MS/MS measurement process, Rh and Re elements were added online as internal standards to compensate for the matrix effect, respectively. By comparing the gas-free mass in situ mode (M-SQ-N/A), helium collision and kinetic energy discrimination mode (M-SQ-KED), oxygen mass transfer mode (M-TQ-O-2), ammonia mass transfer mode (M-TQ-NH3), it was found that the detected value in M-SQ-KED mode is closer to the standard value with minimum mass spectrum interference. The recovery rate of REEs (compared to values reported in standard coal ash) is 92. 49%similar to 112. 88 %, the detection limit is lower than 0. 005 2 mu g center dot g(-1), and the relative standard deviation is less than 1. 71%. Tracing Eu in a high-Ba solution revealed that the M-SQ-KED mode can greatly reduce the ion interference because the He was likely to collide with interfering polyatomic ions with a larger collision probability than that of the target analytes. Finally, the established method was applied analyse fly ash samples from different coal-fired power plants. The results showed that the distribution of the normalized REE curves (relative to UCC) was uniform and smooth, indicating that the results were stable and reliable. Compared with microwave digestion, the proposed method has the advantages of low cost, simple operation, and high efficiency and can be used for vast amounts of ash sample determination. The dynamic reaction cell technology can eliminate the interference of mass spectrometry online, which greatly improves the determination efficiency.

Automated summary

Coal ash is an important alternative source of rare earth elements (REEs). However, there is currently no standard method for determining REEs in coal ash. This study proposed a new efficient and accurate method using graphite digestion-ICP-MS/MS. The method demonstrated low cost, simple operation, and high efficiency.