固相萃取-电感耦合等离子体质谱法测定地表高盐水体中的痕量稀土元素

基于电感耦合等离子体质谱（ICP-MS）建立测定地表高盐水体中痕量稀土元素的分析方法，提出UV/H₂O₂去除地表水体中有机物的新策略。地表水体中的溶解态稀土浓度极低，质量浓度在ng·L-1数量级，分析测试非常困难。当水体中总溶解性固体超过1 g·L-1时，样品直接测试会造成严重的质谱干扰，同时可能会导致雾化器、截取锥和样品锥阻塞。因此，测试高盐水体中的稀土元素需要先去除水体中的盐类。为提高测试的准确度，样品测试前通常需要对水体进行预富集以增加待测物质量浓度。但是，内陆地表水体中有机物浓度较高，在进行预富集的过程中有机物和稀土元素发生络合作用，使得溶解态稀土在富集时发生分异，对预富集工作也是一项挑战。首先在样品中加入H₂O₂，将样品放入紫外消解系统中氧化去除水体中的有机物，消解后水体中有机碳的浓度可降低至0.5 mg·L-1。然后采用NobiasPA1固相树脂对样品进行预富集，步骤如下：首先使用流速为2.2 mL·min-1的硝酸，超纯水和醋酸铵缓冲液依次分别清洗预富集系统，去除预富集系统中可能残留的稀土元素；然后用流速为2.0 mL·min-1的醋酸铵缓冲液、样品和醋酸铵缓冲液依次分别通过固相萃取柱，富集样品中的稀土元素并去除吸附在树脂柱上的盐类；最后使用流速为0.7 mL·min-1的HNO₃溶液淋洗树脂柱并收集样品。ICP-MS测试样品时，选择115In为内标校正基体效应。研究结果表明在pH 4.6±0.1的情况下，各稀土元素的检出限和空白值分别在0.34～22.0和0.34～12.9 ng·L-1之间；稀土元素检出的相对标准偏差（n=5）<5%；稀土标准溶液的加标回收率在97%～101%之间。将该方法用于渤海海水、海河河口水体和西藏雅根错水体，加入Tm作为内标，样品的加标回收率在98%～101%, 相对标准偏差（n=3）<5%。这说明该方法适用于地表高盐水体中溶解态稀土元素的定量分析。

Determination of Rare Earth Elements in High-Salt Water by ICP-MS After Pre-Concentration Using a Chelating Resin

Based on inductively coupled plasma mass spectrometry (ICP-MS), a novel method for the accurate determination of ultra-trace rare earth elements (REEs) in high-salt surface water was established. The interfering with organic matter in the surface water was eliminated by UV/H₂O₂. The concentration of REEs in surface water is at the ng·L-1level, making the quantitative determination of dissolved REEs very difficult. The matrix effect of ICP-MS is serious when the total dissolved solid concentration is higher than 1 g·L-1. Moreover, nebulizer, sampler cone, and skimmer Cone may be blocked in the process of measuring. Therefore, it is necessary to remove salt from the water when the concentration of REEs in high-salt water is determined. REEs in water is needed to preconcentrate before measurement. However, the concentrations of organic matter are usually high in the surface water. The complexation of organic matter can lead to a fraction of REEs during -pre-concentration. The preconcentration of dissolved REEs is also a challenge. In this work, H₂O₂was added to the sample before the preconcentration. The sample was subsequently irradiated with a digester, which destroyed the organic ligands of REEs. The dissolved organic carbon (DOC) concentration in water could be reduced to approximately 0.5 mg·L-1. The REEs in water were pre-concentrated through a Nobias PA1 chelating resin column. Proceed as follows: initially, the preconcentrated system was respectively rinsed with HNO₃, pure water, and NH₄AC solution in sequence at a flow rate of 2.2 mL·min-1 to remove the possible residual REEs. Then, the column was respectively rinsed with NH₄AC solution, sampler, and NH₄AC solution in sequence at a flow rate of 2.0 mL·min-1 to preconcentrate REEs and remove the loaded salts. Finally, the REEs were eluted with HNO₃ at a flow rate of 0.7 mL·min-1 and analyzed by ICP-MS. A 115In internal standard was used to correct instrument fluctuation and matrix effect. Results showed that the procedural blanks, detection limits, and relative standard deviations (RSD) of the REEs were 0.34～12.9 and 0.34～22.0 ng·L-1, and <5% (n=5), respectively, at a pH of 4.6±0.1. All REEs could be quantitative, and their recoveries were 97%～101%. The results from applying this method to coastal water, estuary water, and saline lake water showed that the recoveries of Tm internal standard were 98%～101%, and RSD of the samples (n=3) were <5%. It indicates that the method is suitable for analysing REEs in high-salt surface water.