Cadmium Isotope Analysis Of Environmental Reference Materials Via Microwave Digestion–Resin Purification–Double-Spike MC-ICP-MS

Abstract

Cadmium isotope fractionation is a promising indicator for tracing the source, transport, and transformation of Cd in the environment; therefore, a high-precision method for the Cd isotope analysis of environmental samples is urgently required. In this study, eight environmental reference materials (NIST 2711a, GSS-1, GSS-4, GSS-5, GSD-11, GSD-12, GSD-30, and BCR-679) with different matrices were digested under microwave irradiation and purified via anion exchange. Thereafter, their Cd isotope ratios were analyzed using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with double-spike correction. The samples digested under microwave irradiation exhibited high Cd recovery (> 96%). One step of anion-exchangebased purification can remove most interfering elements without any detectable loss of Cd. If the purified solution contained Zn/Cd > 0.04, Zr/Cd > 0.01, Mo/Cd > 0.2, Pd/Cd > 4 × 10 −5 , In/Cd > 0.02, or Sn/Cd > 0.1, a secondary step using the same purification procedure would be necessary. The measured δ 114/110 Cd values of reference materials (from −0.558 to 0.550‰) were in adequate agreement with those of previous studies, suggesting that this method can be used to analyze the Cd isotope ratios in soil, sediment, and plant samples. In addition, the large variation in the Cd isotope ratios of these reference materials implies that the Cd isotope ratio is promising for identifying pollution sources and the biogeochemical cycle of Cd.

Revealing the Sources of Cadmium in Rice Plants under Pot and Field Conditions from Its Isotopic Fractionation

“…The dry weight of plant tissues was recorded. The plant samples (0.25 g) were digested with HNO 3 , H 2 O 2 , and HF (6:2:0.5, v/v) using a microwave digester . For some plant samples under slaked lime treatment with low Cd concentrations, 0.3–0.5 g samples were weighted and digested for total Cd and isotopic composition analysis.…”

“…All extracted or digested solutions were evaporated to dryness in the presence of 111 Cd– 113 Cd double spike (double spike/sample ≈ 0.5) and then redissolved in 2 mL of HCl (2 mol L –1 ) for chemical purification. This process was used to remove matrix elements from samples following a previously reported protocol . In short, the sample solution was loaded into a microcolumn filled with 3 mL of AG-MP-1 M anionic exchange resin.…”

“…The Cd isotopic compositions of all samples were determined by a high-resolution multiple-collector ICP-MS in "dry" plasma conditions (Neptune Plus, Thermo Scientific, Bremen, Germany). 28 Four reference Cd solutions (AAS Cd, Thermo-Cd, GSB-Cd, and NIST 3108 with a double spike) were analyzed to monitor the precision and accuracy of Cd isotopic composition. The long-term precision of NIST 3108 was ±0.05% (2SD).…”

“…The soil (0.1 g) was digested with HCl, HNO 3 , and HF (6:2:2, v/v) using a microwave digester (MARS 6, CEM, Matthews, NC) for total Cd analysis. 28 The plant-available Cd pools were extracted as watersoluble Cd and Ca(NO 3 ) 2 -extractable Cd. The water-soluble Cd was extracted by dissolving the soluble fraction, including free Cd ions and Cd complexed with dissolved organic matter and other ligands, in deionized water.…”

“…This process was used to remove matrix elements from samples following a previously reported protocol. 28 In short, the sample solution was loaded into a microcolumn filled with 3 mL of AG-MP-1 M anionic exchange resin. Matrix elements such as Ca, Zr, Mo, Zn, and Sn were removed using different concentrations (2, 0.3, 0.06, and 0.012 mol L −1 ) of HCl.…”