Magnesium Hydroxide Coprecipitation for Separation of Trace Elements from Calcium Matrix of Fish Otoliths and Biominerals for ICP-MS Determination

Metals Analysis and Remediation
Oral Presentation

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Fish otoliths are calcium carbonate biominerals located in the head of the fish that aid in hearing and balance. These biominerals contain major and trace elements that originate from the local water environment of the fish and are incorporated into the calcium carbonate growth rings over the course of the fish’s life. The elemental composition of an otolith serves as a natural biomarker in elucidating the fish’s life history, including natal homing grounds, migration, and mixing patterns. For most elements, excluding a few alkali and alkaline earth elements, trace element levels in otoliths are at the sub-µg/g levels. Inductively coupled plasma mass spectrometry (ICP-MS), owing to its high sensitivity, is a powerful technique for otolith trace element analysis, but accurate measurement of trace transition and rare earth elements (REEs) in otoliths solutions is very difficult. The elevated calcium matrix (ca. 0.5 to 1% Ca2+) not only degrades the instrumental detection power but also results in the formation of numerous polyatomic ion interferences that can confound the measurement accuracy. In this work, a matrix removal approach based on Mg(OH)2 coprecipitation is developed for elimination of the calcium matrix of the otolith solution prior to ICP-MS analysis. Trace elements (10 µg/L) and REEs (1 µg/L) added into a simulated otolith solution (0.5% Ca2+ as nitrate) were coprecipitated in small amounts of Mg(OH)2 by adding 0.1 mL triethylamine. A 200 to 250 µg/mL Mg2+ concentration in solution was sufficient to initiate Mg(OH)2 precipitation and to achieve coprecipitation of trace elements quantitatively. After adding triethylamine, precipitates were centrifuged in 2-mL microcentrifuge tubes. Supernatant solution was discarded, and the pellet was washed gently with water and then dissolved in 0.5 mL 10% HNO3. The volume was completed to 2 mL with 2% HNO3 and analyzed by ICP-MS. Recoveries for trace transition and REEs varied from 88 to 101%. More than 99.5% of the calcium matrix was eliminated; Ca levels in the analysis solution varied from 4 to 20 µg/mL. The method was validated by analysis of an otolith certified reference material (National Institute for Environmental Studies Certified Reference Material NIES CRM 22) and applied to the elemental analysis of various natural fish otoliths and clam shells.