Study of EPR Signals in Corals - EPR (ESR) Applications

The name coral comes from the Old Persian sanga (stone), which is the common name for the coral worm community and their skeleton. Coral polyps are corals of the phylum Acanthozoa, with cylindrical bodies, which are also called living rocks because of their porosity and branching growth, which can be inhabited by many microorganisms and fish. Corals flourish in tropical ocean. The chemical composition of white coral is mainly CaCO₃ and contains organic matter, called carbonate type. Golden, blue, and black coral is composed of keratin, called keratin type. Red coral (including pink, flesh red, rose red, light red to deep red) shell has more keratin and CaCO₃. Cora, based on  the skeletal structure characteristics, can be divided into plate bed coral, four-shot coral, six-shot coral, and eight-shot coral. Modern coral is mostly the latter two categories. Coral is an important carrier to record the marine environment, for the determination of paleoclimatology, ancient sea level change and tectonic movement and other studies have important significance.

Electron paramagnetic resonance spectroscopy is a powerful tool for studying substances with unpaired electrons. This technique utilizes microwave irradiation to probe the energy separation of unpaired electrons created by an external magnetic field.  A special application of EPR spectroscopy is developed for analyzing corals, which is a valuable method marine environmental studies.   The information regarding paleoclimate is reflected in the Mn²⁺ concentration in coral reef. Using EPR spectroscopy, the signal of Mn²⁺ can be easily analyzed and interpreted, since its concentration is relatively high during a warm period and decreases sharply during a period of rapid cooling. Another set of substances in coral that can be measured by EPR spectroscopy is lattice defects and impurities produced by natural irradiation. Such lattice defects trap unpaired electrons, which produces observable EPR signals. The lineshape of such signals are indicative to the composition of minerals and trapped impurities, and therefore can be used for inferring the information about age of formation as well as crystallization condition of samples.

The EPR signal in the coral will be analyzed using a CIQTEK X-Band EPR (ESR) spectroscopy EPR100 to provide information on the composition and defect vacancies in the coral.

CIQTEK X-Band EPR100

Experimental Sample

The sample was taken from white coral in the South China Sea, treated with 0.1 mol/L dilute hydrochloric acid, crushed with a mortar, sieved, dried at 60°C, weighed about 70 mg, and tested on the CIQTEK EPR100.

White Coral Sample

Electron Paramagnetic Resonance Spectroscopy

The CIQTEK EPR100 was used to test the EPR signal in white coral. To achieve accurate measurement of the EPR signal, the specific experimental conditions were as follows.

Experimental Conditions

Experimental Results & Analysis

The structure of corals varies depending on the species, and unresolved spectral line are usually present in the EPR spectra of corals. The experimentally measured signals in coral bone samples may be assigned to CO^2 - ion free radicals by comparing information from the literature. Mn²⁺ in corals is usually associated with the recrystallization of corals and secondary calcite deposition. Since even trace amounts of Mn²⁺ produce sharp EPR lines, it is used for determine the recrystallization of samples. The lack of the signature sixfold peak signal of Mn²⁺ is indicates there was no recrystallization of samples. 

EPR Spectra of White Corals Detected by CIQTEK EPR100

EPR100 in Research

Coral EPR (ESR) measurements are currently used mainly for marine environmental climate analysis and dating. In addition, because carbonate lattice and impurity defects in corals trap unpaired electrons, and these unpaired electron concentrations can be correlated with natural and annual irradiation dose rates, they are also commonly used for EPR dating in the million-year range.

The CIQTEK EPR100 features high sensitivity, high magnetic field uniformity, and high stability, and can be equipped with a variety of high-performance EPR probes and optical devices , low-temperature, and goniometer to meet customers' needs for a variety of general continuous-wave and pulsed EPR measurements, and the EPR-Pro software provides a fast experimental operation process and scientific data analysis functions to help users quickly establish EPR experimental methods.

References:

1. Strzelczak, Grazyna , et al. "Multifrequency EPR study of carbonate- and sulfate-derived radicals produced by radiation in shells and corallite." Radiation Research 155.4(2001):619- 624.

2. Seletchi, E. D. , and O. G. Duliu . "Comparative study on esr spectra of carbonates." Romanian journal of physics 52.5-7(2007):p.657-666. 

3. Ye Yuguang, Zhou Shiguang, Liu Xinbo. "The ESR Signal of Mn²⁺ in Coral Reefs and Its Paleoclimatic Implications." Ocean and Limnology 000.005(1998):547. 

4. Li Jianping, Diao Shaobo, Liu Chunru, He Xingliang. "Application of ESR dating in marine carbonate dating." Frontiers in Marine Geology 31.010(2015):65-70.