DETERMINATION OF CD, CO, CU, FE, MN, NI AND ZN IN CORAL SKELETONS BY CHELATION ION CHROMATOGRAPHY

Cadmium, Co, Cu, Fe, Mn, Ni and Zn incorporated in the aragonitic skeletons of corals (Porites) were analyzed by chelation ion chromatography (CIC). A 500-mg amount of bleached, oven-dried coral powder was dissolved in 1 ml of 14.4 M HNO3 and buffered at pH 5.4 +/- 0.1 using 50 ml of 2 M ammonium acetate. A 30-g amount of sample was concentrated on-line by pumping the solution through a Dionex MetPac CC-1 chelating concentrator column at a rate of 3 ml/min, and the MetPac was rinsed with 2 M ammonium acetate to elute alkaline earth metals to waste. The transition metals were eluted on a second concentrator column (Dionex TMC-1) using 2 M HNO3, and the TMC-1 was then converted to a salt form using 0.1 M NH4NO3 and switched on-line with the analytical column, the Dionex IonPac CS-5. The metals were then eluted from the separator column using 6 mM pyridine-2,6-dicarboxylic acid-0.4 M NaOH; the pH of this eluent was 4.4. The separated metals were, after they had left the column, complexed by 0.5 mM 4-(2-pyridylazo)resorcinol, and the absorbance was measured in a Dionex ultraviolet-visible detector at 520 nm. The use of two separator columns in series improved the metal separation. This is especially important for Ni and Co because coral samples typically contained an order of magnitude more Zn than Ni or Co. With two columns in series, Ni, Co and Zn were baseline-separated even when a 30-g sample containing 5 ppb Ni, 5 ppb Co and 100 ppb Zn was concentrated. The absolute sensitivity of the instrument was approximately 10 ng Cd. Although Cd was successfully determined in several corals using CIC, most coral samples contain on the order 1 ng/g Cd or less and, thus, the quantitative measurement of Cd in most corals by CIC would require tens of grams of coral powder. Unfortunately, it is not possible to obtain such large quantities of coral material from annual bands. The chelation system may be useful, however, to pre-concentrate coral digests for Cd analyses using more sensitive methods of detection (e.g. graphite furnace atomic absorption spectrometry or inductively coupled plasma mass spectrometry).