Reversed-phase chromatographic separation and analysis of marine metal—organic complexes

Abstract

Metal—organic complexes were isolated from seawater by adsorption on reversed-phase chromatographic supports such as C 18 Sep-Pak cartridges and XAD-2 resin and subsequent elution with methanol, acetonitrile or tetrahydrofuran. Preliminary results indicate that Sep-Pak cartridges packed with Styragel are also suitable, although back pressures are higher and low blank values are difficult to achieve. Analysis of the extracts by high-performance liquid chromatography with atomic fluorescence detection of trace metals showed that methanol extracts remain unchanged for up to three months in polypropylene at temperatures between −20°C and 20°C and in glass at −20°C. No changes were observed in acetonitrile extracts stored for one week at −20°C, even though the wet extract separated into two phases. Rapid freezing of C 18 Sep-Pak cartridges to /t-195°C also seemed to be a suitable method for preserving samples for future analysis. High-intensity lamps improved the sensitivity of the atomic fluorescence detector by 1/2-2 orders of magnitude and zinc, magnesium, copper, nickel and iron could be determined simultaneously with detection limits ranging from 0.05 μg l −1 (zinc) to 7 μg l −1 (iron). The chromatograms were calibrated by standard additions of trace metals. All the chromatographic columns investigated (C 18 Resolve, C 18 Nova-Pak, PRP-1 and Protein-Pak 125) contained impurity sites that could interfere with the analysis of metal—organic compounds at the ultra-trace level. As interference was most pronounced for magnesium—organic complexes, magnesium was used as an indicator of the presence of coordinating sites on the chromatographic support. Interference effects could be reduced by the addition of modifying agents containing alkali metal salts at a concentration of 50 m M. The most suitable salt was potassium trifluoromethanesulfonate. Metal—organic compounds were isolated on XAD-2 resin (waters from Storm Bay, Tasmania), and on C 18 Sep-Pak cartridges (waters from south-east Australian waters and the Fiji Basin). The concentrations of copper (410 p M), zinc (650 p M) and iron (800 p M) organic complexes from south-east Australian waters were higher than the values obtained for copper (60/2-120 p M), zinc (50/2-150 p M and iron (150/2-180 p M) organic complexes isolated from the Fiji Basin.