Determination of cobalt and iron in estuarine and coastal waters using flow injection with chemiluminescence detection† Vincenzo Cannizzaro, Andrew R. Bowie, Anton Sax,‡ Eric P. Achterberg and Paul J. Worsfold* Department of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, Plymouth, UK PL4 8AA Received 21st September 1999, Accepted 2nd November 1999 Flow injection with chemiluminescence detection (FI-CL) was used to determine cobalt and iron in estuarine and coastal waters. Cobalt(II) was determined by means of a pyrogallol–hydrogen peroxide–sodium hydroxide reaction in the presence of methanol and the surfactant cetyltrimethylammonium bromide (CTAB). With pyrogallol, the sensitivity was enhanced compared with the traditional reagent, gallic acid. The practical limit of detection in sea-water was 5 pM (3s) and there was good agreement with certified values for the sea-water CRMs NASS-5 (0.16 ± 0.01 nM), CASS-3 (0.60 ± 0.09 nM) and SLEW-2 (0.93 ± 0.13 nM). Results for an Irish Sea sample gave good agreement with data obtained using cathodic stripping voltammetry. Iron(ii + iii) was determined using a luminol reaction with dissolved oxygen as the oxidant. The practical limit of detection was 40 pM (3s) and results from shipboard analysis of the CRM NASS-4 (1.95 ± 0.14 nM) were in good agreement with the certified value (1.88 ± 0.29 nM). Field evaluation of the instrumentation and analytical methods was achieved through a series of local surveys in the Tamar Estuary (UK), from which environmental data are presented. Introduction The oceanic concentrations of cobalt are extremely low and it has been suggested that the element may act as a (co-)limiting nutrient for marine phytoplankton. 1 Cobalt demonstrates a scavenged-type vertical distribution in the open ocean, typically 18–300 pM in surface waters and 20–50 pM at depth. 2 Cobalt concentrations in estuarine and coastal waters are significantly higher. For example, Knauer et al. 1 reported 0.85–20 nM in north San Francisco Bay and Achterberg et al. 3 found 140–310 pM in coastal waters near the Wash and Humber Estuaries on the eastern coast of the UK. High concentrations of Co (up to 3.5 nM) have also been reported in hydrothermal plumes 4 and the element has been used as a chemical indicator of hydrothermal activity. 5 Cobalt is a co-factor in the vitamin B 12 complex and is known to accumulate in manganese nodules. It is only toxic to plants and mammals at relatively high concentrations ( > 17 mM), which are rarely observed in the aquatic environment. 6,7 Iron has played a key role in oceanographic research over the past decade and is now thought to act as a limiting micronutrient regulating biological productivity in ca. 40% of the world’s oceans. 8210 Consequently, Fe has been intimately linked to the ocean–atmosphere carbon dioxide exchange 11 and transitions in climate from glacial to interglacial times. 12 Recent advances in analytical and sampling techniques for trace metal determina- tions have led to an increased understanding of the bio- geochemical role of Fe in sea-water, although its distribution in many remote seas and across ocean margins is poorly constrained. Iron has been reported to demonstrate a nutrient- like profile in many regions of the open ocean, with dissolved Fe typically existing at < 0.2 nM in surface waters and converging to 0.7–0.8 nM in deep waters. 13 However, recent studies suggest that Fe distributions show high temporal and spatial variability through oceanic provinces where atmospheric fluxes are high (e.g., in the Atlantic Ocean 14 ). In coastal and shelf seas, trace metals are delivered laterally by rivers and through diffusion from reductive shelf sediments, resulting in elevated Fe levels (e.g., 1.7 nM in the North Sea 15 ). Estuaries, however, act as a filter, effectively trapping elevated riverine concentrations of metals and leading to large property gradients existing at the land–sea margin. 16 Commonly used laboratory techniques for sub-nanomolar Co and Fe determinations include electrothermal atomic absorption spectrometry (ETAAS) following filtration and extractive complexation 17 and inductively coupled plasma mass spectrom- etry (ICP-MS). 18 There is a need, however, to develop shipboard analytical methods owing to the problems of sample instability and contamination during transport and storage. ICP- MS and ETAAS are impractical for shipboard use because the instrumentation is bulky, expensive and sensitive to the ship’s vibrations, and the preferred methods for Co and Fe are based on voltammetry, 15,19,20 catalytic spectrophotometry 21 and flow injection with liquid-phase chemiluminescence detection (FI- CL). 14,22–30 All require preconcentration utilising solvent extraction, electrochemical deposition of a metal–ligand com- plex or chelating resin column separation. Advantages of the FI- CL approach are the ability to perform in-line matrix removal and preconcentration, low-cost detection, wide dynamic range, rapid analysis (seconds), robustness and portability. In this work, the relative sensitivities of seven polyhydroxy aromatic compounds were evaluated for the FI-CL determina- tion of Co(ii). A reaction based on the oxidation of pyrogallol was optimised to permit a limit of detection of 5 pM in sea- water. A modified FI-CL method for the determination of Fe(ii + iii) in estuarine waters, based on the luminol chemistry, is also reported. An inert tangential flow device is described for filtration of waters containing high suspended particulate matter. The instrumentation was successfully deployed along an † Presented at SAC 99, Dublin, Ireland, July 25–30, 1999. ‡ Present address: IFA-Tulln, Analytikzentrum, Konrad Lorenz Strasse 20, A-3430 Tulln, Austria. This journal is © The Royal Society of Chemistry 2000 Analyst, 2000, 125, 51–57 51