7 Development of Shipboard Copper Analyses by Atomic Absorption Spectroscopy RICHARD W. ZUEHLKE and DANA R. KESTER Graduate School of Oceanography, University of Rhode Island, Kingston, R I 02881
This chapter describes a technique for automatically preconcentrating dissolved copper in seawater for subsequent analysis by graphite furnace atomic absorption spectroscopy. Copper and other trace metals are chelated by 8-hydroxyquinolineeither immobilized on silica in a chromatographic column or in solution with the oxine complexes isolated on a C reverse-phase liquid chromatography column. Elution with an appropriate solvent provides concentration factors in the range of 25-30 times the concentration in the initial sample. The system is adaptable for removal and preconcentration of organically complexed copper. The methods are evaluated in terms of their analytical chemistry and ability to produce valid profiles through an oceanic water column. The technique is promising if pH and column-loading rates are optimized, and if attention is paid to the role of naturally occurringorganic ligands that can affect the retention of copper by the columns. 18
R E A L - T I M E C H E M I C A L M E A S U R E M E N T S I N T H E O C E A N are receiving increasing attention as a means of advancing our understanding of chemical processes i n the sea. The traditional approach for many types of chemical measurements has been to collect a suite of samples at various depths and locations i n the ocean, to return the samples to a laboratory ashore, and to chemically analyze them. This process produces results months or years after sampling. Measurements of biologically active nutrients (phosphorus, nitrogen, and silicon) and gases (dissolved oxygen and the carbon dioxide system parameters) have been exceptions to this general approach; shipboard techniques have been used extensively for these variables for many 0065-2393/85/0209-0117 $06.50/0 © 1985 A m e r i c a n C h e m i c a l Society
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years. Several advantages are realized by shipboard analytical measurements: biological and chemical effects during storage can be avoided; when the data are available i n nearly real time, the sampling can be modified in view of the results obtained; and experimental studies at sea become possible. This chapter reports our efforts with techniques to measure transition metals at low concentration i n ocean water on board a research vessel. Several criteria that were set forth at the beginning of our work determined our choices i n the analytical approach to this problem. W e ultimately wish to perform measurements with a precision and accuracy of about 5 % at concentrations on the order of 1 0 " mol/kg. Although we have been focusing on a single metal to optimize the techniques i n this stage of the work, we plan to extend the capability to a broad range of metals. W h e n the methods are perfected, the interpretation of the analytical results should be relatively straightforward and comparable to results obtained by using shore-based measurements. The techniques should be sufficiently rapid to allow nearly real-time data acquisition and high spatial resolution of metal distributions i n the ocean. In practice, this requirement sets a limit of 30-45 m i n of total processing time, including any reagent addition, volume measurement, filtration, preconcentration, and dissolution or other phase change. Copper was selected as the first metal for which to attempt to optimize the shipboard analyses because considerable information is available about the marine chemistry of copper, and because this new analytical capability would greatly enhance our ability to study copper i n the ocean. The concentration of copper i n the ocean varies from 0.5 to 5 nmol/kg i n response to biological and geochemical processes (Table I). The chemical speciation of copper has received considerable attention because the biological effects of copper depend on its chemical form (1-3). The principal forms of copper include inorganic complexes such as C u C O j j , C u H C O ^ , C u O H , and organically bound copper (4, 5). A l l of the available analytical methods i n seawater at concentrations on the order of 1 nmol/kg require a preconcentration step prior to analyses. The preconcentration step can serve two purposes: It brings the amount of metal to be detected into the analytical range of the measuring technique, and it can place the metal into a medium more favorable for analysis than seawater. T w o methods are being widely used for copper measurements in seawater: anodic stripping voltammetry (ASV) and heated graphite atomization atomic absorption spectroscopy ( H G A AAS). W i t h A S V , the preconcentration step consists of reducing copper [generally Cu(II) i n seawater] to an amalgam i n a mercury thin-film or drop electrode. Detection is performed electrochemically by oxidizing the copper back into the solutipn i n response to a varying applied voltage at the electrode. W i t h H G A A A S various preconcentration procedures can be used. Some of the most common are coprecipitation of the metal by a scavenging solid phase (6), solvent 9
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