Cadmium in seawater

In the determination of cadmium in seawater, for both operational reasons and ease of interpretation of the results it is necessary to separate particulate material from the sample immediately after collection. The dissolved trace metal remaining will usually exist in a variety of states of complexation and possibly also of oxidation. These may respond differently in the method, except where direct analysis is possible with a technique using high-energy excitation, such that there is no discrimination between different states of the metal. The only technique of this type with sufficiently low detection limits is carbon furnace atomic absorption spectrometry, which is subject to interference effects from the large and varying content of dissolved salts. 

Various workers have discussed the application of graphite furnace atomic absorption spectrometry to the determination of cadmium in seawater [ 115— 124],... 

Batley and Farrah [ 120] and Gardner and Yates [118] used ozone to decompose organic matter in samples and thus break down metal complexes prior to atomic absorption spectrometry. By this treatment, metal complexes of humic acid and EDTA were broken down in less than 2 min. These observations led Gardner and Yates [ 118 ] to propose the following method for the determination of cadmium in seawater. 

Danielson et al. [119] have described a method for the determination of cadmium in seawater. The samples were analysed by graphite furnace atomic... 

As cadmium is one of the most sensitive graphite furnace atomic absorption determinations, it is not surprising that this is the method of choice for the determination of cadmium in seawater. Earlier workers separated cadmium from the seawater salt matrix prior to analysis. Chelation and extraction [ 121— 128], ion exchange [113,124,125,129], and electrodeposition [130,131] have all been studied. 

The direct determination of cadmium in seawater is particularly difficult because the alkali and alkaline earth salts cannot be fully charred away at temperatures that will not also volatilise cadmium. Most workers in the past [125,132-135] who have attempted a direct method have volatilised the cadmium at temperatures which would leave sea salts in the furnace. This required careful setting of temperatures, and was disturbed by situations that caused temperature settings to change with the life of the furnace tubes. 

Campbell and Ottaway [136] also used selective volatilisation of the cadmium analyte to determine cadmium in seawater. They could detect 0.04 pg/1 cadmium (2pg in 50 pi) in seawater. They dried at 100 °C and atomised at 1500 °C with no char step. Cadmium was lost above 350 °C. They could not use ammonium nitrate because the char temperature required to remove the ammonium nitrate also volatilised the cadmium. Sodium nitrate and sodium and magnesium chloride salts provided reduced signals for cadmium at much lower concentrations than their concentration in seawater if the atomisation temperature was in excess of 1800 °C. The determination required lower atomisation temperatures to avoid atomising the salts. Even this left the magnesium interference, which required the method of additions. 

Guevremont et al. [ 117] used a direct, selective volatilisation determination of cadmium in seawater. They used 20 pi seawater samples, 1 g/1 of EDTA, an... 

Guevremont et al. [117] studied the use of various matrix modifiers in the graphite furnace gas method of determination of cadmium in seawater. These included citric acid, lactic acid, aspartic acid, histidine, and EDTA. The addition of less than 1 mg of any of the compounds to 1 ml seawater significantly decreased matrix interference. Citric acid achieved the highest sensitivity and reduction of interference, with a detection limit of 0.01 pg cadmium per litre. 

Sperling [133] has reported extensively on the determination of cadmium in seawater, as well as in other biological samples and materials. He added ammonium persulfate, which permitted charring seawater at 430 °C without loss of cadmium. For workbelow 2 pg/1 cadmium in seawater he recommended extraction of the cadmium to separate it from the matrix [126,134,135]. He found no change in the measured levels over many months when the seawater was stored in high-density polyethylene or polypropylene. 

Three Zeeman-based methods for the determination of cadmium in seawater were investigated. Direct determinations can be made with or without the use of a pyrolytic platform atomisation technique. The wall atomisation methods presented were considerably faster than the platform atomisation technique. For extremely low levels of cadmium, indirect methods of analysis employing a preliminary analyte extraction can be employed. Background levels are minimal in extracted samples, and the total furnace programme time was the lowest of the methods examined. 

Lum and Callaghan [ 140 ] did not use matrix modification in the electother-mal atomic absorption spectrophotometric determination of cadmium in seawater. The undiluted seawater was analysed directly with the aid of Zeeman effect background correction. The limit of detection was 2 ng/1. 

Electrothermal atomic absorption spectrophotometry with Zeeman background correction was used by Zhang et al. [141] for the determination of cadmium in seawater. Citric acid was used as an organic matrix modifier and was found to be more effective than EDTA or ascorbic acid. The organic matrix modifier reduced the interferences from salts and other trace metals and gave a linear calibration curve for cadmium at concentrations < 1.6 pg/1. The method has a limit of detection of 0.019 pg/1 of cadmium and recoveries of 95-105% at the 0.2 pg of cadmium level. 

In a method described by Yoshimura and Uzawa [ 144], cadmium in seawater is coprecipitated with zirconium hydroxide (Zr(OH)4) prior to determination by square-wave polarography. The precipitate is dissolved in hydrochloric acid, and cadmium concentration is determined from the peak height of the... 

Kounaves and Zirino [ 145] studied cadmium-EDTA complex formation in seawater using computer-assisted stripping polarography. They showed that the method is capable of determining the chemical speciation of cadmium in seawater at concentrations down to 10 8 M. 

Ammonium pyrrolidine dithiocarbamate (APDC) chelate coprecipitation coupled with flameless atomic absorption provides a simple and precise method for the determination of nanomol kg 1 levels of copper, nickel, and cadmium in seawater. With practice, the method is not overly time-consuming. It is reasonable to expect to complete sample concentration in less than 20 min, digestion in about 4 h, and sample preparation in another hour. Atomic absorption time should average about 5 min per element. Excellent results have been obtained on the distribution of nickel and cadmium in the ocean by this technique. 

Batley [780] examined the techniques available for the in situ electrodeposition of lead and cadmium in seawater. These included anodic scanning voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen, and in situ electrodeposition on mercury-coated graphite tubes. 

Bruland et al. [785] compared voltammetric and AAS (with preconcentration) methods in the determination of copper, lead, and cadmium in seawater. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

Knowles M (1987) Varian atomic absorption no AA 71 methods for the determination of cadmium in seawater with Zeeman background correction... 

M. C. Yebra-Biurrun, A. Moreno-Cid and L. Puig, Minicolumn field preconcentration and flow-injection flame atomic absorption spectrometric determination of cadmium in seawater, Anal. Chim. Acta, 524(1-2), 2004, 73-77. 

Carbon isotope ratios in benthic foraminifera Cadmium in benthic foraminifera Cadmium in seawater Cd/Ca in foraminifera... 

Cadmium in seawater has a nutrient-like profile, with depleted values in warm surface waters, and... 

Figure 5. Effect of atomization temperature on the absorbance of 5 pi. of a 10-ppb spike of cadmium in seawater...

The riverine and atmospheric inputs to the oceans appear to be about equal but these fluxes are poorly known. With a total input of 10 mol year and an oceanic inventory of 10 mol, the average oceanic residence time of Cd is 10,000 years [18]. The bulk of the cadmium in seawater is in the dissolved phase and only a tiny fraction in suspended solids. The particulate concentration is in the range 0.04-4 pM, corresponding roughly to a concentration in the solid phase of 0.1-1 (jumol Cd mol C (Fig. 4) [19]. 

J. A. Jurado-Gonzalez, M.D. Galindo-Riano, and M. Garda-Vargas. Expjerimental designs in the development of a new method for the sensitive determination of cadmium in seawater by adsorptive cathodic stripping voltammetry. Analytica Chimica Acta 487 229-241, 2003. 

The levels of zinc and cadmium in seawater are 10 and 10 ppm, respectively, which represent mean concentration factors of 10 for zinc and 10 for cadmium. 

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