Cadmium seawater analysis

Scarponi et al. [93] concluded that filtration of seawater through uncleaned membrane filters shows positive contamination by cadmium, lead, and copper. In the first filtrate fractions, the trace metal concentration maybe increased by a factor of two or three. During filtration, the soluble impurities are leached from the filter, which is progressively cleaned, and the metal concentration in the filtrate, after passage of 0.8 -11 of seawater, reaches a stable minimum value. Thus it is recommended that at least one litre of seawater at natural pH be passed through uncleaned filters before aliquots for analysis are taken... 

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. 

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. 

In similar work, Sturgeon et al. [125] compared direct furnace methods with extraction methods for cadmium in coastal seawater samples. They could measure cadmium down to 0.1 pg/1. They used 10 pg/1 ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont [116] for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/1 in a seawater sample containing 0.07 pg/1. Generally, he charred at 300 °C and atomised at 1500 °C. This method required compromise between char and atomisation temperatures, sensitivity, heating rates, and so on, but the analytical results seemed precise and accurate. Nitrate added as sodium nitrate delayed the cadmium peak and suppressed the cadmium signal. 

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. 

A Cis column loaded with sodium diethyldithiocarbamate has been used to extract copper and cadmium from seawater. Detection limits for analysis by graphite furnace atomic absorption spectrometry were 0.024 pg/1 and 0.004 xg/l, respectively [283]. 

Olsen et al. [660] used a simple flow injection system, the FIAstar unit, to inject samples of seawater into a flame atomic absorption instrument, allowing the determination of cadmium, lead, copper, and zinc at the parts per million level at a rate of 180-250 samples per hour. Further, online flow injection analysis preconcentration methods were developed using a microcolumn of Chelex 100 resin, allowing the determination of lead at concentrations as low as 10 pg/1, and of cadmium and zinc at 1 pg/1. The sampling rate was between 30 and 60 samples per hour, and the readout was available within 60-100 seconds after sample injection. The sampling frequency depended on the preconcentration required. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Cabezon et al. [662] simultaneously separated copper, cadmium, and cobalt from seawater by coflotation with octadecylamine and ferric hydroxide as collectors prior to analysis of these elements by flame atomic absorption spectrometry. The substrates were dissolved in an acidified mixture of ethanol, water, and methyl isobutyl ketone to increase the sensitivity of the determination of these elements by flame atomic absorption spectrophotometry. The results were compared with those of the usual ammonium pyrrolidine dithiocarbamate/methyl isobutyl ketone extraction method. While the mean recoveries were lower, they were nevertheless considered satisfactory. 

Zhuang et al. [664] used palladium salts as a coprecipitation carrier for the concentration of cadmium, cobalt, and lead in seawater prior to analysis by atomic absorption spectrometry. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

Field et al. [747] used ICP high-resolution mass spectrometry to determine vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, and lead in seawater. Each analysis required 50 p,l sample and a 6 minute analysis time. 

Scarponi et al. [781] studied the influence of an unwashed membrane filter (Millpore type HA, 47 mm diameter) on the cadmium, lead, and copper concentrations of filtered seawater. Direct simultaneous determination of the metals was achieved at natural pH by linear-sweep anodic stripping voltammetry at a mercury film electrode. These workers recommended that at least 1 litre of seawater be passed through uncleaned filters before aliquots for analysis are taken the same filter can be reused several times, and only the first 50-100 ml of filtrate need be discarded. Samples could be stored in polyethylene containers at 4 °C for three months without contamination, but losses of lead and copper occurred after five months of storage. 

Nygaard et al. [752] compared two methods for the determination of cadmium, lead, and copper in seawater. One method employs anodic stripping voltammetry at controlled pH (8.1,5.3 and 2.0) the other involves sample pretreatment with Chelex 100 resin before ASV analysis. Differences in the results are discussed in terms of the definition of available metal and differences in the analytical methods. 

Drabek et al. [803] applied potentiometric stripping analysis to the determination of lead, cadmium, and zinc in seawater. The precision was evaluated by several duplicate determinations and was found to be in the range 5-16% relative, depending on the concentration level. The accuracy of the method was evaluated by comparison with other conventional methods, e.g., AAS and ASV, and good agreement between the methods was found. 

Holzbecker and Ryan [825] determined these elements in seawater by neutron activation analysis after coprecipitation with lead phosphate. Lead phosphate gives no intense activities on irradiation, so it is a suitable matrix for trace metal determinations by neutron activation analysis. Precipitation of lead phosphate also brings down quantitatively the insoluble phosphates of silver (I), cadmium (II), chromium (III), copper (II), manganese (II), thorium (IV), uranium (VI), and zirconium (IV). Detection limits for each of these are given, and thorium and uranium determinations are described in detail. Gamma activity from 204Pb makes a useful internal standard to correct for geometry differences between samples, which for the lowest detection limits are counted close to the detector. 

Although ICP-ES is a multielement technique, its inferior detection limits (relative to GFA-AS) necessitate the processing of relatively large volumes of seawater. 250 mL aliquots were found to be useful for the analysis of iron, manganese, zinc, copper, and nickel. Extension of the method to include cadmium, cobalt, chromium, and lead would require improvements in the preconcentration procedure. 

A significant proportion of the needs for reference materials for seawater trace metal studies would be addressed by the preparation of these materials. Although the total iron concentration of these reference materials should be provided, these materials clearly will be useful for studies of other important metals such as zinc, manganese, copper, molybdenum, cobalt, vanadium, lead, aluminum, cadmium, and the rare earth elements. With careful planning, such water samples should be useful for analysis of dissolved organic substances as well. The collection sites should be chosen carefully to provide both a high and a low concentration reference material for as many metals as possible. 

Coale, K. H., and Mart, L. (1985) Analysis of Seawater for Dissolved Cadmium, Copper and Lead An Inter-comparison of Voltammetric and Atomic Absorption Methods. Mar. Chem. 17, 285-300. 

Anderson (11) was the first to report on the use of FIA for the analysis of seawater micronutrients. He developed a method for the simultaneous determination of nitrate and nitrite. The chemical reactions for the analysis of nitrate were based on the reduction of nitrate to nitrite by a copper-ized cadmium column placed in the flow path. The nitrite was then analyzed as an azo dye (11). This reaction sequence is conventionally used in both segmented CFA and manual analyses of nitrate and nitrite in seawater (2, 6, 7). The detection limits are 0.1 fxM for nitrate and 0.05 juM for nitrite. 

A rapid technique has been developed for quantitatively concentrating several trace metals from aqueous solution. The metals are co-precipitated as dithiocarbamate chelates by adding an excess of another dissolved metal. This technique has been coupled with atomic absorption analysis for the precise determination of nmol/kg quantities of copper in seawater. Radiotracer experiments show that nickel, iron, and cadmium are also co-precipUated by this technique under proper experimental conditions. 

In order to show the effect of total salt quantity in the atomizer, a series of injections were made for cadmium and manganese analysis with different volumes of solution but with the same total quantity of the analysis metal present per injection. Three series of injections were made—in distilled water, in seawater, and in seawater diluted to maintain the total salt quantity per injection constant. The results are shown in Figures 15 and 16 for manganese and cadmium, respectively. It is... 

Figure 22. Recorder signal for cadmium analysis in seawater and seawater spiked with 0.5 ppb of cadmium (recorder scale expansion, 5X chart speed, 160 mm/min)...

Stock Solutions and Standard Additions. One-liter, 10 M stock solutions were made by dissolving the appropriate amounts of copper, zinc, cadmium, and lead or their salts in nitric acid and diluting to volume. From these, two sets of standard solutions were prepared by dilution with deionized, quartz-distilled water a quadruple standard, used in the zinc analysis, containing 5.0 X 10 M copper, 2.5 X 10 M zinc, and 2.5 X 10" M cadmium and lead and a triple standard, 2.0 X lOr M in copper, 1.0 X 10 M in cadmium, and 2.0 X 10 M in lead, used in the analysis of these three metals. Additions of 100 /J. or multiples thereof were made to 100- or 200-ml seawater samples. 

To summarize, the analysis of seawater samples of representative composition for copper, zinc, cadmium, and lead by AASV with standard addition should yield reasonably accurate values for the concentrations of the metals. Although nickel and silver are present in seawater in concentrations high enough to interfere with the determinations of zinc and copper, the error caused by these metals is expected not to exceed 10 or 15%. Nevertheless, the composition of samples cannot always be guaranteed, and the analysis is always made on the assumption that the standard partitions are present in the sample and in the film in exactly the same manner as the metals originally present in the sample. Because this cannot be known with certainty, particularly when a field survey is being conducted, automated ASV with the thin film must at present be considered a semiquantitative indicator of trace metal activity in the water. Thorough intercomparisons between thin-film voltammetry and other techniques are needed to establish fully the quantitative aspects of this method. 

Potentiometric stripping analysis, as stated in one review,92 "is not as general an analytical technique for the determination of metal traces as is graphite-furnace atomic absorption spectroscopy." It is used as a complementary technique for assay of some toxic metals in water (zinc, cadmium, lead, and copper in potable water and wastewater,93 94 and lead and thallium in seawater.95 The advantage of anodic stripping voltammetry (ASV) is summarized in two steps, which include electrolytic preconcentration and the stripping process. There are a number of interfering ions that can affect the... 

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