Seawater cation preconcentration

Matthews and Riley [336] determined indium in seawater at concentrations down to 1 ng/1. Preconcentration of metals on a cation exchanger was fol-... 

In general, this technique does not have adequate sensitivity for the determination of the low levels of cations likely to occur in seawater. Coupling the technique with a preconcentration procedure can, however, enable some analyses to be carried out at the pg/1 level. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

Ordinary anion and cation ion-exchange resins are of limited use for the analytical concentration of trace elements from water, because of their lack of selectivity. This is especially so with strong electrolytes such as seawater. In this case the major ions sodium, magnesium, calcium and strontium, are retained preferentially. However, the recent advent of commercial chelating resins based mainly on iminodiacetic acid-substituted cross-linked polystyrene, makes it possible to concentrate trace elements from waters. In consequence, a number of researchers have used chelating resins for trace-metal preconcentration from seawater and natural waters. 

Chelation and Metal Recovery. The competition of protons with trace metals for chelation sites is well established. Figure 2 was constructed from stability constants determined in simple, well-defined solution media the effect of seawater medium on the stability constants and the influence of other cations, both major (e.g., Ca " and Mg " ) and minor (e.g., trace metals), was neglected. The effect of pH on the retention of metals by the preconcentration cartridges probably differs somewhat from that shown in Figure 2 because of medium effects and because of 8-HQ distortion upon immobilization. 

Figure 23-3 Cation-exchange column containing 1.2 mL of resin used for shipboard preconcentration of traces of Fe from 10 L of filtered seawater. [F. Lacan, A. Radic,...

Figure 23-3 shows a column containing 1.2 mL of cation-exchange resin used to preconcentrate traces (11-330 ng) of Fe from 10 L of filtered seawater. Elution of Fe from the resin with 1.5 M HNO3 into a final volume of 10 mL gives a 1 000-fold increase in concentration. When done with extreme care in a shipboard clean-room, a blank has only 1 ng of Fe. 

Another paper has suggested solid-phase extraction (SPE) for the preconcentration and spe-ciation of Cr in waste water samples prior to AAS determination [106]. Interferences of di- and tri-valent cations and diverse anions were studied. The detection limits for these species are about 1 pg/L. Other examples for SPE procedures are given in [107] for seawater analysis using on-line ICP-MS and in [108] for the separation and determination of precious metals. 

The above-mentioned preconcentration of transition metals on MetPac CC-1 was originally used by Kingston [626] for the AAS analysis of transition metals in seawater. It is based on the selective concentration of transition metals on a macroporous iminodiacetic acid resin, where divalent cations are retained by chelation. Their affinity toward the stationary phase decreases in the order... 

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