Cations, pre concentration

Cation Pre-concentration method Analytical finish Detection limit Reference... 

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

Barium Barium pre-concentrated on cation exchange resin, then extraction with nitric acid Graphite furnace AAS [78]... 

Table 5.15. Miscellaneous solids used in the pre-concentration of cations in seawater...

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Toxic trace elements were isolated from water samples by extraction with di-ethyldithiocarbamate (Table 2.1.2). Following this pre-concentration step the metal ions were adsorbed on a cation-exchange resin using a mixture of tetrahydro-furan-methylglycol-6 M HCl as sorption solution. The succesive elution was treated with 6 M HCl, 1 M HCl and 2 M HNO3 for fractional separation. In another application hexane-isopropanol-HCl mixture was used as the adsorption medium An analytical scheme which provides quantitative results, is described for ion-exchange separation of fifteen major, minor and trace elements in silicates For concentration and separation of copper, chromium, lead and iron an ion-exchanger in phosphate or OH -form was used in various combinations ... 

The introduction of microprocessor technology, in connection with modem stationary phases of high chromatographic efficiencies, makes it a routine task to detect ions in the medium and lower ppb concentration range without pre-concentration. The detection limit for simple inorganic anions and cations is about 10 ppb based on an injection volume of 50 pL. The total amount of injected sample lies in the lower ng range. Even ultrapure water, required for the operation of power plants or for the production of semiconductors, may be analyzed for its anion and cation content after preconcentration with respective concentrator columns. With these pre-concentration techniques, the detection limit could be lowered to the ppt range. However, it should be emphasized that... 

Ion flotation has been used as a method for pre-concentration of heavy metal ions in water. Anionic complexes of these elements are formed by adding complexing agents, and then floated by using a cationic surfactant and nitrogen. The amount of the surfactant should be greater than the stoichiometric amount, but excessive concentration may decrease trace recoveries [93]. 

A simple and fast flow injection fluorescence quenching method for the determination of low levels of iron(III) in water has been developed. For this purpose, a preconcentration minicolumn consisting of cation-exchange resin was coupled to the FIA system. The use of mini-column in the system provided an improvement in sensitivity and the developed FIA method was successfully applied to the on-line determination of low levels of iron in real samples without the pre-concentration process. Fluorimetric determination was based on the measurement of the quenching effect of iron on salicylic acid fluorescence. An emission peak of salicylic acid in aqueous solution occurs at 409 nm with excitation at 299 nm. The effect of interferences from various metals and anions commonly present in water was also studied. The method was successfully applied to the determination of low levels of iron in real samples (river, sea, and spring waters). 

Many applications of cationic clays are directed toward the detection of heavy metals [1]. hi this case, the analytical method followed is very similar to an anodic stripping voltammetric technique a step reducing pre-concentrated metal ions to the zero valence state is performed and the analytical signal is collected in the subsequent anodic re-oxidation step [61]. The selectivity of the clay toward a target metal ion can be improved by a functionalization of the material with suitable alkylsilane derivatives [64, 65, 80] or by intercalation of a specific organic ligand inside the clay interlayers [66, 67, 81]. 

At variance with the high number of applications of cationic clays as pre-concentrating materials, only a few examples are found about the use of anionic clays [77-79]. They involve both LDHs as such, for the adsorption of electroactive anions [76], and the modification of conventional anionic clays for the analysis of organic species in this last case, anionic surfactants are adsorbed in the interlayer region of the clay, to increase the distance between adjacent brucite-like layers and to induce a higher hydrophobic character that is more suitable for the adsorption of organic species [84, 85]. 

Most of the instrumental methods currently available do not provide the sensitivity or freedom from matrix interferences to determine trace elements in seawater at the picomolar and nanomolar levels directly. Therefore, in most cases a pre-concentration step (and separation from the matrix for some methods) is necessary before instrumental detection. These pre-concentration steps may be selected from a wide variety of selective techniques for example, liquid-liquid extraction with chelating agents (such as dithiocarbamates), chelating cation exchange with resins (such as Chelex-100), electrochemical pre-concentration in ASV and CSV, hydride generation for the measurement of metalloids or co-precipitation with Mg(OH)2 or selected metal chelates. Some of these techniques will be discussed in more detail in conjunction with the analytical procedures outlined in Sections 12.2-12.4. 

Ion chromatography can also be applied for ultra-trace analysis of transition metals. In comparison to ICP-MS it offers the advantageous ability to specify the oxidation state of metals and, moreover, to carry out multi-element determinations in the lowest ng/L range after pre-concentration. Transition metal separation is performed with ion exchangers that have defined anion and cation exchange capacities detection is carried out photometrically after derivatization... 

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