Chelates analytical methods

An outline of the history of analytical methods based on complex formation. F. Szabadvary and M. T. Beck, Chelates Anal. Chem., 1971,1,1-14 (69). 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

Metal ions, especially transition metal ions, form coloured complexes with chelating ligands that are the basis of the analytical methods and indicators for these ions that have been used for many decades. " The term metallochromism has been apphed to this phenomenon, and hence metallochromic indicators. Typical bidentate ligands are dimethylglyoxime, 1,2-dihydoxybenzenes and 1-hydroxyanthraquinones, 8-... 

The peroxy complex of titanium(IV), Ti(02)(edta)2, has been used as part of an analytical method for either titanium(IV) or H202. More recently the structural nature of TiO(edta)2- in solution and the reaction of TiO(edta)2- with H202 to form Ti(02)(edta)2 have been investigated over the pH range 2.0-5.2. Two distinctly different primary coordination spheres in terms of spectral properties and reactivities towards H202 are observed for TiO(edta)2" solutions in this pH range. As shown in Scheme 2, these are the aquated forms (A), TiO(edtaH )(H20), 2, having two coordinated carboxylates and two amines and one coordinated water, and the fully chelated forms (B), TiO(edtaH ) 2, with three carboxylates and two amines coordinated to titanium(IV).115... 

The use of europium chelates, with their unusually long fluorescence decay times, as labels for proteins and antibodies has provided techniques that are referred to as time-resolved fluoroimmunoassays (TRFIA). Fluorophores as labels for biomolecules will be the topic of Sect. 3. Nevertheless, TRFIAs always have to compete with ELISA (enzyme-linked immunosorbent assays) techniques, which are characterized by their great versatility and sensitivity through an enzyme-driven signal amplification. Numerous studies have been published over the past two decades which compare both analytical methods, e.g., with respect to the detection of influenza viruses or HIV-1 specific IgA antibodies [117,118]. Lanthanide luminescence detection is another new development, and Tb(III) complexes have been applied, for instance, as indicators for peroxidase-catalyzed dimerization products in ELISAs [119]. 

Determination of lanthanide ions with a fairly low detection limit of 10-5-10-6 wt% is achieved with Ln111 /i-diketonates. Relying on this fact new, rapid, selective, and highly sensitive analytical methods essentially based on the luminescence properties of these complexes have been developed. After a preliminary work on Smm and Eum chelates (Topilova et al.,... 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

The presence of water has been postulated to cause the thermal instability of many of the hydrates (3, 4, 20), and attempts to chromatograph neodymium (III) trifluoroacetylacetonate dihydrate have failed (23). There is some evidence that hydrolysis occurs at elevated temperatures (3, 4, 20). Furthermore, there are indications that certain complexes undergo hydrolysis when allowed to stand in vacuo, even at room temperature (20). For these reasons the chelates are difficult to dehydrate by conventional means. Many of the claims in the older literature that anhydrous tris chelates were obtained must be considered questionable because the assignments of composition were either arbitrary or were based on analytical methods relatively insensitive to the amount of water present. Some investigators, however, have reported reasonably well-characterized anhydrous tris complexes 4,11,15), but most of these are not sufficiently volatile and stable to be chromatographed. 

The lack of suitable chelating agents for some analytes of interest is the main limitation of GC/MS. However, Reamer and Veillon (1981b) have developed a suitable chelate and method for the determination of selenium in biological materials. This method has been further adapted to a double-isotope method for studies of selenium metabolism (Reamer and Veillon, 1983 Swanson et al., 1983a, 1983b Moser-Veillon et al., 1992). Under routine conditions this method has proven to be well suited to determine total selenium in foods, diets, human breast milk, infant formulae, plasma and serum, red blood cells, feces and urine. The method has the accuracy expected for a definitive method, the precision is about 2% at the 100/absolute detection limit is about 50 pg. 

Questions frequently arise, especially during the development phase of analytical methods, about how to optimize parameters, such as reaction time, concentration of reagents, etc. and how to optimize analysis procedure, e.g. extraction method, choice of reagent, etc. Questions constantly crop up, e.g. "What effect do pH and EDTA level have on the extraction of a metal ion by means of a chelating agent " - or "What significance do pH and redox potential have in the desorption of heavy metals from sands "... 

Kulmala S, Eskola J, Laaksonen T, Korpela T, Hakansson M, Suomi J, (2010) Integrated carbon electrode chips for electrical excitation of lanthanide chelates and analytical methods utilizing these chips. Patent application FI 20100253... 

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