Chelate solvent extraction

Determination of chromium by many of the methods cited above is problematic. Variable and nonquantitative recovery with chelation-solvent extraction... 

Use of immobilised chelating agents for sequestering trace metals from aqueous and saline media presents several significant advantages over chelation-solvent extraction approaches to this problem [193,194], With little sample manipulation, large preconcentration factors can generally be realised in relatively short times with low analytical blanks. 

Owing to inadequate detection limits by direct analysis, various workers examined preconcentration procedures, including dithiocarbamate preconcentration [447,732-734], ion exchange preconcentration [735-737], chelation solvent extraction [736], coprecipitation [738], and preconcentration in silica-immobilised 8-hydroxyquinoline [129]. 

Table 5.13. Pre-concentration of metals in sea water chelation-solvent extraction techniques followed by direct AAS and graphite furnace AAS... 

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. 

Ashbrook, A. W. Commercial chelating solvent extraction reagents. 1. o-Hy-droxyoximes Purification and isomer separation/extraction. Metallurgy Division, Mines Branch, Dept. Energy, Mines and Resources, Canada, Report EMA 73-10, 1973. 

The use of chelating solvent extractants such as the hydroxyoximes for the flotation of oxidized copper ores has been reported 8 9 unfortunately, although laboratory tests have shown that the use of chelating collectors has potential, they are not yet in widespread commercial use. 

There are four widely used methods for preconcentrating trace metals from water, namely evaporation, chelation—solvent extraction, ion-exchange and coprecipitation. 

The low level of aluminium in some samples may require concentration by chelation—solvent extraction prior to introduction into the flame. A procedure described for wines [36] involves digestion with HN03—H2S04, extraction with 8-hydroxyquinoline in MIBK and determination with a nitrous oxide—acetylene flame. Aluminium has also been extracted with 2,4-pentanedione [185], EAAS has been applied to measuring aluminium in beer [186]. Gorsuch [139] proposes wet oxidation to be preferred and mentions that adverse comments reported in respect of sample destruction have usually applied to dry ashing. 

The procedures described above using FA AS are suitable for samples with chromium content > 2 mg kg-1. To determine the very low (jug kg-1) natural levels, resort must be made to techniques involving chelation—solvent extraction [189,195] and/or EAAS [196—198]. No procedure for jug kg-1 levels of chromium in biological tissues has yet reached official status. 

The sample is brought into solution by wet digestion or dry ashing. Nickel is concentrated if required by chelation—solvent extraction, and is determined by FAAS using an air—acetylene flame. Vegetable oils are dissolved in MIBK prior to FAAS measurement. 

With the majority of food products, copper can usually be determined by FAAS. In the few instances of low copper levels, resort has commonly been made to chelation—solvent extraction prior to FAAS measurement [8d, 34, 37m, 217] recent reports on the application of EAAS to analysis of foodstuffs and biological materials have appeared [193, 201, 204, 205, 223, 224], The importance of non-atomic absorption in the determination of copper in plant digests by FAAS has been studied by Simmons [225]. Correction was necessary to arrive at good results, with a continuum light source providing more accurate data than use of a nearby non-absorbing line. 

The sample is wet digested, molybdenum is concentrated by chelation-solvent extraction, and determined by FAAS using a nitrous oxide-acetylene flame. 

F. Chelation—solvent extraction method for determining trace metals in water samples... 

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