Separation iron-exchange chromatography

An example of the application of dynamic ion-exchange chromatography for the direct separation of rare earths is shown in Fig. 1.22. The sample was a sodium hydroxide leach solution from an aluminium processing operation and contained high concentrations of sodium, iron and aluminium. Due to matrix interference, these solutions could not be accurately analysed by inductively coupled plasma emission spectroscopy. Fig. 1.22 shows the chromatogram when the sample was separated by dynamic ion-exchange... 

Anion exchange chromatography was used to separate iron from other cations. The remainder of the acid solutions were added to mini-columns (4 ml Pasteur pipettes, Fisher Scientific Co.) containing anion-exchange resin (Bio-Rad AG 1-X8). In 6 N HCl, iron is anionic (FeCl/ ) and binds to the resin. After washing with 25 ml of 6 N HCl to remove cations, iron was eluted as a cation with 0.5 N HCl. 

F. W. E. Strelow, Separation of titanium from rare earths, beryllium, niobium, iron, aluminum, thorium, magnesium, manganese and other elements by cation exchange chromatography. Anal Chem., 35,1279,1963. 

If, however, phosphoric acid is used, the iron(III) ions are rapidly removed from the column, and a sharp separation results. Subsequently, the copper(II) ions can be removed with hydrochloric acid. Clearly the phosphate ions form a much more stable complex with iron(III) ions, which are rendered colourless, than with copper(II). Complex formation is undoubtedly an important factor in other types of chromatography, particularly in inorganic separations on paper, but in no other technique has it been exploited to quite the same extent as in ion-exchange chromatography. 

HPLC and CZE methods have been developed to resolve all of the Tf sialoforms present in serum. An HPLC method based on anion-exchange chromatography with direct detection at 420 nm was developed by Jeppsson et al. [188] to individually separate Tf sialoforms in about 16 min. The serum required to be saturated with iron and lipoproteins to be precipitated. Several authors followed the method with minor changes and improvements [186,189,190]. 

Similar to the bis(bidentate)copper(I) centers in the earlier examples, the octahedral bis(tridentate)iron(II) component centers are configurationally achiral, Ihe molecule forms as a racemic mixture of the two chiral heUcal forms, and there are many examples of double-stranded helices of this genre." In this particular instance above, the two enantiomeric forms were separated by cation-exchange chromatography, using an eluent with a chiral anion." ... 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Note that the strong acid paper gives a better separation than the weak acid type. The order of elution on the ion exchange paper is opposite to that in normal chromatography (Figure 9.2). Notice also that on P81 paper the iron does not move, because of the high stability of the iron-phosphate complex. The approximate Rf values are given in Table 9.1. 

Separation of carrier-free lead samples from inert or radioactive contaminants present in macro or tracer quantities can be accomplished by any of a number of the waiys that have been outlined in the foregoing sections on techniques. Ion exchange (k8), solvent extraction (R7), and "filter paper chromatography (P10)(P11) have all been used for separation of carrier-free radioisotopes. A precipitation separation of carrier-free lead from thallium oxide cyclotron targets has alsq been proposed (06) (W8). In this procedure, the target material is dissolved In nitric acid, sulfurous acid is introduced to reduce the thallium to the univalent state, the lead is carried on a Iron-III hydroxide precipitate by addition of iron carrier and ammonium hydroxide. After dissolution of the hydroxide in hydrochloric acid. Iron was extracted Into ethyl ether leaving the carrier-free lead in aqueous solution. 

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