Development of complexation ion chromatography for the determination of metal ions
Bashir, Wasim (2002) Development of complexation ion chromatography for the determination of metal ions. PhD thesis, Dublin City University.
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A simple ion chromatographic method was developed for the determination of Pb(II) in river and polluted water samples. The method was based upon the use of a colourforming complexing eluent and direct visible detection of the eluting Pb(II) complex. Using the combination of a strong cation exchange column and an eluent consisting of 20 mM sodium acetate-acetic acid buffer and 0.2 mM xylenol orange (XO) (~pH = 4.2), Pb(II) was detected at 572 nm eluting in under 6.5 min. The developed method proved both sensitive and selective and eliminated the need for post-column reaction, replacing this with the novel approach of on-column complexation within the eluent.
This concept of on-column complexation was further investigated with the development of an ion-interaction reversed-phase liquid chromatographic method for the determination of Cd(II) at low pg/L concentrations in environmental water samples. Cd(II) and other matrix metals were separated through on-column complexation with 8 -hydroxyquinoline sulphonate, using an octadecylsilica column and an eluent containing 15% acetonitrile, 10-13 mM tetrabutylammonium hydroxide (TBAOH), 5 mM 8 -hydroxyquinoline 5-sulphonic acid (8 -HQS) and 10 mM acetic acid-acetate buffer (pH 4.8-5.4). Detection was achieved using fluorescence, although once again the inclusion of the fluorescent complexing reagent within the eluent removed the requirement for post-column reaction.
Another form of complexation, in this case stationary phase complexation, was then investigated. The chromatographic behaviour of alkaline earth metals on a chelating/cation exchange high-performance stationary phase was studied using iminodiacetic acid (IDA) bonded silica. It was found that the ionic strength and pH of the eluent greatly affected both retention time and selectivity by controlling the extent to which either simple cation exchange or surface complexation was responsible for retention. The effect of the ionic strength of the sample was also studied and it was found that utilising the highly selective nature of the IDA phase and by matching the eluent cation with that of the sample matrix, efficient separations of alkaline earth metals in 1.0 M NaCl and KC1 brines could be obtained without matrix system peaks. The developed chelation ion chromatography was applied to the determination of trace levels of Ca(II) and Mg(II) in medicinal NaCl saline solution and laboratorygrade KC1, using post-column reaction with o-cresolphthalein complexone (o-CPC).
As a result of the success of the previously developed system, the highly selective nature of the IDA stationary phase was further utilised in the development of a chelation ion chromatographic method for the determination of trace Be(II) in water samples. The resolution of Be(II) from excess concentrations of alkali, alkaline earth and transition metal ions was possible when using the IDA phase through careful control of eluent parameters. The optimum eluent conditions were found to be 0.4 M KNO3, adjusted to pH 2.5 using HNO3, with detection of Be(II) achieved using visible detection at 590 nm following post-column reaction with 0.26 mM chrome azurol S (CAS), 2% Triton-X-100, 50 mM 2-(A?-morpholino)ethanesulfonic acid (MES), pH 6.0. The concentration detection limit for Be(II) was found to be 3 flg/L in a standard solution and 4 (Xg/L in a tap water sample, using a 250 (il injection loop. The method was successfully applied to a number of fresh water samples of varying matrix complexity, including simulated seawater, and also to a natural freshwater certified reference material (NIST 1640).
Following the above work with alkaline earth metal ions, a further investigation into the selectivity of the same IDA-silica column for transition and heavy metal ions using non-chelating inorganic eluents was carried out. A number of parameters were investigated to determine the exact retention mechanism taking place when using such a chelating ion exchange phase for transition and heavy metal ions and to investigate the control of cation selectivity. The parameters studied were eluent ionic strength and the nature of the inorganic salt used, eluent pH and column temperature. A greater understanding of the above finally led to the development of an isocratic method using a 0.035 M KC1, 0.065 M KNO3 (pH 2.5) eluent for the determination of Mn(II), Cd(II), Co(II) and Zn(II) at concentrations between 20 and 121 |i,g/L in a freshwater certified reference material (NIST 1640).
Finally, a new polymeric chelating ion exchange phase was investigated for the separation of alkaline earth and transition metal ions. The polymer phase was functionalised with a methylenesuccinic acid (itaconic acid) chelating ion exchange group, which like IDA showed a high degree of selectivity for divalent metals ions. As with previous work, the retention behaviour of transition, heavy and alkaline earth metals was investigated under a range of eluent conditions in an effort to evaluate the dominant retention mechanism. Mg(II), Ca(II), Mn(II), Cd(II), Zn(II) and Co(II) could be separated isocratically within 30 min using the itaconic acid phase, with results indicating stationary phase complexation and not ion exchange being responsible for retention of both sets of metal ions.
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