344
c. S arzanini
required to avoid precipitation when an acidic eluent is not suitable for the columns
selected. A low-capacity silica-based cation-exchange column was used for the separation of transition metals (Co, Cu, Fe) and coupled with post-column chemiluminescence detection to enhance sensitivity (Yan and Worsfold 1990). In this case attention
has been paid to eluent composition not only to improve separation but also for its
compatibility with the post-column reaction.
Ion chromatographic separation of metal ions based on anionic exchange offers the
potential for a different selectivity, reduced problems for metal ion hydrolysis and for
application to complex sample matrices. Notwithstanding the fact that many organic
acids from mono-, di-, tricarboxylic acids to chelating agents such as a-hydroxyisobutyric acid (HIBAa), tartaric, citric, oxalic, pyridine-2,6-dicarboxylic acid (PDCA),
1,2-diaminocydohexanetetraacetic acid (DCTA), and diethylenetriaminopentaacetic
acid (DTPA) have been evaluated for simultaneous ion-chromatography of anions,
alkali, alkaline earth and heavy metals (Yan and Schwedt 1990; Cardellicchio et a1.1997),
ethylenediaminetetraacetic acid (EDTA) plays a fundamental role. EDTA has also been
used as a masking agent, to avoid metal ion interference arising from possible precipitation due to eluent pH. Since EDTA forms, at the proper pH, negatively charged
complexes with divalent or trivalent metal ions, the possibility for simultaneous separation of anions from metal ions as well as the speciation of metal ions is also feasible.
In these procedures complexes can be obtained in two ways: the first one is through
their formation before the chromatographic separation (precolumn complexation,
complexes must be stable enough to avoid decomposition during separation or ligand
must be added to the eluent); the second way is based on the complexation in the chromatographic column itself. Some examples of applications of an EDTA eluent have been
reported, and experiments were also performed with binary eluent systems comprising EDTA as complexing agent. For seawater samples (LeGras 1993), silica based anion exchange analytical columns have enhanced sensitivity and enabled detection limits to be reached from 20 flg rl for Mg2+ to 0.4 mg rl for Ca 2 + with UV and conductivity detection and eluent pH at 4.8.
18.3.4
Ion Interaction Chromatography
As mentioned above, the elution of cations is achieved by their complexation with an
eluent ligand and ion-pairing of a negatively charged complex formed with IIR or their
cation exchange with the counter-ion of IIR. Research in this field is devoted to the
evaluation of the nature and concentration of proper ligands and IIR as well as an
organic modifier and eluent pH. Ion-interaction chromatography is one of the most
suitable techniques for metal ion determination in complicated samples. The main
approaches involve metal complex formation by adding the ligand to the sample or in
situ complexation by reaction of metal ions with the ligand added to the eluent. Naturally the eluent must contain a proper ion pairing reagent and an organic modifier, if
required, enabling the separation of complexes. The sequence mentioned is useful but
it is not sufficient when trace metals have to be determined. In this case a preconcentration step is required (see below).
An extensive study on lIC of metal complexes of nitrosonaphthol sulfonates ionpaired, with liquid-liquid extraction and on-line derivatization, has been carried out
c. S arzanini
required to avoid precipitation when an acidic eluent is not suitable for the columns
selected. A low-capacity silica-based cation-exchange column was used for the separation of transition metals (Co, Cu, Fe) and coupled with post-column chemiluminescence detection to enhance sensitivity (Yan and Worsfold 1990). In this case attention
has been paid to eluent composition not only to improve separation but also for its
compatibility with the post-column reaction.
Ion chromatographic separation of metal ions based on anionic exchange offers the
potential for a different selectivity, reduced problems for metal ion hydrolysis and for
application to complex sample matrices. Notwithstanding the fact that many organic
acids from mono-, di-, tricarboxylic acids to chelating agents such as a-hydroxyisobutyric acid (HIBAa), tartaric, citric, oxalic, pyridine-2,6-dicarboxylic acid (PDCA),
1,2-diaminocydohexanetetraacetic acid (DCTA), and diethylenetriaminopentaacetic
acid (DTPA) have been evaluated for simultaneous ion-chromatography of anions,
alkali, alkaline earth and heavy metals (Yan and Schwedt 1990; Cardellicchio et a1.1997),
ethylenediaminetetraacetic acid (EDTA) plays a fundamental role. EDTA has also been
used as a masking agent, to avoid metal ion interference arising from possible precipitation due to eluent pH. Since EDTA forms, at the proper pH, negatively charged
complexes with divalent or trivalent metal ions, the possibility for simultaneous separation of anions from metal ions as well as the speciation of metal ions is also feasible.
In these procedures complexes can be obtained in two ways: the first one is through
their formation before the chromatographic separation (precolumn complexation,
complexes must be stable enough to avoid decomposition during separation or ligand
must be added to the eluent); the second way is based on the complexation in the chromatographic column itself. Some examples of applications of an EDTA eluent have been
reported, and experiments were also performed with binary eluent systems comprising EDTA as complexing agent. For seawater samples (LeGras 1993), silica based anion exchange analytical columns have enhanced sensitivity and enabled detection limits to be reached from 20 flg rl for Mg2+ to 0.4 mg rl for Ca 2 + with UV and conductivity detection and eluent pH at 4.8.
18.3.4
Ion Interaction Chromatography
As mentioned above, the elution of cations is achieved by their complexation with an
eluent ligand and ion-pairing of a negatively charged complex formed with IIR or their
cation exchange with the counter-ion of IIR. Research in this field is devoted to the
evaluation of the nature and concentration of proper ligands and IIR as well as an
organic modifier and eluent pH. Ion-interaction chromatography is one of the most
suitable techniques for metal ion determination in complicated samples. The main
approaches involve metal complex formation by adding the ligand to the sample or in
situ complexation by reaction of metal ions with the ligand added to the eluent. Naturally the eluent must contain a proper ion pairing reagent and an organic modifier, if
required, enabling the separation of complexes. The sequence mentioned is useful but
it is not sufficient when trace metals have to be determined. In this case a preconcentration step is required (see below).
An extensive study on lIC of metal complexes of nitrosonaphthol sulfonates ionpaired, with liquid-liquid extraction and on-line derivatization, has been carried out