C. Sarzanini
mechanism in which the ion exchanger is dynamically generated by the retention of
the counterion in the stationary phase (Sacchero et al. 1991).
Studies for the separation and determination of metal ions (Cu, Co, Fe, Ni, V(V),
Pd) as s-Br-PADAP chelates (S-Br-2-pyridylazo-s-diethylaminophenol) by RP-HPLC
showed that only the retention of the Co(III)-s-Br-PADAP complex is affected by varying the concentration of surfactant added to the eluent (Zhao and Fu 1990). In addition a stronger interaction resulted for TBA in respect to CTA and cetylpyridinium
(CP). An alkyl group, such as that of CTA or TBA, may interact with the C18 chain on
the stationary phase by molecular interaction so that the charged part of surfactant is
exposed on the surface, increasing its polarity so that the Co chelate is eluted earlier.
18.3.5
Chelation Ion Chromatography
For samples of high ionic strength such as sea waters or concentrated brines, the selectivity of the chromatographic separation can be enhanced by the use of chelating
chemically bonded phases. Complexation reactions in the stationary phase, ion exchange due to free or protonated chelating groups which act as ion exchange sites, and
in some cases complexation in the eluent, are responsible for the separation. Two main
approaches can be followed to obtain proper stationary phases:
i. chemical bonding of the chelating group to the substrate
ii. coating of a substrate with a ligand which is permanently trapped onto the substrate
Voloschik et al. (1994a) used a silica gel-based sorbent with chemically bonded
amidoxime functional groups for the selective determination of metals in waters (Mn,
Cd, Co, Cu, Ni, Pb, Zn and Hg). Since this kind of resin showed a weak affinity for Mg
and Ca and a very strong affinity for Fe it was possible to eliminate their interference
in the determination. The separation of mono- and divalent cations on a polybutadiene
maleic acid (PDMA) stationary phase coated on silica materials has been performed
with different eluents, and a detailed study on this material (Nair et al. 1994) enabled
the determination of some transition metals by conductivity detection within detection limits from 3S to 410 Jlg 1-1 for Cd and Pb respectively.
The most widely used resin for pre concentrating and for separating elements and
groups of elements in seawater, applied for the first time almost three decades ago
(Riley and Taylor 1968), is an iminodiacetate resin (i.e. Chelex-lOo). The low degree
of cross-linking of the polystyrene-divinylbenzene (PS-DVB) supporting polymer
made the Chelex -100 unsuitable for high pressure liquid chromatography applications.
A more highly cross-linked macroporous PS-DVB resin containing the iminodiacetate
functional group that allows operation at high pressure without physical degradation
(MetPac CC-l column, Dionex) has been more recently developed (Siriraks et al. 1990).
The alternative approach to ligands chemically bound to silica or polymer phases
refers to a technique, applied for a long time in metal ions pre concentration and matrix removal before spectroscopic determinations, based on the permanent loading
of sorbents by chelating agents (Abollino et al. 1990,1993; Porta et al. 1992; Sarzanini
et al. 1992). Jones and co-workers widely investigated this field in order to improve
detect ability of both alkaline-earth and heavy metals in brine samples. Using high
mechanism in which the ion exchanger is dynamically generated by the retention of
the counterion in the stationary phase (Sacchero et al. 1991).
Studies for the separation and determination of metal ions (Cu, Co, Fe, Ni, V(V),
Pd) as s-Br-PADAP chelates (S-Br-2-pyridylazo-s-diethylaminophenol) by RP-HPLC
showed that only the retention of the Co(III)-s-Br-PADAP complex is affected by varying the concentration of surfactant added to the eluent (Zhao and Fu 1990). In addition a stronger interaction resulted for TBA in respect to CTA and cetylpyridinium
(CP). An alkyl group, such as that of CTA or TBA, may interact with the C18 chain on
the stationary phase by molecular interaction so that the charged part of surfactant is
exposed on the surface, increasing its polarity so that the Co chelate is eluted earlier.
18.3.5
Chelation Ion Chromatography
For samples of high ionic strength such as sea waters or concentrated brines, the selectivity of the chromatographic separation can be enhanced by the use of chelating
chemically bonded phases. Complexation reactions in the stationary phase, ion exchange due to free or protonated chelating groups which act as ion exchange sites, and
in some cases complexation in the eluent, are responsible for the separation. Two main
approaches can be followed to obtain proper stationary phases:
i. chemical bonding of the chelating group to the substrate
ii. coating of a substrate with a ligand which is permanently trapped onto the substrate
Voloschik et al. (1994a) used a silica gel-based sorbent with chemically bonded
amidoxime functional groups for the selective determination of metals in waters (Mn,
Cd, Co, Cu, Ni, Pb, Zn and Hg). Since this kind of resin showed a weak affinity for Mg
and Ca and a very strong affinity for Fe it was possible to eliminate their interference
in the determination. The separation of mono- and divalent cations on a polybutadiene
maleic acid (PDMA) stationary phase coated on silica materials has been performed
with different eluents, and a detailed study on this material (Nair et al. 1994) enabled
the determination of some transition metals by conductivity detection within detection limits from 3S to 410 Jlg 1-1 for Cd and Pb respectively.
The most widely used resin for pre concentrating and for separating elements and
groups of elements in seawater, applied for the first time almost three decades ago
(Riley and Taylor 1968), is an iminodiacetate resin (i.e. Chelex-lOo). The low degree
of cross-linking of the polystyrene-divinylbenzene (PS-DVB) supporting polymer
made the Chelex -100 unsuitable for high pressure liquid chromatography applications.
A more highly cross-linked macroporous PS-DVB resin containing the iminodiacetate
functional group that allows operation at high pressure without physical degradation
(MetPac CC-l column, Dionex) has been more recently developed (Siriraks et al. 1990).
The alternative approach to ligands chemically bound to silica or polymer phases
refers to a technique, applied for a long time in metal ions pre concentration and matrix removal before spectroscopic determinations, based on the permanent loading
of sorbents by chelating agents (Abollino et al. 1990,1993; Porta et al. 1992; Sarzanini
et al. 1992). Jones and co-workers widely investigated this field in order to improve
detect ability of both alkaline-earth and heavy metals in brine samples. Using high