CHAPTER 18 • Metals Analysis by High Performance Liquid Chromatography
345
by Siren (1991). A modification of the methods proposed consists of an on-line
derivatisation of metal ions; in this procedure metals are injected into a methanolwater eluent containing a quaternary ammonium bromide (e.g. cetyltrimethylammonium, CTA) and, after the column, they are mixed with a ligand solution (1nitroso-2-naphthol-6-sulfonate) (Siren and Riekkola 1992). The metal ion separation
by this procedure is governed by the kinetics of formation of complexes and ion-pairs
and retention onto the post-column mixer-reactor system. The chromatographic
behaviour of 3- (s-chloro-2-hydroxyphenylazo) -4,S-dihydroxy-naphthalene-2,7disulfonic acid (Plasmocorinth B) and its metal ion complexes has been studied in ionpairing reactions for metal preconcentration and separation by HPLC (Sarzanini et al.
1993a,b). The separation of analytes was optimized with a flow-gradient elution and
the method, successfully applied to river water samples, enabled analyte metals to be
separated from alkaline and alkaline earth elements.
Ohtsuka et al. have separated several ion pairs of anionic metal chelates with pyridylazosulfoaminophenol derivatives (PAPS) on a CIS stationary phase (Ohtsuka et al.
1991,1992). They elucidated the retention behaviour of metal chelates (PAPS) in IIC as
a function of mobile phase composition (Ohtsuka et al. 1994) with respect to the significant differences found in methanol-water and acetonitrile-water systems as a function of the volume fraction of water (Alvarez-Zepeda et al. 1992).
Octadecyl-bonded silica permanently coated with sodium dodecylsulfate in the
presence of complexing agents was considered for the separation of transition metals
(Janos and BroulI992). In the work mentioned an ion-exchange mechanism similar
to that of fixed sites exchangers seems to occur; both the pushing effect of the eluting
cation and the pulling effect of the complexing anion take place but the latter plays a
dominant role in the process of elution. A significant example of the approach mentioned, is the detailed study by Cassidy and Sun (1993). They compared the performance of an anion separation with a cation separation both based on an ion-interaction system that used cetylpyridinium chloride or n-octanesulfonate to modify a reversed stationary phase. In the first case transition metals (Mn, Co, Ni, Cu and Zn) were
eluted with an oxalate eluent. The anion-exchange system provided column efficiences
comparable with that for the cation system. This approach may be attractive for solving analytical problems taking into account the considerably different order of separation obtained by the two systems.
Reversed-phase ion-pair procedures involving EDTA have also been considered in
optimizing separation and detection of metal species. Different techniques like
precolumn derivatization without a complexing agent in the eluent or on-column
derivatization may be less efficient and give rise to peak broadening. Ion-pair reversed
phase high performance chromatography has been investigated by coupling EDTA with
tetraethylammonium (TEA), tetrapropylammonium (TPA) (Marina et al. 1993) and
tetrabutylammonium (TBA) (Sacchero et al.1991; len and Chen 1992; Marina et al.1993)
bromide ion pairing agents. TBA proved to be the most suitable ion pairing agent in
all cases, and the use of EDTA in the eluent (Sacchero et al. 1991; Marina et al. 1993),
together with high complexation constants, shifted the equilibrium in favour of chelate formation attaining lower detection limits. The data obtained (Marina et al. 1993)
clarify some aspects of the separation mechanism of ion-interaction chromatography
for different oxidation states of metal ions and confirm that the retention of divalent
and trivalent metal ions complexed with EDTA takes place through an ion exchange
345
by Siren (1991). A modification of the methods proposed consists of an on-line
derivatisation of metal ions; in this procedure metals are injected into a methanolwater eluent containing a quaternary ammonium bromide (e.g. cetyltrimethylammonium, CTA) and, after the column, they are mixed with a ligand solution (1nitroso-2-naphthol-6-sulfonate) (Siren and Riekkola 1992). The metal ion separation
by this procedure is governed by the kinetics of formation of complexes and ion-pairs
and retention onto the post-column mixer-reactor system. The chromatographic
behaviour of 3- (s-chloro-2-hydroxyphenylazo) -4,S-dihydroxy-naphthalene-2,7disulfonic acid (Plasmocorinth B) and its metal ion complexes has been studied in ionpairing reactions for metal preconcentration and separation by HPLC (Sarzanini et al.
1993a,b). The separation of analytes was optimized with a flow-gradient elution and
the method, successfully applied to river water samples, enabled analyte metals to be
separated from alkaline and alkaline earth elements.
Ohtsuka et al. have separated several ion pairs of anionic metal chelates with pyridylazosulfoaminophenol derivatives (PAPS) on a CIS stationary phase (Ohtsuka et al.
1991,1992). They elucidated the retention behaviour of metal chelates (PAPS) in IIC as
a function of mobile phase composition (Ohtsuka et al. 1994) with respect to the significant differences found in methanol-water and acetonitrile-water systems as a function of the volume fraction of water (Alvarez-Zepeda et al. 1992).
Octadecyl-bonded silica permanently coated with sodium dodecylsulfate in the
presence of complexing agents was considered for the separation of transition metals
(Janos and BroulI992). In the work mentioned an ion-exchange mechanism similar
to that of fixed sites exchangers seems to occur; both the pushing effect of the eluting
cation and the pulling effect of the complexing anion take place but the latter plays a
dominant role in the process of elution. A significant example of the approach mentioned, is the detailed study by Cassidy and Sun (1993). They compared the performance of an anion separation with a cation separation both based on an ion-interaction system that used cetylpyridinium chloride or n-octanesulfonate to modify a reversed stationary phase. In the first case transition metals (Mn, Co, Ni, Cu and Zn) were
eluted with an oxalate eluent. The anion-exchange system provided column efficiences
comparable with that for the cation system. This approach may be attractive for solving analytical problems taking into account the considerably different order of separation obtained by the two systems.
Reversed-phase ion-pair procedures involving EDTA have also been considered in
optimizing separation and detection of metal species. Different techniques like
precolumn derivatization without a complexing agent in the eluent or on-column
derivatization may be less efficient and give rise to peak broadening. Ion-pair reversed
phase high performance chromatography has been investigated by coupling EDTA with
tetraethylammonium (TEA), tetrapropylammonium (TPA) (Marina et al. 1993) and
tetrabutylammonium (TBA) (Sacchero et al.1991; len and Chen 1992; Marina et al.1993)
bromide ion pairing agents. TBA proved to be the most suitable ion pairing agent in
all cases, and the use of EDTA in the eluent (Sacchero et al. 1991; Marina et al. 1993),
together with high complexation constants, shifted the equilibrium in favour of chelate formation attaining lower detection limits. The data obtained (Marina et al. 1993)
clarify some aspects of the separation mechanism of ion-interaction chromatography
for different oxidation states of metal ions and confirm that the retention of divalent
and trivalent metal ions complexed with EDTA takes place through an ion exchange