342
C. Sarzanini
A recent study of the chromatographic behaviour of metal ions in lIC when the stationary phase is modified with various alkanesulfonates (1-10 carbon atoms in
the alkyl chain) must be mentioned; the paper shows that a very good resolution is
achieved even if in the ion interaction mode the number of theoretical plates of the column is lower than that obtained in the reversed-phase mode (Zappoli et al. 1996).
18.2.4
Chelation Ion Chromatography
Chelation ion chromatography, or, more correctly, high performance chelation ion
chromatography (HPCIC) is based on the use of high-performance substrates for trace
metal separation and determination. Chelation exchange involves an ion-exchange
process and the formation of coordinate bonds. The different kinds of stationary
phases, available in the market or laboratory made, have recently been reviewed (Jones
and Nesterenko 1997) and are silica or polymer based materials with chelating agents
grafted onto the surface. Alternatively, chelating ligands can be immobilized by adsorption onto a styrene-divinylbenzene copolymer, silica gel or other synthetic polymers. These precoated columns use for the separation of metal ions an aqueous mobile phase at a relatively high concentration of an inorganic salt, e.g. 0.5-1.0 M KN0 3 •
More recently a graphitic porous carbon reversed phase column has been coupled with
a mobile phase containing a selective metallochromic ligand for the separation of alkaline earth metals (Paull et al. 1996) and a polymeric reversed phase column with a
mobile phase containing methylthymol blue for the separation of Mn, Zn, Cd and Pb
(Paull et al. 1998). Since the ligand is a part of the eluent, we can say that, in analogy
with the dynamic ion exchange, a dynamic chelating chromatography has been developed. For the HPCIC the acting separation mechanisms, as a function of pH, ionic
strength and organic modifier content of the eluent, are chelation and ion exchange.
Each of the techniques mentioned is suitable for metal separations and determination in different kinds of samples, and the main problems encountered are due to matrix composition and analyte concentration. In some cases, as detailed below, more than
one mechanism or column is coupled in order to enhance chromatographic resolution
and so called multimode or multidimensional techniques are used for complicated samples.
We will consider hereafter some of the procedures that have been developed, focusing our attention on the methods dedicated to seawater analysis.
18.3
Procedures for Metal Separation
18.3.1
Normal Phase Chromatography
The current approach for normal phase chromatography (NPC) is based on the formation of metal chelates (e.g. diacetylbisthiobenzohydrazones, dithizone, diethyldithiocarbamate), their extraction from the sample, injection and their elution, performed with organic solvent mixtures of n-heptane/benzene, toluene, diethyl ether/acetonitrile or similar. A typical reaction for heavy metal ions is with ammonium tetramethylenedithiocarbammate (ATDC), which gives neutral complexes. Since this kind
C. Sarzanini
A recent study of the chromatographic behaviour of metal ions in lIC when the stationary phase is modified with various alkanesulfonates (1-10 carbon atoms in
the alkyl chain) must be mentioned; the paper shows that a very good resolution is
achieved even if in the ion interaction mode the number of theoretical plates of the column is lower than that obtained in the reversed-phase mode (Zappoli et al. 1996).
18.2.4
Chelation Ion Chromatography
Chelation ion chromatography, or, more correctly, high performance chelation ion
chromatography (HPCIC) is based on the use of high-performance substrates for trace
metal separation and determination. Chelation exchange involves an ion-exchange
process and the formation of coordinate bonds. The different kinds of stationary
phases, available in the market or laboratory made, have recently been reviewed (Jones
and Nesterenko 1997) and are silica or polymer based materials with chelating agents
grafted onto the surface. Alternatively, chelating ligands can be immobilized by adsorption onto a styrene-divinylbenzene copolymer, silica gel or other synthetic polymers. These precoated columns use for the separation of metal ions an aqueous mobile phase at a relatively high concentration of an inorganic salt, e.g. 0.5-1.0 M KN0 3 •
More recently a graphitic porous carbon reversed phase column has been coupled with
a mobile phase containing a selective metallochromic ligand for the separation of alkaline earth metals (Paull et al. 1996) and a polymeric reversed phase column with a
mobile phase containing methylthymol blue for the separation of Mn, Zn, Cd and Pb
(Paull et al. 1998). Since the ligand is a part of the eluent, we can say that, in analogy
with the dynamic ion exchange, a dynamic chelating chromatography has been developed. For the HPCIC the acting separation mechanisms, as a function of pH, ionic
strength and organic modifier content of the eluent, are chelation and ion exchange.
Each of the techniques mentioned is suitable for metal separations and determination in different kinds of samples, and the main problems encountered are due to matrix composition and analyte concentration. In some cases, as detailed below, more than
one mechanism or column is coupled in order to enhance chromatographic resolution
and so called multimode or multidimensional techniques are used for complicated samples.
We will consider hereafter some of the procedures that have been developed, focusing our attention on the methods dedicated to seawater analysis.
18.3
Procedures for Metal Separation
18.3.1
Normal Phase Chromatography
The current approach for normal phase chromatography (NPC) is based on the formation of metal chelates (e.g. diacetylbisthiobenzohydrazones, dithizone, diethyldithiocarbamate), their extraction from the sample, injection and their elution, performed with organic solvent mixtures of n-heptane/benzene, toluene, diethyl ether/acetonitrile or similar. A typical reaction for heavy metal ions is with ammonium tetramethylenedithiocarbammate (ATDC), which gives neutral complexes. Since this kind