CHAPTER 18 • Metals Analysis by High Performance Liquid Chromatography
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of chelate is usually uncharged, it can be eluted in the same manner as neutral compounds do on normal phase silica columns (e.g. 1% propanol in hexane).
The main problems encountered in NPC are connected with metal complex solubility, the need of extraction and the risk of pollution of the sample when traces must
be detected. Among the mechanisms mentioned, normal phase chromatography is not,
at present, the most widely used.
18.3.2
Reversed Phase Chromatography
This technique has been widely used for the separation of neutral or weakly charged
metal complexes, but the more extensive applications are based on the ion pairing
mechanism, so related procedures will be detailed hereafter.
Analysis for trace metals is carried out by the formation of metal chelates with separation by RPC on CIS columns and the use of organic-based mobile phases. Dithiocarbamates are the most frequently reported complexing agents due to the strong chelating ability of their sulfur groups and their ability to form neadywater-insoluble metal
salts with all metals except sodium and other alkali and alkali earth metals. A comprehensive study on reversed-phase HPLC behaviour of diethyldithiocarbamate
(DEDTC) complexes of Cu, Co, Cr, Ni and Hg with a variety of columns and mobile
phases was completed (Dilli et al.I990). In this case DEDTC complexes were preformed
off-column (60°C, 15 min), extracted into chloroform and finally dissolved in CH30H
and injected for the separation onto a Cwcolum (fJ.Bondapak, Waters). The study
showed that the ligand must also be present in the mobile phase for low concentration of chelates, to avoid their dissociation.
Various azo dyes have also been considered for the chromatographic separation of
metal chelates on a reversed phase RP-18 column, and the study focused on the separation and determination ofV(V} at trace levels in natural waters (Miura 1990). The
originality of this investigation is due to optimization of the RP column selectivity by
introducing a tetraalkylammonium salt into the system. The metal chelates considered are neutral or cationic and ion-paired complexes are not involved, whereas other
metal ions (e.g. Fe, AI) do not interfere in the determination.
8-Quinolinol (HQ) is another extensively used ligand for the separation of metal
ions by HPLC. For this ligand also, methods are based on metal ion complexation, usually by heating the sample in the presence of HQ, one or two step extraction with a
compatible eluent solvent and injection of complexes into the chromatographic system. To give an example, the simultaneous determination of Mo(VI), V(V}, Cu(II} and
Fe(III} at ppb level in sea water can be mentioned (Ohashi et al. 1991).
18.3.3
Ion Chromatography
Ion exchange chromatography (IEC) of metal ions is performed by using both cation
and anion exchangers. In the cation-exchange technique the metal ions are normally
reacted with an anion of a weak acid to reduce their charge density in the eluent solution before entering the separation column, where they are separated according to their
respective affinities toward the active sites of the separating resin. Ligands are also
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of chelate is usually uncharged, it can be eluted in the same manner as neutral compounds do on normal phase silica columns (e.g. 1% propanol in hexane).
The main problems encountered in NPC are connected with metal complex solubility, the need of extraction and the risk of pollution of the sample when traces must
be detected. Among the mechanisms mentioned, normal phase chromatography is not,
at present, the most widely used.
18.3.2
Reversed Phase Chromatography
This technique has been widely used for the separation of neutral or weakly charged
metal complexes, but the more extensive applications are based on the ion pairing
mechanism, so related procedures will be detailed hereafter.
Analysis for trace metals is carried out by the formation of metal chelates with separation by RPC on CIS columns and the use of organic-based mobile phases. Dithiocarbamates are the most frequently reported complexing agents due to the strong chelating ability of their sulfur groups and their ability to form neadywater-insoluble metal
salts with all metals except sodium and other alkali and alkali earth metals. A comprehensive study on reversed-phase HPLC behaviour of diethyldithiocarbamate
(DEDTC) complexes of Cu, Co, Cr, Ni and Hg with a variety of columns and mobile
phases was completed (Dilli et al.I990). In this case DEDTC complexes were preformed
off-column (60°C, 15 min), extracted into chloroform and finally dissolved in CH30H
and injected for the separation onto a Cwcolum (fJ.Bondapak, Waters). The study
showed that the ligand must also be present in the mobile phase for low concentration of chelates, to avoid their dissociation.
Various azo dyes have also been considered for the chromatographic separation of
metal chelates on a reversed phase RP-18 column, and the study focused on the separation and determination ofV(V} at trace levels in natural waters (Miura 1990). The
originality of this investigation is due to optimization of the RP column selectivity by
introducing a tetraalkylammonium salt into the system. The metal chelates considered are neutral or cationic and ion-paired complexes are not involved, whereas other
metal ions (e.g. Fe, AI) do not interfere in the determination.
8-Quinolinol (HQ) is another extensively used ligand for the separation of metal
ions by HPLC. For this ligand also, methods are based on metal ion complexation, usually by heating the sample in the presence of HQ, one or two step extraction with a
compatible eluent solvent and injection of complexes into the chromatographic system. To give an example, the simultaneous determination of Mo(VI), V(V}, Cu(II} and
Fe(III} at ppb level in sea water can be mentioned (Ohashi et al. 1991).
18.3.3
Ion Chromatography
Ion exchange chromatography (IEC) of metal ions is performed by using both cation
and anion exchangers. In the cation-exchange technique the metal ions are normally
reacted with an anion of a weak acid to reduce their charge density in the eluent solution before entering the separation column, where they are separated according to their
respective affinities toward the active sites of the separating resin. Ligands are also