C. Sarzanini
primary column is passed to a secondary column for a defined period of time. The two
following examples can summarize the application of multidimensional chromatography.
Transition and heavy metals have been determined in seawater samples by multidimensional liquid chromatography (Voloschik et al. 1994b). The separation was
achieved by coupling a dynamically coated sorbent column, a preconcentrator and a
chelating column. The metal concentrations (Cu, Ni, Co, Mg, Ca, Sr, Fe and Pb, Zn, Mn,
Cd) in the seawater sample were 5.0-50 Jlg rl.
Samples containing alkali metals, alkaline earth cations and ammonium ion are
difficult to analyse; environmental samples, at low levels of ammonium in matrices
with a high concentration of sodium, are a typical case. This is mainly due to the similar
selectivities, of ammonium and sodium ions, for the common stationary phases containing sulfonate or carboxylate cation-exchange functional groups. This problem has
been solved by a column-switching technique which enables the determination of trace
concentrations of the common inorganic cations (Li, Na, K, Mg, Ca) and ammonium
in the presence oflarge concentrations of either sodium or ammonium (Rey et al.I997).
It must be mentioned that a great number of papers referring to column switching
or multidimensional liquid chromatography concern methods not actually using two
chromatographic columns, but simply a short column (preconcentrator) coupled with
an analytical column.
18.3.7
Preconcentration
To overcome the problems arising from off-line complexation and preconcentration
(e.g. sample poisoning, extraction procedures) great efforts have been devoted to the
development of on-line preconcentration procedures. The main approaches, for metal
ion determinations, are based on the retention of metals on a preconcentrator as such
(e.g. on a chelating microcolumn which does not retain alkali and alkaline earths) or
after their complexation, usually with negatively charged sulfonated dyes. In this case,
metal anionic complexes can be retained as such on an anion exchanger or on a reversed
phase material preloaded with an ion pairing agent, enabling a specific retention through
ion pairing and their hydrophobic interaction with the network of the stationary phase.
In all cases attention must be paid to ensure the compatibility of the preconcentration
step (strength of the retention of analytes on the preconcentrator) with the composition
of the eluent used for the subsequent recovery of the analytes and their separation.
18.4
Analytical Applications
To give an example, a column switching technique has been proposed (Ryan and
Meaney 1992) where two compatible eluents of different eluotropic strengths were
selected, one (CH 3 CN/H 2 0) to concentrate the metal-8-quinolinol (HQ) derivative
complexes on a precolumn (Nucleosil CI8), and the other (CH 3CNIH20/HQ) to elute
the analytes from the precolumn onto the analytical column (CI8). The linear dynamic
range is from 5 ppb to 10 ppm for Al and from 40 ppb to 5 ppm for Cu and Fe.
DEDTC chelate formation-preconcentration and RP-HPLC separation have been
used, on-column, for the determination of Cd, Cu and Ni in seawater samples (Comber
primary column is passed to a secondary column for a defined period of time. The two
following examples can summarize the application of multidimensional chromatography.
Transition and heavy metals have been determined in seawater samples by multidimensional liquid chromatography (Voloschik et al. 1994b). The separation was
achieved by coupling a dynamically coated sorbent column, a preconcentrator and a
chelating column. The metal concentrations (Cu, Ni, Co, Mg, Ca, Sr, Fe and Pb, Zn, Mn,
Cd) in the seawater sample were 5.0-50 Jlg rl.
Samples containing alkali metals, alkaline earth cations and ammonium ion are
difficult to analyse; environmental samples, at low levels of ammonium in matrices
with a high concentration of sodium, are a typical case. This is mainly due to the similar
selectivities, of ammonium and sodium ions, for the common stationary phases containing sulfonate or carboxylate cation-exchange functional groups. This problem has
been solved by a column-switching technique which enables the determination of trace
concentrations of the common inorganic cations (Li, Na, K, Mg, Ca) and ammonium
in the presence oflarge concentrations of either sodium or ammonium (Rey et al.I997).
It must be mentioned that a great number of papers referring to column switching
or multidimensional liquid chromatography concern methods not actually using two
chromatographic columns, but simply a short column (preconcentrator) coupled with
an analytical column.
18.3.7
Preconcentration
To overcome the problems arising from off-line complexation and preconcentration
(e.g. sample poisoning, extraction procedures) great efforts have been devoted to the
development of on-line preconcentration procedures. The main approaches, for metal
ion determinations, are based on the retention of metals on a preconcentrator as such
(e.g. on a chelating microcolumn which does not retain alkali and alkaline earths) or
after their complexation, usually with negatively charged sulfonated dyes. In this case,
metal anionic complexes can be retained as such on an anion exchanger or on a reversed
phase material preloaded with an ion pairing agent, enabling a specific retention through
ion pairing and their hydrophobic interaction with the network of the stationary phase.
In all cases attention must be paid to ensure the compatibility of the preconcentration
step (strength of the retention of analytes on the preconcentrator) with the composition
of the eluent used for the subsequent recovery of the analytes and their separation.
18.4
Analytical Applications
To give an example, a column switching technique has been proposed (Ryan and
Meaney 1992) where two compatible eluents of different eluotropic strengths were
selected, one (CH 3 CN/H 2 0) to concentrate the metal-8-quinolinol (HQ) derivative
complexes on a precolumn (Nucleosil CI8), and the other (CH 3CNIH20/HQ) to elute
the analytes from the precolumn onto the analytical column (CI8). The linear dynamic
range is from 5 ppb to 10 ppm for Al and from 40 ppb to 5 ppm for Cu and Fe.
DEDTC chelate formation-preconcentration and RP-HPLC separation have been
used, on-column, for the determination of Cd, Cu and Ni in seawater samples (Comber