CHAPTER 18 • Metals Analysis by High Performance Liquid Chromatography
341
18.2.3.1
Ion Exclusion Chromatography
This kind of IC is based on the use of both cation and anion exchangers as stationary
phases. The separation mechanism is governed by the Donnan effect, that is, strongly
ionized species are repelled by the charge on the surface of the sationary phase and
neutral or weakly dissociated analytes are retained by interaction with the matrix.
Charge and solute size are the dominant factors affecting the separation, but hydrophobic adsorption also plays a role for the retention mechanism of ion-exclusion
chromatography (IEC).
The main advantage of IEC is that the degree of ionization of the solute can be
modified, as can its retention, by modulating the pH value of the eluent.
18.2.3.2
Ion Interaction Chromatography
The above mentioned secondary chemical equilibria (SCE) refer to all other equilibria (the primary equilibrium is the distribution of the solute between the mobile phase
and the stationary phase), providing additional mechanism{s) for control of retention
and selectivity during a separation, e.g. ionization, metal complexation, solute-micelle
association and ion pairing.
Ion interaction chromatography (IIC), also called soap chromatography, ion pair
chromatography and dynamic ion exchange chromatography, is a typical example of
a chromatographic process based on secondary equilibria. Eluents containing an ion
interaction reagent (IIR) (e.g. alkylammonium salts, alkylsulfates or alkylsulfonates)
are used, and stationary phases (conventional RP or polymers) are dynamically modified into low-capacity ion-exchangers. During the separation, the retention of neutral
analytes, analytes having the same or opposite charge with respect to the IIR, will not
be affected, decreased or increased. Elution of cations is achieved by their complexation with the eluent ligand and ion-pairing of the negatively charged complex formed
with IIR or their cation exchange with the counter-ion of IIR. Research in this field is
devoted to the evaluation of the nature and concentration of proper ligands and lIR,
as well as the organic modifier and eluent pH. The pH of the mobile phase affects retention and resolution of chelates since it can alter their stoichiometry and over-all
charge.
Another approach to lIC is the use of common reversed phase stationary phases
permanently coated with suitable hydrophobic agents such as alkylsulfonates or
alkylsulfates with a sufficiently long alkyl group. The mechanism of elution is governed
by the mobile phase in the following two ways:
i. eluents containing a strong driving cation and a small amount of complexing agent
("push-pull" method, e.g. mobile phase containing ethylenediammonium cation and
tartaric acid)
ii. eluents containing a very weak driving cation and higher concentrations of complexing agent ("pure pull" mechanism)
341
18.2.3.1
Ion Exclusion Chromatography
This kind of IC is based on the use of both cation and anion exchangers as stationary
phases. The separation mechanism is governed by the Donnan effect, that is, strongly
ionized species are repelled by the charge on the surface of the sationary phase and
neutral or weakly dissociated analytes are retained by interaction with the matrix.
Charge and solute size are the dominant factors affecting the separation, but hydrophobic adsorption also plays a role for the retention mechanism of ion-exclusion
chromatography (IEC).
The main advantage of IEC is that the degree of ionization of the solute can be
modified, as can its retention, by modulating the pH value of the eluent.
18.2.3.2
Ion Interaction Chromatography
The above mentioned secondary chemical equilibria (SCE) refer to all other equilibria (the primary equilibrium is the distribution of the solute between the mobile phase
and the stationary phase), providing additional mechanism{s) for control of retention
and selectivity during a separation, e.g. ionization, metal complexation, solute-micelle
association and ion pairing.
Ion interaction chromatography (IIC), also called soap chromatography, ion pair
chromatography and dynamic ion exchange chromatography, is a typical example of
a chromatographic process based on secondary equilibria. Eluents containing an ion
interaction reagent (IIR) (e.g. alkylammonium salts, alkylsulfates or alkylsulfonates)
are used, and stationary phases (conventional RP or polymers) are dynamically modified into low-capacity ion-exchangers. During the separation, the retention of neutral
analytes, analytes having the same or opposite charge with respect to the IIR, will not
be affected, decreased or increased. Elution of cations is achieved by their complexation with the eluent ligand and ion-pairing of the negatively charged complex formed
with IIR or their cation exchange with the counter-ion of IIR. Research in this field is
devoted to the evaluation of the nature and concentration of proper ligands and lIR,
as well as the organic modifier and eluent pH. The pH of the mobile phase affects retention and resolution of chelates since it can alter their stoichiometry and over-all
charge.
Another approach to lIC is the use of common reversed phase stationary phases
permanently coated with suitable hydrophobic agents such as alkylsulfonates or
alkylsulfates with a sufficiently long alkyl group. The mechanism of elution is governed
by the mobile phase in the following two ways:
i. eluents containing a strong driving cation and a small amount of complexing agent
("push-pull" method, e.g. mobile phase containing ethylenediammonium cation and
tartaric acid)
ii. eluents containing a very weak driving cation and higher concentrations of complexing agent ("pure pull" mechanism)