CH CH CH CH CH Figure 12.33

CH 2 CH CH 2 CH CH 2 CH CH 2 CH CH 2 CH Figure 12.33

Structures of styrene, divinylbenzene, and a styrene-divinylbenzene co-polymer modified for use as an ion-exchange resin. The ion- exchange sites, indicated by R, are mostly in the para position and are not necessarily

bound to all styrene units.

Table 12.5 Examples of Common Ion-Exchange Resins

Type

Functional Group

Examples

strong acid cation exchanger

sulfonic acid

–SO 3 –

–CH 2 CH 2 SO 3 –

weak acid cation exchanger

carboxylic acid

–COO –

–CH 2 COO –

strong base anion exchanger

quaternary amine

–CH 2 N(CH 3 ) 3 +

–CH 2 CH 2 N(CH 2 CH 3 ) 3 +

weak base anion exchanger

amine

–NH 3 +

–CH 2 CH 2 NH(CH 2 CH 3 ) 2 +

592 Modern Analytical Chemistry

The ion-exchange reaction of a monovalent cation, M + , at a strong acid ex- change site is

–SO 3 – –H + (s) + M + (aq) t –SO 3 – –M + (s) + H + (aq)

The equilibrium constant for this ion-exchange reaction, which is also called the se- lectivity coefficient, is

where the brackets { } indicate a surface concentration. Rearranging equation 12.31 shows that the distribution ratio for the exchange reaction

amount of M in stationary phase + { SO

3 – − M } { − SO 3 – − H }

[ H + ] is a function of the concentration of H + and, therefore, the pH of the mobile phase.

amount of M in mobile phase

Ion-exchange resins are incorporated into HPLC columns either as micron- sized porous polymer beads or by coating the resin on porous silica particles. Selec- tivity is somewhat dependent on whether the resin includes a strong or weak ex- change site and on the extent of cross-linking. The latter is particularly important because it controls the resin’s permeability and, therefore, the accessibility of the ex- change sites. An approximate order of selectivity for a typical strong acid cation ex- change resin, in order of decreasing D, is

Al 3+ > Ba 2+ > Pb 2+ > Ca 2+ > Ni 2+ > Cd 2+ > Cu 2+ > Co 2+ > Zn 2+ > Mg 2+ > Ag + >K + > NH 4 + > Na + >H + > Li +

Note that highly charged ions bind more strongly than ions of lower charge. Within a group of ions of similar charge, those ions with a smaller hydrated radius (Table 6.1 in Chapter 6) or those that are more polarizable bind more strongly. For

a strong base anion exchanger the general order is

SO 4 2– >I – > HSO 4 – > NO 3 – > Br – > NO 2 – > Cl – > HCO 3 – > CH 3 COO – > OH – >F – Again, ions of higher charge and smaller hydrated radius bind more strongly than

ions with a lower charge and a larger hydrated radius.

The mobile phase in IEC is usually an aqueous buffer, the pH and ionic com- position of which determines a solute’s retention time. Gradient elutions are possi- ble in which the ionic strength or pH of the mobile phase is changed with time. For example, an IEC separation of cations might use a dilute solution of HCl as the mo- bile phase. Increasing the concentration of HCl speeds the elution rate for more strongly retained cations, since the higher concentration of H + allows it to compete more successfully for the ion-exchange sites.

Ion-exchange columns can be substituted into the general HPLC instrument shown in Figure 12.26. The most common detector measures the conductivity of the mobile phase as it elutes from the column. The high concentration of electrolyte in the mobile phase is a problem, however, because the mobile-phase ions dominate the conductivity. For example, if a dilute solution of HCl is used as the mobile

phase, the presence of large concentrations of H 3 O + and Cl – produces a background conductivity that may prevent the detection of analytes eluting from the column.

ion-suppressor column

To minimize the mobile phase’s contribution to conductivity, an ion-suppressor

A column used to minimize the

column is placed between the analytical column and the detector. This column se-

conductivity of the mobile phase in ion- exchange chromatography.

lectively removes mobile-phase electrolyte ions without removing solute ions. For

Chapter 12 Chromatographic and Electrophoretic Methods

the mobile phase, the suppressor column contains an anion-exchange resin. The ex- change reaction

H + (aq) + Cl – (aq) + Resin + –OH – Resin + –Cl – t +H 2 O(l)

replaces the ionic HCl with H 2 O. Analyte cations elute as hydroxide salts instead of

as chloride salts. A similar process is used in anion ion-exchange chromatography in which a cation ion-exchange resin is placed in the suppressor column. If the mo-

bile phase contains Na 2 CO 3 , the exchange reaction

2Na + (aq) + CO 3 2– (aq) + 2Resin – –H + 2Resin – –Na + t +H 2 CO 3 (aq)

replaces a strong electrolyte, Na 2 CO 3 , with a weak electrolyte, H 2 CO 3 .

Ion suppression is necessary when using a mobile phase containing a high con- centration of ions. Single-column ion chromatography, in which an ion-suppressor

single-column ion chromatography

column is not needed, is possible if the concentration of ions in the mobile phase

Ion-exchange chromatography in which

can be minimized. Typically this is done by using a stationary phase resin with a

conditions are adjusted so that an ion-

low capacity for ion exchange and a mobile phase with a small concentration of suppressor column is not needed. ions. Because the background conductivity due to the mobile phase is sufficiently

small, it is possible to monitor a change in conductivity as the analytes elute from the column.

A UV/Vis absorbance detector can also be used if the solute ions absorb ultravi- olet or visible radiation. Alternatively, solutions that do not absorb in the UV/Vis range can be detected indirectly if the mobile phase contains a UV/Vis-absorbing species. In this case, when a solute band passes through the detector, a decrease in absorbance is measured at the detector.

Ion-exchange chromatography has found important applications in water analysis and in biochemistry. For example, Figure 12.34a shows how ion-exchange chromatography can be used for the simultaneous analysis of seven common an- ions in approximately 12 min. Before IEC, a complete analysis of the same set of anions required 1–2 days. Ion-exchange chromatography also has been used for the analysis of proteins, amino acids, sugars, nucleotides, pharmaceuticals, con- sumer products, and clinical samples. Several examples are shown in Figure 12.34.