Page 226 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

conductance for quantifying eluted ionic species may be illustrated by the simple
example of the separation and determination of sodium and potassium in a
sample in which these are the only cations present. Complete separation of these
two cations may be achieved using a strong acid cation exchange resin with
aqueous hydrochloric acid as eluant. However, the high conductance of the
hydrochloric acid in the effluent effectively 'swamps' the lower conductance due
to the sodium and potassium ions, so preventing their measurement by electrical
conductance.
In IC this problem of electrolyte background is overcome by means of eluant
suppression. Thus in the above example of sodium and potassium analysis, if
the effluent from the separating column is passed through a strong base
anion exchange resin in the hydroxide form (suppressor column) the following
two processes occur:
(a) Neutralisation of the hydrochloric acid
HCl + OH - (resin) + Cl - (resin) + H20
(b) NaCl (KCl) + OH - (resin) + NaOH (KOH) + Cl- (resin)
A consequence of these ion exchange processes is that the sample cations are
presented to the conductivity detector not in a highly conducting backgrourid
but in the very low conductivity of de-ionised water. It may be noted here that
de-ionised water is not always the product of eluant suppression, the essential
feature being that a background of low electrical conductivity is produced.
Analogous schemes can be devised for anion analysis, in which case a strong
acid cation exchange resin (H + form) is employed in the suppressor column.
Ion chromatography permits the determination of both inorganic and organic
ionic species, often in concentrations of 50 pg L-' (ppb) or less. Since analysis
time is short (frequently less than 20 minutes) and sample volumes may be less
than 1 mL, IC is a fast and economical technique. It has found increasing
application in a number of different areas of chemical analysis and particularly
for the quantitative determination of anions. The state-of-the-art has been
re~iewed.'~
A flow scheme for the basic form of ion chromatography is shown in Fig. 7.3,
which illustrates the requirements for simple anion analysis. The instrumentation
used in IC does not differ significantly from that used in HPLC and the reader
is referred to Chapter 8 for details of the types of pump and sample injection
system employed. A brief account is given here, however, of the nature of the
separator and suppressor columns and of the detectors used in ion chromatography.
Separator column. The specific capacity of the separating column is kept small
by using resins of low capacity. For example, low-capacity anion exchangers
have been prepared by a surface agglomeration method in which finely divided
anion exchange resin is contacted with surface-sulphonated styrene-divinyl-
benzene copolymer; the small particles of anion exchanger are held tenaciously
on the oppositely charged surface of the sulphonated bead~.'~ These resins are
stable over a wide range of pH, in which respect they are superior to glass- or
silica-based pellicular resins.
Suppressor column. Where electrical conductance is used for detection of sample
ions in the effluent from the columns, an eluant background of low conductivity
is required. The function of the suppressor column is to convert eluant ions