Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 264

SOLUTION: (a) In reverse phase HPLC, the mobile phase is polar and the stationary phase
is nonpolar, the polar analytes will be eluted first. Hence, the polarities of three compounds
aremostlikelyintheincreasingorderofC< B < A. (b) Increasing the polarity of the
mobile phase will help the column retain the analyte better, and, hence, a longer retention
time. Therefore, the percentage of water should be increased, such as 70% or even 90%. A
trial run will help decide the optimal mobile phase composition. If this does not work, then
try a gradient flow to obtain better-spaced peaks with a higher resolution.
10.2.3 Ion Chromatography
IC is one of the applications of HPLC. A schematic diagram showing the major
components of IC is omitted because IC is similar to HPLC, in that it employs a
standard HPLC apparatus, including (a) an eluent reservoir holding the mobile
phase, (b) a pump, (c) a sample injection system, (d) a column (analytical column
and a suppressor column), (e) a detector, and (f) a data-acquisition system (refer to
Fig. 10.4). However, since the separation in commonly used IC is based on ion
exchange of ionic species (inorganic and organic) rather than adsorption or partition,
as described earlier for normal or reverse phase HPLC, the mobile phase, column
stationary phase, and detector are fundamentally different from those described for
HPLC.
IC is a combination of ion exchange chromatography, eluent suppression, and
conductivity detection. Rather than using organic solvents of various polarities, IC
uses acid/base or salt buffer solutions as the eluent (mobile phase) to affect the ion
exchange process. The type and strength of such eluent is important so that various
ionic analytes will have different affinities (retention times).
There are two essential columns arranged in sequence in an IC instrument. The
first analytical column is used to separate various anions or cations. If anions
(negatively charged) are separated, this column is packed with a positively charged
cationic exchange resin, and is termed anion exchange column. If cations (positively
charged) are to be separated, then the analytical column is packed with a negatively
charged anionic exchange resin, and is termed cationic exchange column. For both
types of ion exchange resins, there is an organic polymer to which functional groups
are attached. It is either a negatively charged stationary phase (SO 3 HorCOOH)
for the cation exchange resin, or a positively charged stationary phase (NH 3 or
NR 3þ ) for the anion exchange resin.
The second column, called a suppressor column, is essential and used to reduce
the background conductivity of the eluent to a low or negligible level. This sup-
pression column is placed downstream from the separation column. For anion
analysis (A ), the suppressor column is a large capacity anionic exchange resin used
to retain cations and at the same time convert anions into their corresponding acids
(HA). For cation analysis (M þ ), the suppressor column is a large capacity cationic
exchange resin used to retain anions and at the same time convert cations into metal
hydroxides (MOH). A schematic diagram showing the principles of ion
chromatograph for both anion analysis (left) and cation analysis (right) is given in
Figure 10.6.