Page 228 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS
P. 228
7 ION EXCHANCE
thus allowing one column to be regenerated while the other is in use. A more
convenient arrangement, however, is the use of a hollow-fibre suppressor, such
as that developed by Dionex Ltd, which allows continuous operation of the ion
chromatograph. In the case of anion analysis, the suppressor incorporates a
tubular cation exchange membrane which is constantly regenerated by dilute
sulphuric acid flowing through the outer casing.
It is appropriate to refer here to the development of non-suppressed ion
chromatography. A simple chromatographic system for anions which uses a
conductivity detector but requires no suppressor column has been described by
Fritz and CO-~orkers.'~ The anions are separated on a column of macroporous
anion exchange resin which has a very low capacity, so that only a very dilute
solution (CU 10-4M) of an aromatic organic acid salt (e.g. sodium phthalate)
is required as the eluant. The low conductance of the eluant eliminates the need
for a suppressor column and the separated anions can be detected by electrical
conductance. In general, however, non-suppressed ion chromatography is an
order of magnitude less sensitive than the suppressed mode.
Detectors. Although electrical conductance has been widely used for detecting
ions in ion chromatography, the scope of the technique has been considerably
extended by the use of other types of detector. It is convenient broadly to classify
detectors into two series.
1. Detectors employing electrochemical principles:
(a) Conductimetric detectors. Conductance is a fundamental property of ions
in solution making it an ideal technique for monitoring ion exchange
separations because of its universal and linear response. It is the optimum
mode of detection for strong acid anions (pK, < 7), providing high
sensitivity in the absence of background electrolyte.
(b) Amperometric detectors. This type of detector may be used to detect ions
which are electrochemically active but not readily detected by conductance
measurement, e.g. weak acid anions such as CN-, HS- (pK, > 7). The
detector commonly features interchangeable silver or platinum working
electrodes and may be used alone (no suppressor then being required)
or simultaneously with a conductivity detector.
2. Detectors based on established optical absorption and emission techniques,
are typified by:
(a) Spectrophotometric detectors. The operation of spectrophotometric
detectors is based on the absorbance of monochromatic light by the
column effluent in accordance with the Beer-Lambert law (Section 17.2).
As most organic species have significant absorption in the UV region of
the spectrum, these detectors have wide application. Sensitivity clearly
depends on how strongly the sample absorbs at the wavelength of
maximum absorption, but detection limits in the low- (or even sub-)
nanogram range may be achieved in favourable conditions.
An alternative approach using a spectrophotometric detector has been
described by Small and Millerzg The essential feature here is that the
eluting ion, which is commonly phthalate, must absorb ultraviolet light
thus allowing its concentration to be monitored at a suitable wavelength
as it emerges from the column. When a non-absorbing sample ion elutes
from the column, the concentration of eluting ion decreases and the
detector registers a negative response which is proportional to the