Introduction - Determination of Anions in Natural and Treated Waters - page 35

A limitation of gas chromatography is that it cannot handle less volatile compounds
and these comprise a high proportion of the total organics content of the sample. For this
reason increasing attention is being paid to the application of high-performance liquid
chromatography in water analysis. Again, when positive identifications are required, a
mass spectrometer is connected to the outlet of the chromatograph.
Supercritical fluid chromatography is a recent development which may find increasing
use in water analysis.
A major breakthrough in recent years for the determination of anions is ion
chromatography and, even more recently, electrostatic ion chromatography.
1.1.13.1 Ion chromatography
This technique developed by Small et al. in 1975 [19] is usually extensively employed
for the separation and determination of mixtures of 31 inorganic anions in water.
However, applications have also been found for the determination of some 50 organic
anions in water.
The technique uses specialised ion exchange columns and chemically suppressed
conductivity detection. Advances in column and detection technologies have expanded
this capability to include wider ranges of anions as well as organic ions. These recent
developments, discussed below, provide the chemist with a means of solving many
problems that are difficult, if not impossible, using other instrumental methods. Ion
chromatography can analyse a wide variety of organic and inorganic anions more easily
than either atomic absorption spectrometry or inductively coupled plasma techniques.
At the heart of the ion chromatography system is an analytical column containing an
ion exchange resin on which various anions (and/or cations) are separated before being
detected and quantified by various detection techniques, such as spectrophotometry,
atomic absorption spectrometry (metals) or conductivity (anions).
Ion chromatography is not restricted to the separate analysis of only anions or cations,
and, with the proper selection of the eluent and separator columns, the technique can be
used for the simultaneous analysis of both anions and cations.
The original method for the analysis of mixtures of anions used two columns attached
in series packed with ion exchange resins to separate the ions of interest and suppress the
conductance of the eluent, leaving only the species of interest as the major conducting
species in the solution. Once the ions were separated and the eluent suppressed, the
solution entered a conductivity cell, where the species of interest were detected.
The analytical column is used in conjunction with two other columns, a guard column
which protects the analytical column from troublesome contaminants, and a
preconcentration column. The intended function of the preconcentration column is
twofold. First, it concentrates the ions present in the sample, enabling very low levels of
contaminants to be detected. Second, it retains non-complexed ions on the resin, while
allowing complexed species to pass through.
Dionex Series 400001 Ion Chromatographs
Some of the features of this instrument are:
• chromatography module;