Chemistry Reference
In-Depth Information
Waste
water:
chloride, carbonate and phosphate.
Unlike the pellicular packings used for ion exchange, the packings used in ion exclusion
are derived from totally sulphonated polymeric materials. Separation is dependent upon
three different mechanisms: Donnan exclusion, steric exclusion and
adsorption/partitioning. Donnan exclusion causes strong acids to elute in the void
volumes of the column. Weak acids which are partially ionised in the eluent are not
subject to Donnan exclusion and can penetrate into the pores of the packing. Separation is
accomplished by differences in acid strength, size and hydrophobicity. The major
advantage of ion exclusion lies in the ability to handle samples that contain both weak
and strong acids. A good example of the power of ion exclusion is the routine
determination of organic acids in sea water. Without ion exclusion, the high chloride ion
concentration would present a serious interference.
Ion suppression chromatography
Ion suppression is a technique used to suppress the ionisation of compounds (such as
carboxylic acids) so they will be retained exclusively by the reversed-phase retention
mechanism and chromatographed as the neutral species. Column packings with an
extended pH range are needed for this application as strong acids or alkalis are used to
suppress ionisation. In addition to carboxylic acids, the ionisation of amines can be
suppressed by the addition of a base to the mobile phase, thus allowing chromatography
of the neutral amine.
Micelle exclusion chromatography
This technique has found limitations in the determination of bromide mixed halides,
iodide, iodate, nitrite, nitrate, sulphide, sulphite, thiocyanate, thiosulphate and isobutyrate
in non saline waters.
1.1.13.4
Ion exchange chromatography
This technique has been used to determine the following types of organic compounds in
water.
Non saline waters:
sulphate, nitrite, nitrate, bromide, chloride and fluoride.
Aqueous precipitation:
chloride, bromide.
Potable water:
nitrate and nitrite.
Ion exchange chromatography is based upon the differential affinity of ions for the
stationary phase. The rate of migration of the ion through the column is directly
dependent upon the type and concentration of ions that constitute the eluent. Ions with
low or moderate affinities for the packing generally prove to be the best eluents.
Examples are hydroxide and carbonate eluents for anion separations.
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