Chemistry Reference
In-Depth Information
The stationary phases commonly used in high performance liquid chromatography are
typically derived from silica substrates. The instability of silica outside the pH range 2 to
7.5 represents one of the main reasons why ion exchange separations have not been
extensively used in high performance liquid chromatography. To overcome this, some
manufacturers (eg Dionex) supply columns under the trade name Ion-Pac which contain a
packing which is derived from cross-linked polystyrene which is stable throughout the
entire pH range. This pH stability allows eluents of extreme pH values to be used so that
weak acids such as carbohydrates (and bases) can be ionised.
1.1.13.5
Gas chromatography
The numerous anions that have been determined by gas chromatography or gas
chromatography-mass spectrometry are listed below.
Non saline
waters:
arsenate, bromide, chloride, free cyanide, total cyanide, fluoride, iodide,
nitrate, nitrite, phosphate, selenate, selenite, sulphide and thiosulphate. Also
aminoacetate, carboxylates, hexane carboxylate, malonate, octane dithioate and
octaniioate.
Sea water:
sulphide.
Aqueous
precipitation:
bromide, chloride, nitrate, phosphate and sulphate
Surface and
ground waters:
iodide.
Potable water:
bromide, chloride, iodide and chlorolignosulphonate.
Waste waters:
thiocyanate, free cyanide; total cyanide.
Trade effluents:
free cyanide, polysulphide and sulphide.
The basic requirements of a high performance gas chromatograph are:
• the sample is introduced to the column in an ideal state, ie uncontaminated by septum
bleed or previous sample components, without modification due to distillation effects
in the needle and quantitatively, ie without hold-up or adsorption prior to the column;
• the instrument parameters that influence the chromatographic separation are precisely
controlled;
• sample components do not escape detection; ie highly sensitive, reproducible detection
and subsequent data processing are essential.
There are two types of separation column used in gas chromatography: capillary columns
and packed columns.
Packed columns are still used extensively, especially in routine analysis. They are
essential when sample components have high partition coefficients and/or high
concentrations. Capillary columns provide a high number of theoretical plates, hence a
very high resolution, but they cannot be used in all applications because there are not
many types of chemically bonded capillary columns. Combined use of packed columns
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