Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 636

detection. These workers carried out considerable exploratory work on the development
of ion chromatographic conditions for separating sulphide and cyanide in a basic medium
(to avoid losses of toxic hydrogen cyanide and hydrogen sulphide) and on the
development of a suitable amperometric detector.
12.2.2 Arsenate, arsenite, selenate and selenite
Hoover and Jager [22] have described a procedure for the determination of traces of
arsenite, selenite and selenate in the presence of major interferences (chloride, nitrate,
and sulphate) in potable waters, surface waters and ground waters. By collecting a
selected portion of the ion chromatogram, after suppression, on a concentrator column
and reinjecting it under the original chromatographic conditions, it was possible to
separate selenate, selenite and arsenite from chloride, nitrate and sulphate. Statistical
detection limits varied from 0.02 to 1.2µg of trace element depending on the minor
components to be separated and on the water matrix. The maximal reliably separated
molar ratio was 2300 for sulphate/selenate.
Ion chromatography can readily determine arsenate, selenite, and selenate species in
water in the absence of interferences. However, they will usually be completely obscured
by the major anions. Arsenic and selenium are ordinarily at their highest oxidation state
in surface waters but that is not necessarily the case in ground waters.
Apparatus
Dionex Model 10 ion chromatograph equipped with AG-1 guard column, AS-3 'fast run'
separator column and ASC-1 suppressor. AG-3 concentration columns were used for
introduction of the sample and for collection of and recycling of selected portions of the
chromatogram. For recycle chromatography, modify the plumbing as shown in Fig.
12.12. The original injection valve was connected to two additional dual four-port slider
valves. Solid lines in the figure correspond to the 'Down' position of the toggle switches.
The dummy concentrator in the waste line of the recycle valve was added to maintain
approximately constant back pressure on the conductivity detector because the base line
conductance of the suppressed eluent is highly pressure sensitive.
Concentrator columns were loaded in place with a Harvard Apparatus infusion pump at
1.5mL − 1 using a Hamilton 5ml glass syringe with a gas tight Teflon tipped plunger and
Luer fitting to 0.5ml id. Teflon tubing. The volume of solution loaded was measured by
collecting the effluent in a volumetric flask, shown in Fig. 12.12. All recycle runs were
made at 15% pump rate (1.51ml min −1 ) and at room temperature 25±1°C).