Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 637

Fig. 12.12 Valving: (1) eluent supply; (2) sample loop; (3) to separator; (4)
injection port; (5) from detector; (6) dummy concentrator column; (7)
syringe pump; (8) concentrator columns; (9) volumetric flask 2.5 or
10ml
Source: Reproduced with permission from the American Chemical
Society [22]
Reagents
Stock solution of 1000mg L −1 of the test ions. Prepared from reagent grade sodium salts
in water that had been purified by reverse osmosis and mixed bed ion exchange. The
stock solutions were adjusted to the composition of the Dionex standard eluent (3.0m mol
L −1 NaHCO 3 , 2.4m mol L −1 Na 2 CO 3 ) and membrane filtered through 0.45µm pore size
Metricel GA-6. Solutions of 1 or 10mg L −1 were prepared by dilution of the stock with
the water under test and further dilutions were made from these shortly before injection.
Potable water had to be boiled briefly to remove available chlorine before stable dilute
solutions of selenite in it could be obtained.
Results
An eluent composition of 2.0m mol L −1 sodium bicarbonate and 1.5m mol L −1 sodium
carbonate was found to elute arsenate about midway between nitrate and sulphate.
Selenite eluted between chloride and nitrate in an eluent that was 3.0m mol L −1 in sodium
bicarbonate and 2.0m mol L −1 in sodium carbonate.
Suppressed eluent retained in the concentrator columns produced a large carbonate
peak on direct reinjection. This peak obscured or interfered with everything eluting
before nitrate. In the determination of selenite, the concentrator was washed with
deionised water for 5min after collection and before injection. The wash step was omitted
in the determinations of arsenate and selenate where the carbonate did not interfere.