Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 621

Source: Reproduced with permission from the American Chemical
Society [9]
the wavelength. Identification is then made by running a standard solution of the
suspected species (same retention time) and comparing the peak height ratios at the same
wavelengths as the unknown species.
The ion chromatographic determination of weak acid anions is complicated by ion
exclusion in the suppressor column, resulting in faster elution and sharper peaks, directly
proportional to the degree of exhaustion of the suppressor column, A 10mg L −1 nitrite
standard showed a 37% increase in peak height over an 8h period when monitored with
the conductivity detector while only a minor 2% increase in peak height was observed
over the same time period by using the ultraviolet detector after the separator column.
The ultraviolet detector can also be used in some cases to resolve overlapping peaks.
Fig. 12.4 shows the ion chromatogram of 10mg L −1 nitrite in the presence of 1000-
2000mg L −1 chloride. Determination of the nitrite peak by using the conductivity detector
is complicated by both the ion exclusion effect and incomplete resolution between the
large chloride peak and the much smaller nitrite peak. The ion exclusion interference can
be eliminated for ultraviolet active anions by placing the ultraviolet detector between the
separator and suppressor columns (position 1). In addition, the problem of overlapping
peaks can sometimes be resolved spectrophotometrically by proper choice of wavelength
as shown in Fig. 12.4 (broken line chromatogram).
Sulphide cannot be detected under normal ion chromatographic conditions. It is
converted into hydrogen sulphide in the suppressor column. Hydrogen sulphide acid is a
very weak acid that does not ionise sufficiently to be detected with the conductivity
detector. The ultraviolet detector, however, is able to detect sulphide at low levels. Since
sulphide is a weak acid anion, the ultraviolet detector was placed between the separator
and suppressor columns.
Arsenous acid is a very weak acid and cannot be detected at low levels by the
conductivity detector. However, like sulphide, it is easily detected with the ultraviolet
detector. The simultaneous determination of arsenite and arsenate is possible.
Arsenite also can be separated by ion chromatography using standard conditions. It
elutes early, near the carbonate dip. For this application the ultraviolet detector should be
placed between the separator and suppressor columns.
12.2 Non saline waters
12.2.1 Chloride, bromide, fluoride, iodide, nitrite, nitrate, sulphate, phosphate,
bicarbonate, cyanide and sulphide
Ion chromatography has been used for the measurement of background concentrations of
fluoride, chloride, nitrate and sulphate in non saline waters [10].
Shown in Fig. 12.5 is a schematic diagram of the Dionex ion exchange liquid
chromatograph used by Smee et al. [10]. The instrumentation consists essentially of a
low capacity anion exchange column, the 'separator', a high capacity cation exchange
column, the 'suppressor' and the detection system, a high sensitivity conductivity meter