Multianion analysis, miscellaneous methods - Determination of Anions in Natural and Treated Waters - page 726

for the sample introduction (valve or microsyringe); lep and le, positions for
refilling of the columns used for the first and second stages
respectively
Source: Reproduced with permission from Elsevier Science [47]
The use of a column coupling configuration of the separation unit provides the possibility
of applying a sequence of two leading electrolytes in one analytical run. Therefore, the
choice of optimum separation conditions can be advantageously divided into two steps:
(1) the choice of a leading electrolyte suitable for the separation and quantitation of the
macroconstituents in the first stage (pre-separation column) simultaneously having a
retarding effect on the effective mobilities of micro constituents (nitrite, fluoride,
phosphate); and
(2) the choice of the leading electrolyte for the second stage in which only micro
constituents are separated and quantified (macro constituents were removed from the
analytical system after their evaluation in the first stage).
To satisfy the requirements for the properties of the leading electrolyte applied in the first
stage and, consequently, to decide its composition, two facts had to be taken into account,
ie the pH value of the leading electrolyte needs to be around 4 or less (retardation of
nitrite relative to the macro constituents in this stage) and at the same time the separations
of the macro constituents need to be optimised by other means than adjusting the pH of
the leading electrolyte (anions of strong acids). For the latter reason, complex equilibria
and differentiation of anions through the charge number of the counter ions were tested at
a low pH as a means of optimising the separation conditions for chloride, nitrate and
sulphate in the presence of nitrite, fluoride and phosphate.
The retardation of chloride and sulphate through complex formation with cadmium
enabling the separation of anions of interest [45] to be carried out, was found to be
unsuitable, as the high concentration of cadmium ions necessary to achieve the desired
effect led to the loss of fluoride and phosphate (probably owing to precipitation).
Similarly, the use of calcium and magnesium as complexing co-counter ions [49] to
decrease the effective mobility of sulphate was found to be ineffective as a very strong
retardation of fluoride occurred.
Better results were achieved when a divalent organic cation was used as a co-counter
ion in the leading electrolyte [50,51] employed in the first separation stage when,
simultaneously, the pH of the leading electrolyte was 4 or less, and the steady state
configuration of the constituents to be separated was chloride, nitrate, sulphate, nitrite,
fluoride and phosphate. The detailed composition of the operational system of this type
used for quantitative analysis is given in Table 14.2 (system No. 1.)
The choice of the leading electrolyte for the second stage, in which the
microconstituents were finally separated and quantitatively evaluated, was
straightforward: a low concentration of the leading constituent (low detection limit) and a
low pH of the leading electrolyte (separation according to pK values). The operational
system used throughout this work in the second stage is given in Table 14.2.
An isotachopherogram from the analysis of a model mixture of anions obtained in the
first separation stage is shown in Fig. 14.2. This isotachopherogram merely indicates the