Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 631

expected to be found in non saline waters. Typical k' values were as follows: bicarbonate,
H 2 PO 4 , chloride and nitrite <0.5, sulphate 7.2, nitrate 1.4.

Procedure
A 500µL water sample is injected onto the adsorption column and the eluate collected in
a fraction collector. The fraction (around 2.5ml) which contained the anions, as seen by
the conductometric detector, is evaporated to dryness (80°C overnight) and redissolved in
250µL of water. 100µL aliquots are analysed on the ion chromatographic system using
the ion exchange separation column and conductance detection as described above.
Marko Varga et al. [14] prepared a series of model solutions with 10, 30, 50, 70 and
100mg L −1 of both nitrate and sulphate additionally containing 0.00, 0.09, 0.90, 4.5, 27 or
45mg L −1 of reference humic acid. By use of the procedure described above, nitrate and
sulphate were determined in the solutions. The calibration curve was constructed from
the solutions without added humic acid.
A separate analysis of 45mg L−1 reference humic acid (without anions added) gave the
result: nitrate 3.7mg and sulphate 5.2mg L −1 . This is considered as the background level
of these ions in the reference humic acid. Thus, significant amounts of nitrate and
sulphate are added to the model solutions together with the humic acid.
Considering these background levels, the percent recovery (concentration
found/expected concentration) was calculated for all model solutions analysed and is
presented in Table 12.4. The relative standard deviation within each set of six values was
typically 2-3% both for nitrate and sulphate.
Most of the recoveries in Table 12.4 are not significantly different from 100%. For
sulphate, recoveries tend to be about 10% low at high concentrations of humic acid while
an opposite tendency can be noted for nitrate. One reason for this behaviour might be the
competition between sulphate and nitrate and other ions present in the humic acid for the
binding sites of the humic substances.
In all chromatograms an unknown peak appears. This is probably due to a disturbance
in the phthalate protonation equilibrium created by the injection of aqueous samples. The
retention time of the peak is independent of pH and the concentration of the eluent, but
the intensity is affected by these factors. It is possible to adjust the eluent composition so
the peaks of interest are separated from the 'ghost peak'.
Ion chromatography has been used for the measurement of background concentrations
of fluoride, chloride, nitrate and sulphate in non saline waters [10].

Table 12.4 Percent recovery of nitrate and sulphate from solutions containing humic acid

% recovery for concn. of humic add added (mg L −1 )

Ion concn. mg L −1

0.09 0.9 4.5 9 27 45

NO 3 -
10

102 98 105 94 103 94

30 101 109 99 108 103 105