Best results were obtained by employing an anionexchange
resin for the reversible retention of nitrate. Several
solutions (concentration of 0.6 mol L−1) were evaluated
to elute nitrate from the separation column, including those
containing anions with high (sulfate) or low (acetate, phosphate,
hydroxide and chloride) relative selectivity to the resin
[19]. Perchloric acid and sodium perchlorate solutions were
also evaluated. Sodium hydroxide and sodium acetate solutions
presented strong absorption in the 200–220 nm range.
Chloride and sulfate solutions showed strong absorption at
201 nm but could be employed for measurements in wavelengths
higher than 210 nm. The use of these solutions was
avoided in view of the lessening in sensitivity of at least 20%
by carrying out the measurements out of the nitrate absorption
maximum. The other solutions were evaluated as eluent
and the results are shown in Table 1. Best sensitivity was
achieved with phosphate that presents low relative selectivity
to the anion-exchange resin (5.0 against 65 for nitrate)
[19]. However, best separation from the interfering species
was attained with perchloric acid and this solution was selected
as eluent.