Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 247

chromatographic method for the determination of nitrite and nitrate in non saline water
based on the nitration of an excess of phenol by nitrate or oxidised nitrite ions; the ortho-
nitrophenol produced is extracted, separated on a reversed phase column and quantified
using an amperometric detector in the reduction mode. Nitrite, if present, is first oxidised
to nitrate by addition of 1% hydrogen peroxide. The method has been successfully
applied to waters containing nitrate plus nitrite at the 5µg L −1 level. By using a larger
sample volume (500ml) and injecting a larger aliquot (100µL) onto the column the
sensitivity could be improved, giving a lower limit of about 1µg L −1 .
Fig. 2.35 shows representative chromatograms and illustrates the analysis of nitrate in
water. Recoveries were 82.6±2.37% for nitrate and 76.9±1.94% for nitrite in the 0.01-
1µg mL −1 concentration range.
This method is free from interferences, which when coupled with the specificity and
the high sensitivity of the electrochemical detection mode, renders it suitable for the
determination of trace levels of nitrate and nitrite in surface, ground and main water.
This technique has been applied to the determination of nitrate and nitrite in water
[579]. Chemically bonded amine materials have been used to remove interference by
humic substances prior to the ion chromato-graphic determination of nitrate and sulphate
in non saline waters.
High performance liquid chromatography on a small bore column packed with
microparticulate silica based ion exchange material has been used [580] to determine
down to 0.01mg L −1 nitrate in water without interference from other ions associated with
potable, pond, river and stream water.
Detection of the separated peaks was achieved using a Pye LC-UV variable
wavelength ultraviolet detector fitted with a 8µL flow cell and set at 265nm. The
chromatographic system consisted of a Partisil 10µm SAX column, and a mobile phase
consisting of 10 −3 mol L −1 potassium hydrogen phthalate (pH 3.95) in deionised water.
The method for the detection of non-ultraviolet absorbing ionic species relies on the
technique of indirect photometric detection. In conventional high performance liquid
chromatography where an ultraviolet detector is employed, the mobile phase is selected
for many reasons and among them is the property of low absorbance at the monitoring
wavelength. Compounds which elute and which have a significant absorbance at the set
detector wavelength are thus observed as peaks of high absorbance against the low
background absorbance of the solvent. This situation may be reversed however. Thus the
mobile phase may contain an ultraviolet absorbing substance which will (after
equilibration of the column) give a high, but constant, background absorbance. The
presence of a non-absorbing species (such as an inorganic anion) in the mobile phase
would thus be expected to cause a drop in absorbance. The elution of a non-absorbing
species under these chromatographic conditions is therefore recognised as a peak
corresponding to increased transmission, ie decreased absorption. Indirect detection of
non-absorbing species may thus be achieved using a conventional variable wavelength
ultraviolet high performance liquid chromatographic detector.
Lee and Field [581] have discussed a technique of post column fluorescence detection
of nitrite, nitrate, thiosulphate and iodide anions in high performance liquid
chromatography. These anions react with cerium(IV) to produce fluorescent species in a
post-column packed bed reactor.