Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 232

Hydroxide ion considerably quenches the fluorescence. This is probably due to a
proton transfer between the excited hydrogen-bonded aminopyridine and hydroxide ion,
followed by radiationless decay of the imine anion. Although this procedure is lengthier
than the Griess method, it provides a five-fold increase in sensitivity of nitrite detection.
2.63.3 Chemiluminescence methods
Dunham et al. [487] have described an improved method of flow injection analysis for
aqueous nitrite ion which exploits the sensitivity and selectivity of the nitric oxide
chemiluminescence detector. Trace analysis of nitrite ion in a small sample (5-160µL) is
accomplished by conversion of nitrite ion to nitric oxide by aqueous iodide in acid. The
resulting nitric oxide is transported to the gas phase through a semi-permeable membrane
and subsequently detected by monitoring the photoemission of the reaction between nitric
oxide and ozone. Chemiluminescence detection is selective for measurement of nitric
oxide, and, since the detection occurs in the gas-phase, neither sample coloration nor
turbidity interfere. The detection limit for a 100µL sample is 0.04µg L −1 of nitrite ion.
The precision at the 10µg L −1 level is 2% relative standard deviation, and 60-180
samples can be analysed per hour. The results from a standard colorimetric measurement
are compared with those from the new chemiluminescence method in order to further
validate the latter method. A high degree of selectivity is obtained due to the three
discriminating steps in the process:
(1) the nitrite ion to nitric oxide conversion conditions are virtually specific for nitrite
ion,
(2) only volatile products of the conversion will be swept to the gas phase (avoiding
turbidity or colour in spectrophotometric methods), and
(3) the nitric oxide chemiluminescence detector selectively detects the emission from the
nitric oxide plus ozone reaction. The method is free of interferences, offers detection
limits of low parts per billion of nitrite ion, and allows the analysis of up to 10µL-ized
samples per hour, with little sample preparation and no chromatographic separation.
Much smaller samples can be analysed by this method than in previously reported
batch analysis methods, which typically require 5mL or more of sample and often
need chromatographic separations as well.
2.63.4 Flow injection analysis
This technique has been applied [524] to the fluorometric determination of nitrite in river
water using 3-aminonaphthalene-1,5-disulphonic acid as fluorescing agent. The reagent
solution was prepared by dissolving sodium 3-aminonaphthalene-1,5-disulphonate
(0.035g) in 0.01M hydrochloric acid (100ml) and mixing 3ml of this solution with 0.1M
EDTA, concentrated hydrochloric acid (50ml) and distilled water (244ml). The reagent
stream was then mixed with a carrier stream of distilled water, passed through a mixing
coil and mixed with an alkaline solution stream of sodium hydroxide (20%). The
aromatic amine was derivatised, in acidic medium, with nitrite to form a diazonium salt
which was converted, in alkaline medium, to a fluorescent azoic acid salt. Fluorescence
intensity was recorded (470mm) with a spectrofluorometer. Nitrite levels as low as