Flow-injection analysis of nitrate by reduction to nitrite and gas-phase molecular absorption spectrometry

Abstract

Two flow-injection manifolds have been investigated for the determination of nitrate. These manifolds are based on the reduction of nitrate to nitrite and determination of nitrite by gas-phase molecular absorption spectrophotometry. Nitrate sample solution (300 µL) which is injected to the flow line, is reduced to nitrite by reaction with hydrazine or passage through the on-line copperized cadmium (Cd–Cu) reduction column. The nitrite produced reacts with a stream of hydrochloric acid and the evolved gases are purged into the stream of O₂ carrier gas. The gaseous phase is separated from the liquid phase using a gas–liquid separator and then swept into a flow-through cell which has been positioned in the cell compartment of an UV–visible spectrophotometer. The absorbance of the gaseous phase is measured at 204.7 nm. A linear relationship was obtained between the intensity of absorption signals and concentration of nitrate when Cd–Cu reduction method was used, but a logarithmic relationship was obtained when the hydrazine reduction method was used. By use of the Cd–Cu reduction method, up to 330 µg of nitrate was determined. The limit of detection was 2.97 µg nitrate and the relative standard deviations for the determination of 12.0, 30.0 and 150 µg nitrate were 3.32, 3.87 and 3.6%, respectively. Maximum sampling rate was approximately 30 samples per hour. The Cd–Cu reduction method was applied to the determination of nitrate and the simultaneous determination of nitrate and nitrite in meat products, vegetables, urine, and a water sample.