High-resolution continuum source molecular absorption spectrometry of nitrogen monoxide and its application for the determination of nitrate

The molecular absorption spectrum of the nitrogen monoxide, NO, produced by a 5% (v/v) nitric acid solution in an air-acetylene flame has been investigated using a simultaneous, high-resolution continuum source spectrometer. The vibrational absorption bands of the electronic transition exhibit very complicated fine structure, and each band consists of a multitude of rotational absorption lines. Some of these lines have been evaluated with regard to their applicability for the determination of nitrate using electrothermal vaporization. A method has been developed, which provides a new way for the determination of nitrate. No spectral interference has been observed, mainly because of the low vaporization temperatures of only 700-900 degrees C, where no atomization takes place. However, hydrochloric, sulfuric and phosphoric acid, as well as metal salts, which can form acids by hydrolysis, such as Al, Cr, Cu, Fe and Mn, show strong effects and reduce the NO absorption. This kind of interference can be avoided by using a pyrolysis temperature of only 150 degrees C. Using the NO line 215.360 nm the limit of detection was approximately 5 ng N and the calibration curve was linear at least to 3000 ng N. The method has been validated by analyzing certified reference materials with different nitrate concentrations. The method should also be applicable to the determination of nitrite after oxidizing the nitrite to nitrate and subsequently determining the NO molecular absorption.