Direct detection of nitrates by helium-plasma ionization mass spectrometry

An integral member of the natural nitrogen cycle, nitrate is a primary inorganic nutrient for controlling the biomass production in aquatic and terrestrial habitats. It is a constituent of fertilizers, soils, organic residues, municipal wastewaters and sludges, septic systems, and it plays a role in nitrogen fixation from the atmosphere by legumes, bacteria and lightning. The monitoring of its fate and transport is highly relevant in environmental science and engineering. The numerous nitrate-detection methods available typically sample preparation prior to analysis. In addition, most of these methods are prone to significant matrix interferences which can lead to questionable results. Recently patented helium-plasma ionization-mass spectrometry (HePI-MS), invented at Stevens, is one of the most promising novel methods for detecting volatile and semi-volatile organic compounds at ambient conditions. The work described in this dissertation constitutes a comprehensive method for the direct detection and quantification of inorganic and organic nitrates, without the need for extensive sample preparation, and regardless of matrix or origin. The method described is particularly valuable when complex matrices are involved, such as industrial wastewaters and brackish waters, in which situation commercially available tests such as the Hach test are unreliable. Since most nitrates are too involatile to be observed, an in-source acidification of the samples with a strong mineral acid is a key step for signal generation. The limit of detection from solid inorganic nitrates in any matrix is about 2 ng. The method is equally successful in the direct in situ detection of organic nitrate esters (e.g., pentaerythritoltetranitrate and nitrocellulose), which are well known environmental pollutants at army ranges. Nitrates can be detected by the HePI method even at a distance of at least 50 feet, enabling remote sampling from hazardous areas. The mechanism operative for the generation of nitrate signals from nitrate samples in HePI is discussed and distinguished from that for the generation of similar signals from nitrogen oxide (NO). A "microreactor" was constructed and employed to improve sample delivery to the HePI source. Such a microreactor can be conveniently used in the future for performing and monitoring other in-source reactions to aid the detection of environmentally relevant species, e.g. heavy metals, from difficult and complex matrices, such as natural waters and soils.