Advanced methods for isotopic determination of aqueous nitrate, nitrite, ammonia, and organic matter
Isotopic analysis of nitrogen is one of the most important tools for investigation of the aquatic nitrogen cycle. This dissertation explored new methods to measure the natural abundance levels of nitrogen and oxygen isotopic composition of nitrate, nitrite, ammonia, and dissolved organic nitrogen. Existing methods are limited by precision, concentration range, ability to separate nitrogen species, salinity level, sample throughput, and/or ease of use. The methods developed in this dissertation were targeted for low concentrations of fixed nitrogen species that exist in the open oceans. However, these methods can be applied to other aqueous media, thus increasing their scope to a wide variety of nitrogen isotopic applications.
A variety of methods were examined for natural nitrogen isotopic abundance measurements and are discussed in limited detail. Two methods, the "azide method" and the "UV method" are discussed in detail. The UV method involves the reduction of nitrate to nitrite by UV light. We examined many conditions not previously studied to optimize this reaction and were the first to utilize this approach for the isotopic analysis of nitrate. The azide method involves the reduction of nitrate to nitrite using spongy cadmium followed by the conversion of nitrite by hydrazoic acid to nitrous oxide for oxygen and nitrogen isotopic analysis. The nitrogen and oxygen isotopic composition of nitrite can be measured separately from nitrate using this method. This is the first time such measurements could be made at the natural isotopic abundance level. During the course of this dissertation a biological method was published which used denitrifying bacteria to convert nitrate to nitrous oxide. A direct comparison of the azide and the denitrifier method was performed. The azide method is proving to be a robust procedure for the measurement of the isotopic composition of nitrate and nitrite and is already in use in other laboratories.