AUTOMATED FLOW INJECTION INSTRUMENTATION FOR MONITORING NITROGEN SPECIES IN NATURAL WATERS

Abstract

The provision of high quality analytical data is an essential prerequisite for understanding the biogeochemical cycling of nutrients in the aquatic environment. Due to the instability of samples collected for nutrient determinations however, in situ analysis is preferred. This approach also allows for high temporal and spatial resolution of the data and alteration of the sampling frequency to meet local environmental needs. Chapter One describes the role of nitrogen species, particularly ammonia and nitrate, in the aquatic environment, their sources, and possible environmental effects and summarises analytical techniques for their determination. Solid state miniaturised detectors and their suitability for in situ monitoring are also discussed. The characterisation and evaluation of a miniature Ocean Optics PSD - 1000 spectrometer and its suitability for field deployment is described in Chapter Two. Parameters investigated were optical resolution, wavelength repeatability, photometric linearity and instrumental noise and drift. The incorporation of the Ocean Optics PSD - 1000 miniature spectrometer into a gas diffusion Flow Injection (Fl) manifold for the determination of ammonia in natural waters is detailed in Chapter Three. Optimisation of the Fl parameters and analytical performance are discussed in detail. The development of an immobilised pH indicator and adaptation to a laminar Fl manifold is also considered. Chapter Four describes the use of the miniature spectrometer in a Fl manifold for the determination of nitrate and nitrite, with analytical figures of merit detailed. The increased information potential of the spectrometer (i.e. full spectral acquisition) facilitated the removal of the refractive index problem using dual wavelength correction. Miniaturisation and automation of the optimised nitrate manifold using micro-solenoid pumps and LabView™ graphical programming is described in Chapter Five. Field deployment of the automated system was assessed during a six week British Schools Exploring Society Expedition to Lesotho, S. Africa (in which an intensive biogeochemical survey of the Sehlabathebe National Park was conducted) and a three day campaign on the River Frome, Dorset, U.K. Chapter Six details the ion chromatographic analysis of major anions and cations (including ammonium and nitrate) in precipitation samples. Two sampling campaigns were conducted. One was at an urban site (Plymouth City Centre) from 27/01/98 - 11/05/98, and the influence of aerosol source on the chemical composition of Plymouth precipitation is discussed. The other involved the Austrian precipitation network and wet deposition trends for nitrate, ammonium and sulphate are discussed. The multivariate analysis technique of PCA was applied to both environmental datasets and the interpretation and merits of this statistical approach are considered.