Optical fiber-coupled flow cells and their application in in-situ water analysis

Abstract

ABSTRACT Liquid core wave guides (LCW) are particularly suited as flow-through cells for highly sensitive spectral analysis of small liquid sample volumes as the light propagation is constrained to the liquid core. We report on LCW formed by inside coating of glass capillaries with Teflon® AF 1600 (n § 1.31). The wave guiding losses are between 0.2 and 0.3 dB/cm. On the base of fiber-coupled LCW flow-through cells both, a spectrophotometer and a fluorescence detector have been developed and tested for in-situ water analysis, especially for nitrate and phosphate detection in sea water. Keywords: liquid core wave guides, fiber-coupled flow cells, water analysis 1. INTRODUCTION Nutrients as nitrate, phosphate, and ammonium are essential substances for the metabolism of plants. Due to the intensification of agriculture, the nutrient contents, especia lly in natural water have increased. The presence of nutrients in drinking water reservoirs, lakes, rivers and in the sea infl uences the quality of water and has serious consequences for animal and human health. Therefore, continuous water monitori ng is essential for the evaluation of the water quality. The usual practice is water sampling followed by sample storage, handling and analysis in a laboratory. This procedure is costly and does not provide large data sets of spatial distribution or temporal evolution of dissolved nutrients. To overcome these problems automatic flow analysis (FA) meth ods [1] and equipments are in development. Requirements for such an automated analyzer for in-s itu measurements are high sampling rates, small sample volumes, low reagent and energy consumption, and last but not least, robustness and reliability. Furthermore, various applications, such as marine applications, require lowest detection limits down to the ppb range. The analysis of nutrients in aquatic systems by FA methods is commonly based on the formation of colored chemical reaction products and their photometric or fluorometric detection [2]. Additionally, nitrate contents can be determined directly by UV spectrophotometry without chemical pretreatment [3]. High sensitivity photometric detection requires long optical path length. In any case, FA equipments with optical detection need flow-through cells for low sample volumes. In this paper we report on developments of optical flow-through cells and equipments for nutrient measurements in water with low detection limits both for in-situ and for laboratory use.