Compact Raman Spectrometer for Analysis of the Nitrification Reactions in the Gulf of Mexico Due to Extensive Nutrient Loading

Abstract

A novel, Analytic Non Dispersive Raman analyzer (ANDRaS®) capable of detection of traces of organic compounds with isotopic specificity will be presented. The system is designed to measure the rate of nitrification in the Gulf of Mexico. During nitrification bacteria oxidize ammonium ions ( NH 4 + ) to nitrite ( NO 2 − ); this is subsequently oxidized by other bacteria to nitrate ( NO 3 − ). The primary source of ammonium in the Gulf of Mexico is water discharged by the Mississippi river. The rates of oxidizing reactions are so fast that concentrations of dissolved oxygen in the Gulf can drop below 2 ppm and large hypoxic zones have been observed as a result of oxygen depletion. The Marine Science Institute at Port Aransas has been studying the seasonal variations in nitrification rates in the Gulf of Mexico using isotope labeling techniques for many years. In order to get a reliable global data set multiple places have to be visited by boats and samples collected, preferably several times a day. The collected water is quickly frozen and analyzed in the laboratory at Port Aransas. The established procedure to measure the rate of nitrification by microbes is based on an isotope transfer diagnostic using 15N [1]. The samples are doped with a known amount (4 µM) of 15NH4 and after 24 hours of reaction time, the retention concentration is recorded using high performance cation exchange liquid chromatography (HPLC). Two probes from each sample are analyzed along with a reference sample, and the results of the two probe samples are averaged. This procedure is automated and the basis of our current understanding of the hypoxic zone.