Determination of the abundance of δ15N in nitrate ion in contaminated groundwater samples using an elemental analyzer coupled to a mass spectrometer

Abstract

A rapid method for measuring the δ¹⁵N of nitrate ion in water samples using an isotope ratio mass spectrometer coupled to an elemental analyzer system (EA-MS) was investigated. The water should be removed from the analytical sample before measurement with this system. We investigated the application of a super-absorbent polymer resin powder to various water samples. Each 1 mg of polymer resin powder can absorb about 50–100 mg of solution depending on the concentrations of major ions. Only samples which contain more than 100 mg l⁻¹ of nitrate-nitrogen are suitable to be absorbed by the polymer resin for the determination of δ¹⁵N of nitrate. Preconcentration by rotary evaporation was necessary for dilute samples but the temperature should be kept below 60 °C. The polymer resin (about 8 mg) containing the nitrate was directly analyzed using an EA-MS after being oven-dried at 80 °C. Good accuracy (precision ±0.3‰) for δ¹⁵N measurements of nitrate-nitrogen in a sample without any isotope fractionation effects during pre-treatment was observed. Results for δ¹⁵N of nitrate in contaminated groundwater samples collected in the spring at a tea plantation area in Shizuoka, Japan, were from 9.8 to 10.6‰, which were close to the δ¹⁵N abundance in organic fertilizers.