Abstract
A new sample preparation method for 15N tracer measurement of ammonium regeneration in seawater is described. Ammonium nitrogen is incorporated into indophenol, extracted into dichloromethane and concentrated by evaporation. The isotopic ratio of the indophenol nitrogen is determined by emission spectroscopy. A small amount (0.2 μmol) of unlabeled nitrogen is introduced by reagents used during sample preparation. For samples of equal nitrogen content, the coefficient of variation is constant and the precision of the isotopic ratio determination is inversely related to the relative 15N abundance. Differences of 0.19 atom% 15N can be measured in 5.0 μM NH 4 + samples of 5.0 atom% 15N (SD = 0.07, n = 3, p = 0.05). Interface from forms of dissolved organic nitrogen prevalent in marine environments are negligible.
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