Measure Nitrogen Regeneration

The alternative to bioassays is to use tracer methods of which there are two general approaches. In the first, the dilution of an isotopicaUy labeled pool is measured over time (NH4 regeneration, Glibert et al., 1982). In the second, the flow of is followed into a regenerated N pool directly (e.g., DON release, Bronk and Glibert, 1991). 

Nitrogen regeneration can also be measured by monitoring the flow of a labeled compound from a source pool to a target pool. In the case of DON release, N-labeled NH4+ or NOs is added to a sample and the appearance of N in the DON pool is monitored over time (Bronk and Glibert, 1991). To measure a DON flux, DON must be isolated prior to mass spectrometric analysis. There are three basic approaches used to isolate DON (1) wet chemistry (e.g., Axler and Reuter, 1986 Bronk and Ward, 1999 Slawyk and Raimbault, 1995), (2) ion retardation (Bronk and GHbert, 1991 Nagao and Miyazaki, 1999), and (3) dialysis (Feuerstein et al., 1997) aU three approaches are discussed in more detail in Chapter 28 by McCarthy and Bronk (this volume). 

When measuring any release process one must be aware of three potential artifacts—filtration stress (e.g., Goldman and Dennett, 1985), incubation stress (e.g., Venrick et al., 1977), and loss of DON during the isolation process (Bronk and Ward, 2000) see Chapter 28 by McCarthy and Bronk (this volume) for a more detailed discussion. 

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The surface area of the catalysts was measured by nitrogen physisorption (Sorptometer 1900, Carlo Erba). The fresh and regenerated samples were outgassed at 150°C and the spent samples at 100°C for 3 hours. The specific surface area was calculated with the Dubinin equation. 

Methanol or dimethyl ether feeds were fed from an ISCO positive-displacement pump. Liquid products were collected in a room-temperature trap and gaseous products were analyzed by on-line gas chromatography and volumes measured by a wet-test meter. The catalyst could be regenerated in-situ by switching to a nitrogen/air mixture. 

Slawyk, G., Raimbault, P., and Helquen, S. L. (1990). Recovery of urea nitrogen from seawater for measurement of N abundance in urea regeneration studies using the isotope dilution approach. Mar. Chem. 30, 343—362. 

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