Fractionation effects, nitrogen retention

This method of establishing retention time is valid only if isotopic fractionation effects during nitrogen transformations are unimportant. For example, nitrification results in a 15N enrichment of the residual NH4 + pool... 

It should also be noted that nitrogen is not an unbiased probe adsorbate. Obviously the surface accessibility for irregular materials depends on the size of the probe molecule a large probe cannot foUow the irregularity of the surface. Analyte molecules are usually larger than nitrogen molecules, and may not be able to penetrate aU pores. Thus only a fraction of the surface area is involved in analyte retention. To fulfill lUPAC recommendations for surface characterization methods [9], the most suitable method depends on the specific application. Recently an approach that involves IPR ion adsorption proved effective. The best probe to determine the packing area... 

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