A method for determining the nitrogen isotopic composition of porphyrins

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We describe a new method for analysis of the nitrogen isotopic composition of sedimentary porphyrins. This method involves separation and purification of geoporphyrins from sediment samples using liquid chromatography and HPLC, oxidation of the nitrogen within porphyrin-enriched fractions using a two-step process, and isotopic analysis of the resulting nitrate using the denitrifier method. By analysis of these degradation products of chlorophylls, we are able to measure an isotopic signature that reflects the nitrogen utilized by primary producers. The high sensitivity of the denitrifier method allows measurement of small samples that contain low concentrations of porphyrins. Extraction of only 50 nmol of nitrogen (nmol N) allows the following five analyses to be made (each on ∼10 nmol N): nitrogen concentration, an assessment of potential contamination by nonporphyrin N, and three replicate isotopic measurements. The measured values of δ15N have an average analytical precision of ±0.5‰ (1σ) and an average contribution from Rayleigh fractionation of 0.7‰ from incomplete oxidation of porphyrin N to nitrate. The overall method will enable high-resolution records of δ15N values to be obtained for geological and ecological applications. © 2009 American Chemical Society.

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Analytical Chemistry