The isotopic composition of nitrogen in Australian coals and oil shales

The Kjeldahl method for the quantitative determination of nitrogen in coal is unsuitable for the preparation of nitrogen for isotopic analysis. Consequently, a modified Dumas method was developed which allowed δ ¹⁵N-values to be determined in coals and oil shales with a precision greater than ± 0.3‰. Comparison of nitrogen contents obtained by the Dumas and Kjeldahl methods showed that there was little difference for oil shale samples but that for coals the results obtained with the Dumas method were always higher, the difference increasing with increasing nitrogen content of the sample. This suggests that the poor reproducibility of isotopic data when using the Kjeldahl method results from loss of nitrogen during sample preparation. ¹⁵N/¹⁴N ratios in Australian coals fall within the narrow range reported by other authors for North American and European colas and do not correlate with either the age, rank or depositional environment of the coal. The ¹⁵N content of the oil shales varies according to predominant organic precursors, the concentration and source of the nitrogen (molecular, ammonia, nitrate) and the pathways utilized during metabolism. The different types of Australian oil shales can be classified by nitrogen isotope ratio analysis which, in general, supports a classification based primarily on microscopic techniques.