I have tested HF/HCl on coal samples. Has long as they don't contain minerals
nitrogen isotopic composition is not affected.
It means that kerogen remains unaffected.
But, if your sample contain another nitrogen source, (Clays or soluble
organic compound), it will be removed by a HCl treatment.
The nitrogen isotope ratio measured will be the kerogen one and not the
I have seen this on coals containing 10% of mineral.
So has Scholten S.O. in his PhD: The distribution of nitrogen isotopes in
Laboratoire de Géochimie des Isotopes Stables
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