Dual isotopic (δ¹⁵N-δ¹⁸O) characterization of saltpetre currently prevailing in Lao PDR and its global compilation: new insight into isotope fractionation during production processes

Saltpetre (KNO₃; potassium nitrate) is one of the major ingredients of gunpowder. Simplex saltpetre (total 126 samples) together with gunpowder (total 93 samples) commodities which are currently prevailing in local markets as used for wildlife hunting and rocket festivals (local name in Lao: Boun Ban Fai) were collected from throughout Lao PDR. Dual isotopic composition of nitrates (δ¹⁵N, δ¹⁸O) was analyzed by the microbial denitrification method. Binary plotting of the data was conducted to characterize the mode of formation. In Lao PDR, almost all the commodities were imported from neighbouring Thailand, and to a lesser extent from Israel. Binary plotting of δ¹⁵N vs. δ¹⁸O of nitrates shows their intrinsic origin manufactured by Haber–Bosch and subsequently Ostwald processes. We observed an inverse correlation (δ¹⁵N_nitrate: up to + 12 ‰; δ¹⁸O_nitrate: down to + 15 ‰), deviating from the reservoir values (free air) of δ¹⁵N (dinitrogen; up to 0 ‰) and δ¹⁸O (oxygen; +23.5 ‰), interpreted this as an indication of isotope fractionation during manufacture. The most plausible interpretation for this is the fractionation associated with formation of NO following NO₂ and finally NO₃ products. A nearly comparable inverse relationship is also observed for nitrates in pyrotechnics manufactured in China.