Nitrogen and oxygen isotope ratios (δ
15N, δ
18O) of nitrate have been measured by denitrifier method aiming to analyze N material flows and their sources (ocean, lake, river, and groundwater). The denitrifier method developed in 1990s allows us to measure both δ
15N and δ
18O of nitrate simultaneously employing small amounts (i.e. 30nmol). The instruments including online auto-injection of N
2O gas are still expensive(generally cost more than two hundred thousand US$). Here, we have developed a self-made auto-injection system of N
2O to IRMS connected to the interface(Conflo III). This self-made system is inexpensive(i.e. twenty thousand US$) and has equal precisions of δ
15N and δ
18O with the past systems.
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