In Dumas method for the estimation of N$$_2$$, the sample is heated with copper oxide and the gas evolved is passed over:

In the Dumas method for estimation of nitrogen, an organic compound is heated with excess copper oxide (CuO) in an atmosphere of CO$$_2$$. During this process the nitrogen in the compound is converted to N$$_2$$ gas, while some oxides of nitrogen (like NO, N$$_2$$O) can also form.

To ensure collection of pure N$$_2$$, the gas mixture is passed over heated copper gauze, which reduces any oxides of nitrogen back to N$$_2$$:
$$2Cu + 2NO \rightarrow 2CuO + N_2$$ The pure nitrogen gas is then collected over a KOH solution, which absorbs CO$$_2$$.