For the determination of atom% 15N in NHþ
4 , the solutions were
transferred into glass vials with screw caps. Then 0.5 g MgO was
added and the vial immediately covered with PTFE tape. A filter
paper disc with a diameter of 5mmwas placed on the tape and 7 mL
of KHSO4 (2.5 M) was added. The disc was covered with another
piece of PTFE tape, and the vial closed and gently swirled. The vials
stood at room temperature for 10 days to complete the diffusion
[16]. Afterwards, the paper discs were removed, dried, and placed
into tin capsules for analysis of the atom% 15N using an Elemental
Analyzer Delta C Finnigan mass spectrometer.