• I HAVE USED THIS WEBSITE TO FIND OUT WHICH ELEMENTS MAKE UP NGC 3918

S	16
Fe	26
Ni	28
Zn	30
As	33
Br	35
Tb	65

I have found this article from Monthly Notices of the Royal Astronomical Society. They studied the chemical content of the planetary nebula NGC 3918. They detected very faint lines of several neutron-capture elements (Se, Kr, Rb, and Xe).
They found that Kr is strongly enriched in NGC 3918 and that Se and Xe is less enriched. Also, they detected numerous refractory element lines (Ca, K, Cr, Mn, Fe, Co, Ni, and Cu) and a large number of metal recombination lines of C, N, O, and Ne.

If we compare my study with yours, two elements coincide ( Fe and Ni ).Apart from these chemical elements I have also found S, Zn, As, Br and Tb.