Hi Khadra,
In the Calculator, you would set up the expression something like this:
I have a X in the State in my case because I didn't measure Sn118 in my experiment.
If you're using this channel with 3D Trace Elements, you will need to set the channel properties "Element" and "Mass" by adding properties in the Channels Browser:
You should then be able to run your DRS and get full results.
Please let me know how it goes.
Kind regards,
Bence