After many, many tries, it seems like I finally got it!
Conclusion: Iron is sooooo sensible... it means, it is so complicated!
Interferences, lamp current, fuel flow... I have to repeat the method I achieved to confirm that it works but... I'm happy, for now! The results are not perfect yet but to close.
The secret, fresh standards! Everyday! And an Auto-Zero before samples too!
Thank you again, marquis! If you need some help you in the future, you can count on me.