There are some materials you'll need to solve side quests in the game. So far I've only seen the need for 1 of the type required. Sadly, no option to split stacks (yet). Best thing to do is pay attention when you find something that requires a particular element, make a note, and then come back later once it's in your inventory.
Spoiler below:
Early Game you'll need 1 Uranium and 1 Neutronium for two different side quests.
