@Slypheed that's different. She would need weakness or limations.
Weaknesses? Well, high temperatures in general, sudden or lengthened exposure. Sarah almost jeopardized Sam's life by using her powers at the same time Sam was. (Carbon burns quite easily, you can burn diamonds in the right solution if you heat up the diamond before hand.)
As for limitations...well, there's three(or four if you just want the stamina limitation) to pick from. Limited to manipulating only already existing carbon without the ability to create it. Carbon in its purest form isn't always easy to come by (And thus, making pure carbon would be quite the drain on stamina. Which Sam already has a problem with). And, limited to manipulating certain forms of carbon.
I mainly wanted Sam to just be able to solidify Carbon. That could be perceived as a limitation as well. Unable to control the gas in anyway other than solidifying it.