Do you need a pump, seeing from the volume of the sample?
Most of the spectrofotometers I know can use as little as 10 ml sample.
I used 3.7 V 18650 LiIon batteries. Grouping them 3 in series results in a large capacity. Still, heavy, I doubt the drone can hande.
A less conventional solution might be to use wires from the drone to the power source on the soil. This may give you as well the opportunity to send signals to the drone from surface.
This is the principle of soviet anti-tank missiles. Very effective, yet simple.