What type of sensor could I use to determine the mass of a small object (under 1 kg)? I'm building a robot that needs to pick up a piece of material and then determine its mass. My robot will retrieve the material using a scoop at the ended of a mechanized arm. Once the material is in the scoop, the arm will tilt upwards until the material slides down the arm and into the chassis.
I hope to weigh the material while its in the chassis, but I'm not sure how to do this. I've found that load sensors are useful for taking masses, but I'm not clear on how they're implemented. For example, I've found load beam cells, but how would I implement one in my design? Does it require a metal plate of some sort (like a weighing pan) on top of it like a laboratory scale? The sensor must be able to communicate with an Arduino.

My robot needs to be under 6 lbs and have a footprint of 35 x 35 cm. Thanks!
Just a little brainstorming:

If you have a spring, then you can easily calculate the physical force (mass) needed to stretch the spring to a certain extent. The law is called:
 Hookes's Law
 - you could then just measure the increase in distance between the initial position and the position with load. Use a servo or any distance measure.