It is not easy to detect a specific metal. The best way is with a flourecessent spectrometer but it is not a simple Aarduino project. The sure fire way is with a mass spectrometer again not a portable instrument.Why do you want to do this?
I think the conductivity of blood will increase if Lead levels are present.
Also what about photo reflecticve like those they use in heart-beat sensors
I think the exposure to all that SciFi has left people of an unrealistic expectations of detectors.
Why? Blood is rich in iron and that will swamp out any conductivity effects.