Why do some trace metals require nitrous oxide acetylene flame instead of air acetylene flame?

Answer 1

Nitrous oxide acetylene burns at a higher temperature than air acetylene. You'd really need to ask a metallurgist (someone who know just about everything to do with metals) to get the real answer, but basically some metals are harder than others. The harder the metal, the harder it is to cut with fire.

Brass and gold are very soft, while iron and titanium are hard.

Answer 2

Some trace metals require a nitrous oxide acetylene flame because the higher temperature and reducing environment created by the nitrous oxide flame helps in achieving better atomization and excitation of the metal atoms, leading to improved sensitivity and detection limits compared to an air acetylene flame. This is particularly important for elements that have high excitation energies or are prone to interference from the atmosphere.