High ionisation energy:
The high ionisation energy of helium (24.6 eV) makes it a potential gas to be used in plasma source.
The nitrogen gas is generally not used as a plasma gas in plasma based atomic emission spectrometry because the excited nitrogen emits a complex band spectrum that can cause undesirable spectral interference.
Polychromator is a type of simultaneous spectrometer. It has a fixed diffraction grating monochromator and multiple 'exit slits' with associated photomultipliers. Every of the photomultiplier is optimized to detect a specified wavelength corresponding to an element of interest. The emission radiation from the source is dispersed by the grating and sent through different exit slits to be detected by appropriate photomultiplier tube.