Study of the vertical distribution of intensities with the excitation energy of the measured lines in ICP-AES

Abstract

Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) is a technique widely used in
multielemental analysis. In this work, the intensity of 89 atomic and ionic lines with Total Excitation Potential (PET
in Spanish) ranging from 1,62 to 16,51 eV was studied, while applied power, nebulizer gas flow rate and observation
height above the load coil were varied. In the low zone of the plasma, the relationship between intensity and
excitation energy showed the usual behavior of the thermal excitation mechanisms (e.g. collisions with electrons).
The same behavior was observed when the observation height was increased and plasma was operated in conditions
near to local thermal equilibrium (LTE). In contrast, when plasma was operated far away from LTE and the
observation height increased, the dependence of line intensity with energy drastically changed. Lines having an
upper ionic level, which is more probable to be selectively populated via inelastic collisions between analyte atoms
and excited Ar atoms or Ar ions, showed a less intensity reduction while the observation height increased. Experimental
evidence, not reported before, on the existence of Penning Ionization as excitation mechanism in ICP-AES
is shown. On the other hand, Charge Transfer was confirmed as a non thermal process explaining the behavior of
lines with PET near the first ionization energy of Ar. Some specific reactions describing those processes of energy