Departure from local thermal equilibrium during ICP-AES and FAES: Characterization in terms of collisional radiative recombination activation energy

Abstract

A simplified rate model is presented showing that when analytes are determined by atomic spectroscopy first in the absence, and then in the presence, of easily ionizable elements (EIEs) as interferents, the change in collisional radiative recombination activation energy, ∆Ea, is zero when the system conforms to local thermal equilibrium (LTE). ∆Ea values of –7.462, –7.925, and –8.898 eV were obtained when Ca(II), Mg(II), and Sr(II), respectively, were determined by inductively coupled plasma-atomic emission spectrometry (ICP‑AES) in the absence and presence of excess Li, while ∆Ea values of –6.477 and –7.481 eV were obtained when Mg(II) and Sr(II), respectively, were determined in the absence and presence of excess K as interferent. A value of –2.223 eV for ∆Ea was obtained when Mg(I) was determined by air-acetylene flame atomic emission spectrometry (FAES) in the absence and presence of excess K. The data confirm that all the systems studied were not in LTE, and suggest pre-LTE collisional radiative recombination in the absence of the interferent in all cases, and that collisional radiative recombination involving electrons from the interferent can occur from the ambipolar diffusion state or the LTE state. Possible causes for departure from LTE, and a possible collisional radiative recombination mechanism to account for the ∆Ea values obtained, are discussed.