EASILY IONIZABLE ELEMENT (EIE) INTERFERENCES IN INDUCTIVELY-COUPLED PLASMA-ATOMIC EMISSION-SPECTROMETRY .2. MINIMIZATION OF EIE EFFECTS BY CHOICE OF OBSERVATION VOLUME

Measurements by inductively coupled plasma atomic emission spectrometry (ICP-AES) are expected to be immune to interferences caused by easily ionizable elements. This article demonstrates how observation width, viewing height and viewing volume can be chosen to minimize such interferences and to optimize emission-signal collection in either side-on or end-on configurations. At this time, results must be viewed as a confirmation of previous findings rather than a practical means of performing routine plasma optimizations. Volume data were computed from high-resolution two-dimensional laterally and radially resolved images of Ca I and Ca II emission in the presence and absence of a lithium concomitant. For side-on observation of calcium atomic emission, the customary arrangement, the optimal balance between signal-to-background (S/B) ratio and interference effects arises from use of a 0.5 mm observation width. The optimal position for measurement and the ideal height of the observation volume were dependent on applied r.f. power. More extended contiguous regions of high S/B ratio became available with the proper spatial averaging, which is inherent in viewing an extended volume. Side-on observation of Ca II emission in the presence of a Li concomitant was best with a 5 mm viewing width at low r.f. power (500 and 1000 W) and a narrow width (< 1 mm) at higher power (1500 W). End-on observation of Ca atomic emission was optimal for a 1000 W plasma viewed within 1.5 mm of the central axis with an observation width of less than 6 mm. For end-on Ca II viewing, a 1500 W plasma minimized the lithium interference while maximizing the S/B ratio when the midpoint of the viewing position was within 1 mm of the central channel and the observation width was less than 2.5 mm.