Discharge Characteristics and Non-Spectral Interferences on the Emission of Ca Species in a Medium Power Radiofrequency Capacitively Coupled Plasma Source

Non-spectral interference of easily ionized elements (EIEs) as chlorides and Al as AlCl3 and Al(NO3)3 on the emission of Ca II 393.367 nm, Ca I 422.673 nm, CaOH 554 nm and 622 nm were studied in a medium power radiofrequency capacitively coupled Ar plasma (275 W, 27.12 MHz) with single (SRTrfCCP) and double ring electrode (DRTrfCCP). The mechanisms of interferences were explained based on the matrix energy demand (MED) supposing the local thermal equilibrium (LTE) in plasma, emphasizing also processes contrasting with this model. Matrix effects were found to be dependent on the coupling geometry of the rf power to the torch, observation height, emitting Ca species and EIEs matrix nature but independent from Al as regards its salt origin. The magnitude of the matrix effects was correlated with the electron number density and the discharge temperature. The optimization of the observation height and use of the DRT geometry allowed the reduction of EIEs and Al matrix effects on the emission of Ca species compared to SRT. The best Ca detection limits in SRTrfCCP were at CaOH 622 nm (106–450 ng ml–1), while in DRTrfCCP at Ca I 422.673 nm (92–145 ng ml–1).