Insight into the origin of carbon matrix effects on the emission signal of atomic lines in inductively coupled plasma optical emission spectrometry

Abstract

In inductively coupled plasma optical emission spectrometry (ICP-OES), the presence of carbon in the matrix strongly affects (positively and negatively) the emission signal of atomic lines. However, the emission signal of ionic lines is mostly unaffected by this concomitant. The goal of this work is to gain insight into the origin of carbon matrix effects on the signal of atomic lines in ICP-OES. To this end, the emission signal of a total of 3608 lines (i.e., 1755 atomic and 1853 ionic lines) of 62 elements (Ag, Al, As, Au, B, Ba, Be, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hg, Ho, I, In, Ir, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, Pd, Pr, Pt, Re, Rh, S, Sb, Sc, Se, Sm, Sn, Sr, Tb, Te, Th, Ti, Tm, U, V, Y, Yb, Zn and Zr) were registered for carbon containing solutions (20 g L−1) prepared from glycerol. Results were compared to those obtained operating a 1% w w−1 HNO3 solution. Compared to the HNO3 reference solution, emission signals for As, B, Hg, I, P, Se and Te atomic lines were increased for the carbon containing solution. However, signal suppression was noticed for the signal emission of Ag, Al, Ba, Be, Ca, Cd, Co, Cr, Cs, Eu, Fe, Ga, Ge, K, Ho, In, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Ni, Pb, Pd, Pr, Pt, Re, Rh, S, Sb, Sc, Sn, Sr, Ti, Tm, V, Y and Zr atomic lines. No measurable matrix effects within experimental uncertainties were registered for Au, Cu, Ir, Gd and Zn atomic lines. As regards Ce, Dy, Er, Nd, Sm, Tb, Th, U and Yb no clear conclusion about matrix effects was feasible due to the limited atomic emission wavelengths monitored. Experimental results showed that the intensity of matrix effects depends on both analyte ionization energy and the energy of the electronic upper level involved in the electronic transition. Matrix effects on atomic emission lines can be explained by the simultaneous occurrence of different mechanisms: (i) charge transfer reactions; (ii) collisional ionization; and (iii) collisional excitation with carbon-based species. Because these mechanisms mostly affect analyte atomic population, carbon matrix effects are more significant on the emission signal of atomic lines rather than ionic ones. Finally, the judicious analyte wavelength selection is critical to minimize carbon matrix effects for those elements for which the most sensitive wavelength is atomic (i.e., Se and alkali elements). On this regard, the selection of the internal standard should consider both the ionization energy and the energy of the electronic upper level of the analyte.