Novel insights on the matrix effect of acid solutions in nitrogen microwave-induced plasma optical emission spectrometry: Analytes signal enhancement in formic acid

Abstract

This study evaluated the matrix effect of acid solutions on the nitrogen microwave-induced plasma optical emission spectrometry (N₂-MIP-OES) sensitivity of 24 elements. A model solution of 0.2 % nitric acid was used to compare the element's emission line sensitivity in nitric (1.0 %, and 5.0 %, v v⁻¹), sulfuric (0.2 % and 1 %), hydrochloric, formic, and acetic acid solutions (0.2 %, 1.0 %, and 5.0 %, v v⁻¹). The relative sensitivity was estimated by the linear regression slope (LRS) of signal intensities obtained at eight concentration levels (blank + standards). Principal component analysis (PCA) was applied to the LRS data to distinguish matrix effects. Signal enhancement was noted for a group of elements (Al, As, B, Ba, Cd, Co, Cu, Fe, Mn, Pb, Sr, and Zn) in formic acid solutions (0.2 % and 1 %) and varied from +50 % (Cd, Zn) to +80 % (B, Ba, Pb, Sr) of the corresponding sensitivity in HNO₃ 0.2 %. In contrast, signal suppression occurred for K (−15 %), Ca (−20 %), and Mg (−40 %) and in diluted hydrochloric solutions (0.2 % and 1 %). The ratio values of Mg(II)/Mg(I) and (N₂⁺/OH) used in robustness metrics increased in these solutions, indicating higher robustness. An increase in CO⁺ species formation in the plasma was also observed. Refractory oxide-forming elements (Ce, Cr, La, Mo, Ni, Se, and V) were less impacted by acid matrix effects. This study aimed to contribute to the increasing interest in multi-elemental determination by MIP-OES.