Concomitant ion matrix effects in SCGD-OES enhanced with formic acid

Abstract

Solution-Cathode Glow Discharge Optical Emission Spectrometry (SCGD-OES) is a cost-effective analytical technique due to its miniaturization potential and reduced consumption, useful for on-line and in situ aqueous solution elemental analysis, and to its adequate limits of detection, which are, on average, comparable with those of ICP-OES. The elemental response in SCGD-OES is frequently enhanced making use of low molecular weight compound additives, such as formic acid (HCOOH); however, this enhancement was shown to be affected by the presence of significant amounts of Na⁺ and Cl⁻ ions, which are extensively present in a wide variety of samples. In this context, this work delves into the impact of HCOOH on analyte emission signals when analyzing solutions containing high concentrations of other cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) and anions (SO₄²⁻, Cl⁻, NO₃⁻, HCO₃⁻ and Br⁻), which make up an ample variety of sample matrices, on SCGD-OES. In particular, the atomic emission intensity of certain analytes is observed to decline with increasing HCOOH additive concentration under certain sample matrix conditions. This intensity depression mainly appears when the sample matrix contains alkali cations, regardless of the anion type. Interestingly, elements with notable chemical vapor generation (CVG) efficiencies, such as Hg, Ag, Pb and In, maintain the positive HCOOH enhancement of emission intensity. HCOOH addition enhancement is also evaluated in a more complex sample, artificial seawater, showing similar results compared to those obtained with high NaCl concentration solutions due to the high proportion of these ions in seawater. Finally, the analytical performance of SCGD-OES is evaluated in terms of sensitivity and limits of detection, resulting that the addition of HCOOH for seawater analysis pays off when CVG-prone elements are targeted.