Evaluation of analyte transfer rates and excitation conditions in the SC-GD by absorption and emission spectrometry

The solution cathode glow discharge (SCGD) is a compact atomization/excitation source sustained under ambient air using an aqueous solution as discharge cathode, offering limits of detection (LODs) for many elements in the periodic table within the mid to low ng/mL range when coupled to optical emission spectrometry (OES). Formic acid (HCOOH) is frequently included in the incoming solution as an additive for improved sensitivity; however, some reports have shown that the benefit of this additive depends on the concomitant ions present in the solution. It is widely assumed that the improvement in the analytical sensitivity comes from an enhanced sampling efficiency produced by a higher analyte transfer rate caused by HCOOH; accordingly, the coexisting ions are considered to alter the impact of this additive, probably through changing analyte transfer rates at the liquid-plasma interface. This work aims to provide a perspective of the analyte solution-to-plasma transfer in the SCGD at relevant solution content by means of atomic absorption spectrometry (AAS). For this purpose, a SCGD-AAS system was built and characterized using a pulsed Xenon lamp as radiation source. The absorption signals, together with the emission signals, corresponding to resonant transitions of Mg I (285.2 nm), Cu I (324.7 nm), Ag I (328.1 nm) and Li I (670.8 nm) were recorded at the following situations: (1) cation or anion concentrations up to 60 mM, including Na⁺, Ca²⁺, NO₃⁻ or Cl⁻; (2) HCOOH at concentrations ranging 0–7 %; (3) co-presence of the mentioned ions and HCOOH. The results provide information to discuss the influence of common coexisting cations and anions in real samples, together with the addition of a common sensitivity enhancer, HCOOH, on analyte transfer mechanisms in the SCGD, to understand the general working fundamentals of the technique and possible matrix effects when combined with OES detection.