Elevated-pressure solution cathode glow discharge (SCGD) for more sensitive multi-element analysis by optical emission spectrometry (OES)

Although liquid electrode microdischarges have been studied for over 30 years, the effect of pressure on the characteristics of the generated microplasma has received almost no attention so far. In this work, a laboratory-built pressurized solution cathode glow discharge (SCGD) system was introduced. The microplasma generated in different discharge gases (air, CO₂, Ar, and He), and within a pressure range of 1-2.4 atm, was thoroughly characterized. The new microplasma system was employed as an excitation source in optical emission spectroscopy (OES) for elemental analysis. By increasing the gas pressure, the emission line intensities of target elements were significantly increased; on average, 4-fold in air, 3-fold in CO₂ and Ar, and approximately 70 % in He. The pressure elevation was accompanied by a shift toward the thermodynamic equilibrium in the microplasma, as evidenced by a decrease in the hydrogen excitation temperature and an increase in the vibrational and rotational temperatures of the OH molecules. It was demonstrated that under elevated pressure, the microplasma offered an improved atomization yield, which mitigated matrix effects associated with the formation of poorly dissociating particles (e.g., AgCl and Ca₃(PO₄)₂). The pressurized systems effectively suppressed the N₂ molecular bands, thereby improving the signal-to-noise ratio. Notably, the enhanced intensities of the analytes' emission lines translated into proportionally lower detection limits (DLs). The DLs (3 × σ/a, where "σ" is the standard deviation of the background and "a" is the slope of the calibration curve) obtained for alkali metals using He-SCGD OES at 1.4 atm, i.e., 0.02, 0.01, 0.01, 0.02, and 0.08 μg L^-1 for Li, Na, K, Rb, and Cs, respectively, were 1-4 orders of magnitude lower than those typically achieved with inductively coupled plasma optical emission spectrometry (ICP OES). For the other studied elements, i.e., Ag, Ca, Cd, Co, Fe, Mg, Mn, Ni, Pb, Sr, and Tl, their DLs were 0.3, 6, 0.5, 3, 12, 0.5, 1, 4, 3, 11, and 0.5 μg L^-1, respectively. The trueness of the proposed He-SCGD OES method was validated by analyzing highly mineralized waters and comparing the results with those obtained using the reference method (ICP OES).