Alternative acidic medium to enhance atomic emission intensity in electrolyte cathode atmospheric glow discharges

A shortcoming of the electrolyte cathode atmospheric glow discharge optical emission spectrometry (ELCAD-OES) is the dependence of sensitivity on sample solution pH and acidic medium.

An ELCAD-OES system was studied in various acidifying media (HBr, HCl, HNO₃, H₂SO₄) to compare atomic emission intensities and improve limit of detection (LOD) of selected environmentally relevant elements (Cd, Cu, Hg, Ni, Pb, Zn). HBr-acidified medium increased the emission intensity by 1.5-2.1-fold (standards) and 1.3-1.4-fold (diluted seawater), compared to the other acids studied, thus it was optimized for sample pH, flow rate, plasma current and observation height. Calibration curves were linear up to 90-120 mg/L. LODs ranged from 0.14 mg/L (Hg) to 1.2 mg/L (Pb) at a plasma current of 80 mA. ELCAD-OES results for Cu and Zn in dietary supplements showed good agreement with those observed with flame AAS. Recoveries ranged between 87 and 113%. The ELCAD model was examined for MX-type (mono)oxides and halides of the analytes by considering bond polarizability alongside atomic line intensity ratios determined with different acidic media. The peaks of the vertical analyte profiles of the plasma depended on metal polarizability. Plasma operation with HBr-acidified solution is characterized by a higher total evaporation point, cathode fall voltage, and increased H concentration, compared to the other acidic media. This reflects greater excitation energy, resulting in improved analytical sensitivity.

A new acidifying medium (HBr) was introduced for ELCAD-OES determinations, which provides higher sensitivity and better LODs in low and high salinity matrices.