Improving the analytical performance of electrothermal vaporization coupled to inductively coupled plasma optical emission spectrometry using a mixed-gas plasma

Abstract

A robust Ar–N₂ mixed-gas plasma is proposed to improve the analytical performance of electrothermal vapourization (ETV) coupled to inductively coupled plasma optical emission spectrometry (ICPOES). With 0.4 L min⁻¹ of N₂ in the plasma gas flow and 20 mL min⁻¹ of N₂ sheathing gas in the central channel of the ICP, sensitivity based on peak area increased by up to 32 fold (Ba) and detection limits improved by up to 300 fold (Ba). Detection limits, based on 3 mg aliquots, ranged from 0.007 (Mo, Sb) to 100 (Mg) mg kg⁻¹ with the mixed-gas plasma versus 0.01 (Tl) – 3000 (Al) mg kg⁻¹ with an Ar plasma. Such improvement was in general more important for ionic than atomic emission lines under the optimized ETV conditions: 8 mL min⁻¹ CHF₃ (as chemical modifier), 0.15 L min⁻¹ Ar carrier gas, 0.50 L min⁻¹ Ar by-pass gas, 400 °C pyrolysis for 20 s; cool down to 20 °C for 15 s and 2200 °C vaporization for 30 s. The relative standard deviation ranged from 0.4 to 19% for 3 mg soil replicates (n = 10). Point-by-point internal standardization with Ar I 763.511 nm was systematically applied to compensate for sample loading effects in the ICP. The lower suppression of the Ar I 763.511 nm signal caused by the ETV effluent in the mixed-gas plasma compared to an Ar plasma confirmed the higher robustness of the mixed-gas plasma. Additional studies revealed that only Ar atomic emission lines (at 763.511, 415.859 and 420.067 nm) with similar energy (11.5 eV) to that of Ar metastable species (Ar^m) showed reproducible suppression, suggesting a major role of Ar^m on plasma-related matrix effects. Accurate results were obtained for 41–65% of the elements with certified concentrations in three soil certified reference materials by direct soil analysis using another soil certified reference material for external calibration.