IMPROVING THE ANALYTICAL PERFORMANCE OF ETV-ICP OES VIA MIX-GAS PLASMA

To improve the analytical performance of electrothermal vapourization (ETV) coupled to inductively coupled plasma optical emission spectrometry (ICPOES), i.e., particle decomposition inside the ICP a robust mixed-gas Ar-N2 atmosphere is herein proposed. By simply introducing 0.4 L min-1 of N2 on plasma outer flow and 20 mL min-1 of N2 on the central channel as a sheathing flow and to performing a re-optimization of freon R-23 modifier flow (8 mL min-1), carrier Ar flow (0.15 L min-1) and by-pass Ar flow (0.50 L min-1) provides an peak area improvement of 2.5 compared to a standard Ar-ICP, being it noteworthy for ionic rather than atomic lines. The temperature program of the graphite furnace included a pyrolysis step (400ºC - 20s), cool down (20ºC - 15s) and no ramp vaporization (2200ºC - 30s) following a cooling step (20ºC - 20s). To compensate for sample loading, point-by-point internal standardization with Ar 763.511 nm was applied systematically. Overall, it translated into detection limits (3σ) improvements of an order of magnitude (10-fold) on average for several elements while the slopes of calibration curves were improved by a factor of 4 in comparison of an Ar-ICP. To confirm accuracy of the mix-gas plasma approach external calibration using increasing amounts of SS-2 soil was used to build calibration curves and three soil CRMs (Montana soil NIST2710, Montana soil 2711 and SS-1) were analysed (n=5) where most of the values found agreed with the certified ones according to a Student's t test (95% confidence level). The lower suppression of the argon emission line in the mix-gas plasma atmosphere confirm the higher robustness of mix-gas plasma. Indeed, a further screening study on the suitability of argon emission line for internal standardization suggested that only the emission lines which possess ground energy levels similar to Ar metastable species (Ar*, 11.5 eV) present a high absolute intensity and a reproducible supression on the vapourization step (i.e., Ar(I)-763.511, 415.859 and 420.067 nm) being the high energy level of the line correlated as a sensitivity indicator for plasma-related matrix effects. It suggests a major role of Ar* on plasma-related matrix effects once considering that matrix elements usually quench Ar* level and once such species are abundant with a long lifetime they can, therefore, be used to compensate matrix effects effectively besides being used to simulate the whole ICP on a macroscale.