Evaluation of hydrogen line emission and argon plasma electron concentrations resulting from the gaseous sample injection involved in hydride generation-ICP-atomic emission spectrometric analysis

The simultaneous injection of volatile hydride species and hydrogen gas, or iginating in reagent decomposition, was monitored during the operation of a continuous hydride generation manifold employed for the determination of t race arsenic by HG-ICP-AES. Line and background intensities as well as the FWHM of the hydrogen H gamma and H delta lines were measured, and electron number densities (n(e)) estimated from Stark broadening of the line profile s. Results were compared with those obtained by conventional pneumatic inje ction of aqueous solutions. Overlapping with atomic nitrogen lines at 410 n m and 411 nm tends to distort the H-delta line profile for the hydrogen-see ded plasma, rendering unreliable results. The N I lines seem to be quenched by the presence of water aerosol. More consistent results were obtained wi th the H gamma line. When no solutions are pumped through the hydride gener ation manifold ("dry" plasma), the measured n(e) value was (1.57 +/- 0.22) x 10(15) cm(-3). Conversely, when the reducing reagent flow was replaced by pure water (corresponding to the injection of water vapor in equilibrium t hat is swept by the argon carrier gas passing through the phase separator), the electron concentration is 25% higher. In that case the n(e) value agre es between the experimental error with that obtained for a plasma in which a water aerosol is introduced at a flow rate of 1 mL/min. An enhancement of 52% relative is observed in n(e) when the system is operated under optimiz ed conditions for arsine generation, employing sodium tetrahydroborate in a cidic medium as reducing agent (i.e. hydrogen seeded plasma). It was also o bserved that the continuum emission near 410 nm for the hydrogen containing plasma correlates with the measured electron number density, suggesting th at the background enhancement under hydride generation conditions may respo nd to the ion-electron recombination mechanism.