A preliminary comparison of radial and axial excitation fluorescence in the ICP using non-laser sources

Abstract

This paper describes a preliminary comparison of radial and axial excitation fluorescence within the ICP utilising hollow cathode lamps (HCLs) and narrow band-width light emitting diodes (LEDs) as sources. Univariate searches were used to optimise various plasma parameters, i.e., forward power, viewing height ALC, plasma, nebuliser and auxiliary gas flow rates, and various excitation source parameters, i.e., modulation frequency, duty cycle and peak current for each of the elements studied (Ba, Li, Mg and Na). Calibrations showed excellent linearity over five orders of magnitude (R² = 0.99995–1.0000) with precision better than 5% RSD. Vertical profiles of the plasma, using radial excitation, were obtained for Ba and Mg using HCLs and for Li and Na using LEDs. Limits of detection were found to be 27.6, 0.51, 0.43 and 0.20 µg l⁻¹ for Ba, Li, Mg and Na, respectively. Using the optimum conditions for Li and Na, vertical profiles of the plasma were obtained using axial excitation with the more intense LEDs. Two different excitation lens systems were investigated in the axial mode: a focusing lens and a collimating lens. The relatively sharp profiles and optimum viewing heights ALC obtained for each analyte studied were identical using both radial and axial excitation fluorescence (circa 60–80 mm at forward powers of 475–750 W and injector flow rates of 2.0–2.5 l min⁻¹). The axial LODs obtained under these conditions were found to be 1.12 and 0.70 µg L⁻¹, and 5.3 and 3.1 µg L⁻¹ for Li and Na, using a focusing and collimating lens system, respectively. The potential for increased sensitivity and lower limits of detection is discussed using the novel axial excitation system. ‘Temperature’ measurements (T_exc and T_rot) determined under fluorescence conditions both gave values in the region 2500–3000 K, indicating a plasma operating closer to LTE.