PROBE ATOMIZATION FOR LASER ENHANCED IONIZATION IN A GRAPHITE TUBE FURNACE

Reported here is the first use of laser enhanced ionization (LEI) together with modern furnace technology, for the determination of metals in samples. The experimental arrangement that was used included a pulsed excimer pumped dye laser, a graphite tube furnace, and a graphite probe that was used for both sample introduction and as an electrode. For elements with excitation transitions which promoted the atoms to energy levels within 7000 cm-1 of the ionization limit, such as thallium, indium and lithium, detection limits in the 0.7-2 pg range were obtained. Lead, magnesium and iron, which were excited to levels between 24 000 cm-1 and 31 000 cm-1 from the ionization limit, had detection limits between 10 and 60 pg. The linear dynamic range for each of the elements was between three and four orders of magnitude, and the precision for aqueous standards was between 12 and 16%. Theoretical estimates showed that the final ionization step was collisional ionization, rather than photoionization or radiative ionization by furnace black-body emission, for transitions that were less than 15 000 cm-1 from the ionization continuum. However, for transitions that were more than 25 000 cm-1 from the ionization limit, photoionization and collisional ionization both contributed significantly to the LEI signal. Interferences from easily ionized elements and acids were studied, and indium was chosen as an example analyte. Sodium severely depressed the indium LEI signal in a graphic furnace. Nitric acid and sulfuric acid had little effect upon the indium LEI signal, but the signal was suppressed at high concentrations of hydrochloric acid, probably due to the formation of diatomic indium chloride.