Laser-excited atomic fluorescence spectrometry in a graphite furnace with an optical parametric oscillator laser for sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in Buffalo river sediment

Abstract

It is demonstrated, for the first time, that solid-state lasers based on optical parametric oscillation (OPO) allow relatively rapid sequential multi-element analysis of samples by laser-excited atomic fluorescence spectrometry (LEAFS) in a graphite furnace. These lasers are tunable by facile computer keyboard control over the wavelength region 220–2000 nm. A method is described for the sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in a river sediment standard reference material (NIST SRM 2704) by graphite furnace LEAFS. With a slew rate of 0.125 nm s^–1, the OPO laser could be tuned to cover the wavelength range needed for these elements, from 228 to 304 nm, in 15 min. This allowed each element to be determined sequentially with the analysis time determined primarily by the slow heating cycle of the furnace rather than the laser wavelength tuning. Detection limits in the multi-element mode were 545, 111, 28, 445 and 24 fg for cadmium, cobalt, lead, manganese and thallium, respectively, limited primarily by the low repetition rate of the laser (10 Hz). The multi-element detection limits were within a factor of 2–4 of those in the single element mode. Higher excitation energies, by a factor of 2–5, were required to optically saturate the transitions of the analytes in the sediment sample solution compared with aqueous standards. By use of several aliquots of one sample solution, and simple aqueous calibration, it was possible to analyze the sample, accurately, for the five elements over a concentration range between 1 ng ml^–1 for thallium and 460 ng ml^–1for manganese. Different dilutions were not necessary owing to the long calibration range of the technique. The high sensitivity of LEAFS allowed sufficient initial dilution to remove an interference on thallium that is normally irresolvable by atomic absorption measurements of the same sample in the same graphite furnace.