Ultratrace determination of thulium by laser-excited atomic fluorescence spectrometry with atomization in a rhenium lined tube furnace

Abstract

In the present work, a method for the determination of ultratrace levels of thulium by laser induced atomic fluorescence (LEAFS) was developed. High sensitivity was achieved by using a dye laser pumped by a high repetition copper vapor laser and electrothermal atomization (ETA) in a tube furnace. Four different atomizer surfaces (pyrolytic graphite, tungsten, tantalum and rhenium) as well as coating treatments of the graphite with noble metal solutions were evaluated with the objective of improving the atomization efficiency of thulium. The best results in terms of intensity, reproducibility and lifetime were obtained with rhenium-lined graphite tubes. Since the rhenium surface was more resistant to oxidation under the operational conditions, the sample was atomized under a stop flow regime rather than in an argon miniflow regime, which was required when tantalum and tungsten foils were employed. Atomization under the stop flow regime avoided dilution of the analyte in the analytical zone, thereby improving sensitivity. In addition, the lifetime of the rhenium foil lined atomizer was longer than those observed with tantalum or tungsten linings. Furnace parameters were optimized to maximize the fluorescence signal–background ratio. The limit of detection for the rhenium lined atomizer was 200 fg with a precision of 6.0% at the 10 ng ml −1 level. Thulium was determined with excellent results in urine and coal fly ash samples using the new method.