Determination of selenium by tungsten coil atomic absorption spectrometry using iridium as a permanent chemical modifier

Permanent chemical modifiers have been shown to prolong graphite tube lifet ime while reducing the furnace cycle time, thus improving cost-effectivenes s. In this work, iridium is used as a permanent chemical modifier for the f irst time in the determination of selenium by tungsten-coil atomic absorpti on spectrometry (W-Coil AAS). The iridium modifier is thermally coated onto the tungsten coil. After coating, the coil can be used for 300-400 firings without further application of the modifier. Thermal treatment with iridiu m permits operating with higher pyrolysis temperature and coil lifetime is extended up to 1600 firings. The sensitivity and linearity of the method is improved, and the analytical procedure allows the use of analyte solutions containing up to 8% nitric acid. The short-term stability of the absorbanc e measurements is demonstrated by the reproducibility in the measurements o f a Se amount (6 ng) 30 times higher than the limit of detection (0.2 ng). A 7% relative standard deviation (R.S.D) was observed for 10 consecutive me asurements of 6 ng Se. The long-term stability is almost as good: less than 9% R.S.D. over a 3-week period and 1500 firings. The surface of the tungst en-coil treated with iridium is examined before and after intensive use by scanning electron microscopy. Finally, the thermal treatment of the tungste n-coil with iridium appears to delay the appearance of selenium atoms by ap proximately 0.2 s although the integrated absorbance measurements are unaff ected. The magnitude of delay decreases with coil age. (C) 2001 Elsevier Sc ience B.V. All rights reserved.