MECHANISM OF VOLATILIZATION OF TUNGSTEN IN THE GRAPHITE-FURNACE INVESTIGATED BY ELECTROTHERMAL VAPORIZATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

The mechanism of volatilization of tungsten from a graphite substrate has been investigated using electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Vaporization temperatures in the range 800-2700-degrees-C were studied. In this temperature region two distinct vaporization processes occur, resulting in two separate ETV-ICP-MS peaks for tungsten. The earlier peak appears at temperatures as low as 850-degrees-C and is attributed to the volatilization of tungsten oxide. At temperatures above 2500-degrees-C a second peak appears when tungsten carbide is vaporized from the graphite surface. Results show that NaCl and NaF chemical modifiers are ineffective in preventing the formation of tungsten carbide. The signal-to-background ratio (S/N) for tungsten varies with vaporization temperature, with the optimum S/N and minimum limit of detection (0.51 pg) being obtained at vaporization temperatures around 1100-degrees-C.