Preatomization behavior of palladium in electrothermal atomic-absorption spectrometry

Under certain experimental conditions--short pyrolysis times, fast heating rates and presence of reducing agent--increasing vaporization temperatures led to decreasing losses of palladium as the initial mass was increased between 0.3 and 5.0 ng. When palladium is introduced into the tube at mu g levels, as is typical for modifiers, no similar tendency was established and maximum loss-free temperatures approach 1300 degrees C. A procedure for deriving the enthalpy of vaporization has been developed which characterizes low-temperature palladium release during pyrolysis. The evaluated enthalpy of vaporization increases with increasing palladium mass at ng levels. This could be explained by the enhancement of the dimensions of the vaporized particles/droplets. Because of the small mass of the injected sample, the particles obtained are sufficiently small and the behavior of the droplets at higher pyrolysis temperatures could be explained by the Kelvin equation. In such a case the variation of the particle size leads to variation in their vapor pressure and, as a result, the enthalpy of vaporization is changed. At mu g levels the palladium particles obtained are sufficiently large and the vapor pressure and enthalpy of vaporization are not a function of the particle size. Variation of the initial mass of palladium does not influence the shape of pyrolysis curves when longer pretreatment times are used. The prolonged pyrolysis times possibly influence the rates of nucleation and crystal growth and favor the diffusion of palladium into and on to the graphite surface, thus the initial sample distribution is changed. (C) 1997 Elsevier Science B.V.