Volatilization studies of magnesium compounds by a graphite furnace and flame combined atomic absorption method. The use ofa halogenating atmosphere

Abstract

Separate vaporization of samples in a graphite furnace was combined to a flame atomic absorption spectrophotometer for metal specific detection of the evolved species. The internal temperature of the sample holder (graphite cup) was continuously monitored by a recording optical pyrometer in the 560–2400°C range, when solution and solid samples were heated at 7–16°C s −1 rate. A higher initial observation temperature of vaporization ( T f) of magnesium oxide was found with the present method than with the known electrothermal atomic absorption methods. This was explained by considering both the temperature dependence of the non-equilibrium oxygen partial pressure in the furnace and the difference of sensitivity of the detection methods. Calculations were performed to predict T i values for chemical systems of known thermodynamic data, and to determine vaporization heats on the theoretical base of the third-law method. An empirical approach was followed to specify vaporization characteristics of magnesium in the presence of aluminium and aluminium-calcium salt matrices using Ar and Ar + CCl 4 (chlorinating) atmospheres. The vaporization temperature of hydrated magnesium chloride altered significantly with the sample mass. The observations found with a relatively high mass of samples were used to explain certain interference and/or releasing effects involved in conventional electrothermal and flame atomization methods. The effect of excess CCl 4 vapor introduced into the flame, together with the sample aerosol was treated also theoretically in terms of dissociation equilibria. No aluminium interference on magnesium determination was found with the present combined method.