Equilibrium and mass spectrometry of nitrate decomposition in electrothermal atomic absorption spectrometry

Abstract

The decomposition of metal nitrates to oxides is assumed to occur in electrothermal atomic absorption spectrometry during the pre-atomization drying or thermal pre-treatment of nitrate samples. The mechanism of thermal decomposition of nitrates to solid oxides and gaseous NO₂ is well established, but some evidence from recent electrothermal mass spectrometry (ET-MS) experiments has been used to support an alternative mechanism, that of gaseous metal oxide formation, e.g. MNO₃→MO(g)+ NO₂(g) The ET-MS of nickel, cobalt, copper, magnesium and lead nitrate samples have been re-examined using a quadrupole mass spectrometer with cross-beam rather than axial sampling of gas from the furnace. With this new sampling geometry, no gaseous metal oxides were detected over a range of heating rates from slow drying to rapid atomization. These results are in keeping with thermochemical equilibrium calculations, as is the observation of gaseous phosphorus oxides in the thermal decomposition of NH₄H₂PO₄. Earlier observations of gaseous metal oxides of the above elements by ET-MS are attributed to the formation of molecular clusters of oxides and/or nitrates during rapid decomposition of the metal nitrates.