Investigation of chloride salt decomposition and pre-atomization interferences in electrothermal atomic absorption spectrometry

Abstract

The decomposition and vaporization of NaCl, MgCl₂ and NiCl₂ in an electrothermal atomizer have been studied using ion chromatography, radioactive tracers and flame atomic absorption spectrometry (FAAS). Solutions of the individual chloride salts were injected to deposit 50–200 µg on to a pyrolytic graphite platform in a pyrolytic graphite coated graphite tube. The solutions were dried and then pyrolysed at a temperature in the range 200–1200 °C. The residue on each platform was then analysed to determine the amount in micromoles of metal and chloride remaining at each temperature. This allowed formation of pyrolysis curves for the salts, similar to those produced in conventional electrothermal AAS (ETAAS) for analyte elements. Magnesium chloride and NiCl₂ exhibited partial hydrolysis with the heating programme used, but significant vaporization of MgCl₂ and NiCl₂ also occurred. The decomposition and vaporization of 100 or 200 µg masses of the chloride salts caused pre-atomization step losses of Cd, Ga and Pb, in the range 300–1000 °C, as indicated by radioactive tracer analysis and inductively coupled plasma mass spectrometry. The interference was in the order Ni-Cl₂>MgCl₂>NaCl. Partial hydrolysis of MgCl₂ to MgO minimized the pre-atomization losses of the analytes at higher temperatures due to a chemical modification effect similar to that caused by MgNO₃. The study illustrates that, in the presence of high masses (50–200 µg) of chloride salts, pre-atomization loss of analyte elements can be a major cause of chemical interference in ETAAS if too high a char temperature is selected and the use of chemical modification is ignored.