Vaporization of Radium and Other Alkaline Earth Elements in Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry

Reported are the mechanism of vaporization and optimum experimental conditions for the determination of Ra and other alkaline earth elements (Be, Mg, Ca, Sr and Ba) by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Calculated and published data along with new experimental results suggest that these elements are vaporized from the surface of the graphite tube as oxides. These oxides are then transported to the argon plasma where dissociation and ionization take place. Appearance temperatures and maximum pyrolysis temperatures obtained experimentally generally agree with values obtained using graphite furnace atomic absorption spectrometry (GFAAS). For Ra, the optimum pyrolysis and vaporization temperatures were 1400 and 2500 °C, respectively. Diluted (1:500) seawater, used as a physical carrier, was effective in improving sensitivity when used in small quantities, but caused significant suppression of the Ra signal when the analyte was co-vaporized with quantities of salt in excess of 40 µg. An absolute limit of detection of 1.7 fg was obtained corresponding to 34 fg ml -1 in a 50 µl sample.