Determination of rare earth elements in graphite by solid sampling electrothermal vaporization-inductively coupled plasma mass spectrometry

A method is proposed for the determination of trace rare earth elements (REEs) in graphite by solid sampling electrothermal vaporization-inductively coupled mass spectrometry (ETV-ICP-MS). The operating parameters of the ETV system such as gas flow-rate, heating program, the use of a modifier gas (Freon R-12) as well as sample mass were evaluated. The determination of REE in graphite samples was performed by directly weighing a solid sample (0.5 to 2.5 mg) on the graphite platform of the ETV system. Calibration was carried out using aqueous standards. According to the results, the presence of the modifier gas (Freon R-12) promoted an enhancement of vaporization for all analytes, besides the reduction of the vaporization temperature (about 500 degrees C lower). The accuracy was evaluated by an analyte recovery experiment and also by comparison of results with those obtained by microwave-assisted extraction (MAE) with further determination of REE by ICP-MS equipped with an ultrasonic nebulizer (USN-ICP-MS). Agreement was observed between the results obtained by ETV-ICP-MS and MAE with USN-ICP-MS analysis for most of the elements. Analyte recovery ranged from 100.3 to 118.7% and the obtained RSDs were always lower than 24%. The limits of detection were at the ng g(-1) level and were similar to those obtained by USN-ICP-MS with previous sample preparation by MAE. The proposed method was suitable for direct determination of REE in graphite. In addition, direct analysis of graphite by ETV-ICP-MS avoided the laborious sample preparation step of graphite and the use of concentrated acids for digestion, minimizing interferences in the determination step by ICP-MS.