Trace element analysis of precious metals in minerals by time-of-flight resonance ionization mass spectrometry

The TOF-RIMS mass spectrometer developed at AMTEL has been successfully applied for quantitative trace element analysis of Au, Pd and Rh in sulphides, iron oxides and silicates. Quantification of the TOF-RIMS measurements is performed on the basis of calibration curves established with reference samples covering three orders of magnitude in concentration. Attained minimum detection limits are in the 8-17 ppb range with a precision of ±15%. A comparative analysis of minerals by Dynamic SIMS and TOF-RIMS in the overlapping range of detection sensitivities for Au, Pd and Rh shows good correlation of the quantified data. Recently, the TOF-RIMS mass spectrometer has been upgraded with new pulsed ion optics. Fast high voltage drivers and time synchronization electronics provide switching of the polarity and potentials of the acceleration and ion lens optics between the two consecutive steps of laser ablation and laser photoionization. This pulsed mode of operation enables the time-of-flight mass spectrometer to be operated at higher ablation power densities with more efficient suppression of the primary ions as a source of noise and ultimately leads to a better detection sensitivity. The detection sensitivity of the instrument for Au in pulsed mode of operation was determined to be 3.2 ppb