Determination of traces of lutetium in geological samples by resonance ionization spectroscopy

Abstract

Trace amounts of Lu in geological samples have been measured at the 10^–5% level by use of atomization in vacuum. Atoms of Lu are selectively excited by a two-step laser excitation scheme to an auto-ionizing level. This level represents a permanent channel of decay enabling the excited Lu atoms to be ionized effectively. The ions are registered via a time-of-flight mass separator by a secondary electron multiplier. The integral ionic signal recorded is proportional to the amount of Lu in the samples. The calibration is characterized with very good linearity (3% average and 8% maximum deviation from linearity). The results obtained are compared with the concentration of Lu in samples previously measured by neutron activation analysis. For three of the samples a slight deviation (<20%) from the control data is observed. For the fourth sample the deviation is 70%. The possible reasons for this are analysed. If it is assumed that the measurement error originates from the experimental conditions, it might be due to some type of matrix effect (to avoid it, a special procedure of matrix exchange is required). Taking into account the linearity of the calibration and the ionic signal intensity registered, amounts of Lu at least one order of magnitude less than those of the lowest calibration point (1.432 ng of Lu) could be measured.