PRECISION AND ACCURACY OF LASER ABLATION-ICP-MS ANALYSIS OF RARE-EARTH ELEMENTS WITH EXTERNAL CALIBRATION

Solid sample introduction into an ICP-MS by laser ablation is an effec tive method for the total analysis of rare earth elements (REEs) in so ils because no digestion is needed. A problem of the method, however, is the difference of the ablated mass for each laser shot. Therefore, internal standard for the compensation of signal instability, sample p reparation, and the calibration method have to be carefully chosen. Th e analyzed sample was a certified standard provided by IAEA (SOIL-7). The sample was mixed with an internal standard solution and polyethyle ne (PE), dried, homogenized in a ball-mixer/mill, and pressed to a pel let. For the calibration 5 external standards with increasing REEs con centrations (0.4-20 mug/g) as well as a blank were prepared in the sam e way. The analysis of the pellets was performed on a VG PlasmaQuad II + with a LaserLab unit. The laser ablation-cell was modified to impro ve the sample-particle transportation characteristics and to allow a q uicker sample-exchange. The pellets were ablated from six different sp ots for 60 s each with a laser-repetition rate of 4 Hz. The correlatio n coefficients of the calibration curves based on 5 standards, were be tter than 0.995. The concentrations c(A) of the 15 REEs in the soil sa mple were determined with an average relative confidence interval 100( CI)/c(A) of 6.95%, as a figure for the precision. This good precision have been obtained with a new laser ablation cell, which will be descr ibed in detail. With 2 exceptions (Ce and La) the measured concentrati ons were within the confidence intervals (CI) of the certified values. Therefore, with respect to accuracy and precision, the presented meth od offers a convenient way to analyze homogeneous and powdered soil sa mples for REE's without digestion. Since a good calibration for the RE E determination may be obtained, the laser sampling variance (within t he sample) is less significant than the analytical variance. Automatio n of the method is possible by construction of an autosampler based on the modified laser cell.