Fractionation Effects in LA-ICP-MS

A serious problem in LA-ICP-MS described in the literature on many occasions is the time-dependent elemental fraction (so-called ablation fractionation ) occurring during laser ablation and the transport process of ablated material, or during atomization and ionization processes in the inductively coupled plasma. Numerous papers focus on the study of fraction effects in LA-ICP-MS as a function of experimental parameters applied during laser ablation (such as laser energy, laser power density, laser pulse duration, wave length of laser beam, ablation spot size. 

An important parameter in order to avoid elemental fractionation in laser ablation ICP-MS is the laser power density (ch) which is a function of laser energy, of laser pulse duration and focusing conditions, as described in the following equation  

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The influence of various gas pressure conditions within the laser ablation cell on the particle formation process in laser ablation has also been investigated.69 In LA-ICP-MS studies at low pressure (down to 2kPa) a small particle size distribution and a reduction in elemental fractionation effects was obtained. But with decreasing pressure and transport volume of ablated material, a significant decrease in the ion intensities was observed as demonstrated for uranium measurements in the glass SRM NIST 610.69 However, the laser ablation of solid materials at atmospheric pressure in LA-ICP-MS is advantageous for routine measurements due to lower experimental effort and the possibility of fast sample changing in the ablation chamber. Fractionation... 

In analogy to the observed behaviour in LIMS, in LA-ICP-MS increasing fractionation effects are observed with decreasing laser power density for <10 Wcm . These fractionation effects increase significantly if the laser power density is lower than lO Wcm. A stoichiometric laser ablation of sample material is observed at a laser power density between 10 Wcm and 10 °Wcm in the author s laboratory. 

It is widely accepted that the precision or reproducibility of isotope ratio measurements can be improved dramatically by using an MC instrument. However, further correction or careful procedural protocols are required to also improve the trueness of the resulting isotopic data. There are two major considerations for better quality isotope ratio measurements using LA-ICP-MS analysis the isotope fractionation during the LA and/or ionization process in the ICP, and the contribution of the matrix effect (non-spectral interference) to the mass bias effect. The latter is discussed in Section 4.6. First, the level of isotope fractionation during LA or ionization processes is discussed in this section. 

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