Laser ablation of synthetic geological powders using ICP-AES detection: effects of the matrix, chemical form of the analyte and laser wavelength

A systematic study of the influence of the physical and chemical properties of analogues of geological samples and of laser wavelength on laser ablation was conducted. Different chemical forms of several test elements were investigated, first as pure elements, then mixed in various matrices. Synthetic model samples were prepared from different crystalline compounds of Mg, Al and Fe spiked in SiO2 or CaCO3 and manufactured in the form of pressed pellets that were analysed by laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES). This study was performed at two laser wavelengths (1064 and 266 nm) with a Nd:YAG laser and using ICP-AES as an elemental detector. These various steps made it possible to demonstrate that the LA-ICP-AES response factors of the analytes are strongly dependent on their chemical form and on the bulk composition of the matrix. These effects were also found to be laser wavelength dependent and the use of a UV laser did not lead to any improvements to minimise these effects. Ln contrast, there was no influence of the grain size or binding pressure of the pressed pellets. However, normalisation to a matrix element could compensate for differences in the response factors of the analytes. Implications for direct quantitative elemental analysis of geological materials by laser ablation related techniques are that calibration without matching in terms of the mineralogical and chemical composition of the matrix and of the chemical forms of the analytes can lead to systematic errors in the determination of the element concentration.