Determination of lithium in lithium-ionic conductors by laser-enhanced ionization spectrometry with laser ablation

Abstract

Laser-enhanced ionisation (LEI) with laser ablation (LA) into a methane–air flame was used for lithium determination in ferrites. Since there is a lack of certified reference materials matrix-matched to ferrites powders, a set of spinel samples were synthesised by means of aerosol spray pyrolysis (ASP) or by annealing the mixture of Li and Fe(III) nitrates. Characterisation by X-ray diffraction showed that the annealed ceramics contained two phases, such as either α-LiFe₅O₈ and Fe₂O₃ or α-LiFeO₂ and Fe₂O₃, while ASP ferrites consisted mainly of α-LiFe₅O₈. Total lithium content in samples was certified by conventional flame photometry after dissolution. We explained the strong matrix effects on the LA-LEI signal by the discrepancy of surface microstructure (and hardness) due to the difference in the conditions of sample preparation. The optoacoustic signal and emission line of Fe I at 382 nm were recorded simultaneously with the LA-LEI signal in order to compensate for matrix effects. We suggest that the microstructure (ceramics or pellets from spherical microparticles) influences both mass removal (proportional to OA signal) and plasma temperature (proportional to AE signal). Correlation plots between OA and AE signals deviated strongly from sample to sample, which was the basis for the application of a multivariate correction procedure to reduce strong matrix effects. 3D correlation plots (LEI vs. OA + AE) for each sample were built to obtain a new analytical signal, which represented the slope of the correlation line to the plane of the OA + AE signals. This normalised signal was free from matrix effects within a concentration range of 0.6–7.5 Li%. The proposed approach allows the use of ferrites synthesised by the various techniques as reference materials in order to build a unified calibration curve.