A dried droplet calibration approach for the analysis of solid samples through laser ablation – inductively coupled plasma mass spectrometry

A novel calibration method has been developed for the bulk analysis of solid samples through laser ablation - inductively coupled plasma mass spectrometry (LA-ICP-MS). This method has been called dried droplet calibration approach (DDCA). This procedure is thus based on the deposition of a small aqueous standard volume (ca., 1 mu L) on the flat surface of a solid sample. Then, the solvent is allowed to completely evaporate at room temperature and the solid residue is finally ablated. The implicit assumptions of the DDCA are that: (i) a single raster is enough to fully ablate the solid layer originating from the evaporation of the standard; and, (ii) the solid residue does not affect the sample ablation yield. Both hypothesis have been verified in the present work. Therefore, several rasters have been performed on the solid residues and the generated signals have been obtained by using an ICP-MS spectrometer. The calibration lines are obtained by plotting the sum of intensities for a given residue (corresponding to a given analyte concentration) versus the analyte ablated mass per laser shot. This method is similar to a standard addition procedure and, hence, the analyte mass ablated from the sample per laser shot has been obtained by extrapolating the obtained straight line to the y-axis zero value. The method has provided accurate concentration results for a glass CRM and three different catalysts employed for petroleum refining purposes, prepared as fused beads. The obtained results have demonstrated that it is possible to overcome significant matrix effects by applying the DDCA.