Absorption coefficient modified pressed powders for calibration of laser ablation inductively coupled plasma mass spectrometry

Powdered samples have been pressed, utilising a standard KBr infrared (IR) press, to produce mechanically stable 13 mm discs. Three ‘absorbing’, organic based binders have been employed in this work; vanillic acid, pyrazinoic acid and nicotinic acid, chosen because of their high optical absorbance at the wavelength of the incident laser energy (213 nm). Poly(vinyl alcohol) (PVA) was employed as an example of a ‘nonabsorbing’ binder and because its use has been described previously in the literature. Discs of various sample/binder compositions were prepared, and their absorption properties characterised by diffuse reflectance spectroscopy. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was used to investigate the effect of different sample/binder compositions on signal sensitivity; whilst surface profilometry was performed on the resulting tracks to provide an estimate of the ablation depth achieved by the laser beam. It was found that discs prepared with vanillic acid had the highest optical absorbance at the wavelength of the laser system employed, resulting in a lower ablation depth and improved signal sensitivity probably through the formation of smaller particles during the ablation process. Analysis of certified reference materials (CRMs) was performed using simple external calibration standards of similar and dissimilar CRMs. It was found that discs produced using a 40% vanillic acid binder, 60% sample composition gave superior quality analytical data when compared to the use of 40% PVA binder or no binder at all. These findings indicate the potential for good quality analytical data to be obtained when employing external calibration standards, without internal standardisation and without exact matrix matching. These data also provide further evidence that standardisation of ablation conditions and mass flux to the plasma are pre-requisites for robust calibration, particularly in the absence of a suitable internal standard element.