High spatial resolution analysis of steel samples using laser ablation ICP-MS

Introduction The LA-ICP-MS technique has been evaluated with respect to applications relevant for the steel industry, with special emphasis on spatially resolved analysis. Three different commercially available laser ablation units have been used, CETAC LSX 100, CETAC LSX 200 and New Wave Research UP 213. These are all based on NdYAG lasers combined with frequency doubling and frequency mixing crystals in order to obtain UV laser output. The CETAC systems operate at 266 nm, the New Wave system at 213 nm. In addition, some investigations have been carried out using specially designed systems with excimer lasers. The different laser ablation units are coupled to ICP-MS systems with mass analysers of different types, a quadropole, a time-of-flight (TOF) and a magnetic sector system. Samples and sample preparation A number of samples were selected and prepared for different parts of the investigations. For the lateral resolution studies, special welded samples of different steel alloys and a Cu insert in Zn were prepared. For investigations of calibration and bulk analysis techniques in steels, a set of stainless and low alloy steel CRM's was selected. For investigations of spatially resolved non-metallic inclusions, a steel CRM with certified values for total and soluble (metallic) Al was selected. In addition, powder samples, pellets, glass samples and metallic coating samples were prepared. The work on bulk steel analysis showed that sample preparation is not critical for the intensity and stability of the analytical signals. Combined with the extraordinary flexibility in terms of sample size and shape, this confirms that LA ICP-MS is a very practical technique in terms of sampling. Lateral resolution and depth-resolved analysis The lateral resolution was extensively studied, and was found to be about 40 μm under optimal conditions using the commercially available systems, even with a laser spot size of 10 μm. In a continuous linescan, the time delay in the sample cell and transport system imposes a limitation. The "splashing" and re-deposition of ablated material around the ablation crater imposes a more fundamental limit of approximately 20 μm with currently available technology. Ablation of a thin Cu/Fe "sandwich" foil showed that depth resolved analysis is possible with approximately 1,5 μm depth resolution for a thin layer of 5 μm thickness. These investigations also showed that a single shot of 3 mJ in a 200 μm spot ablates approximately 0,6 μm in steel and 0,5 μm in copper. Influence of different gases The study of the influence of different gases shows that variation in particle size distribution is responsible for the so-called elemental fractionation. This effect can lead to inaccurate analytical results A comparison of Ar and He as aerosol gas shows that the transport efficiency, and thereby the ion signal intensities, are about ten times higher using He. Also, using He considerably reduces the elemental fractionation problem. Unfortunately, the cost of operation with He is prohibitively high for most users.