Abstract
Ablative laser technology has been used successfully as a tool in scientific applications particularly to evaluate the homogeneity of materials and to depth-profile samples with the object of attaining elemental distribution at subsurface levels. Pulsed micro-beams strike a target with pinpoint accuracy and produce trace elemental information both spatially and in the substrate. Exploring different strata of a sample can produce data on impurities buried deep within the sample matrix. This is particularly important in cases where hidden impurities can make a difference to the performance of certain samples, such as semi-conductors or biomedical specimens. Stochastic effects such as imperfect crater formation, erratic energy pulses and unpredictable drift in beam energy could significantly affect the results of research applications. These technical features are controlled by sophisticated software, which plays a salient role in stabilizing the instrument. Samples are usually heterogeneous in nature, such as rocks, reservoir cores and concrete structures, and sample heterogeneity, therefore, is a factor that precludes adoption of conventional protocol for standardization of the technique. Soft samples such as gels and waxes could undergo standardization under special conditions. However, the technique is largely semi-quantitative for solids and is particularly attractive for exploring the homogeneity of solid targets, which reflects the level of elemental distribution within the sample matrix. The laser unit is coupled to a Perkin Elmer ICP-MS instrument and maintenance of consistent operation parameters is crucial for accurate and reproducible results. The laser beam wavelength is in the deep UV region and the system is operated with a beam of 213 nm of variable diameter between 5-100 μm, gas flow of 0.8 L/min, energy pulse rate of 60 MHz, and beam energy between 30-60%. Compared to other current instrumental techniques, ablative laser technology is superior for depth-profiling and surface analysis.
Highlights
Research in the area of laser ablation technology (LA-ICP-MS) is useful for specific applications, including particle-size measurements [1], geochronology applications [2,3,4], nuclear studies and bio-imaging of metals [5,6,7,8,9,10]
The instrument employed in our research program consists of a 213-nm deep-UV laser unit attached to an ICP-MS system that operates at 40 MHz with an argon gas flow of 0.8 L/min
The laser radiation is in the form of a micro-beam that can expand in diameter from a “pencil” beam of 5 μm to a wider shaft of 100 μm
Summary
Research in the area of laser ablation technology (LA-ICP-MS) is useful for specific applications, including particle-size measurements [1], geochronology applications [2,3,4], nuclear studies and bio-imaging of metals [5,6,7,8,9,10]. The point reached below the sample surface can be estimated by penetrating a thin copper or mastic film (for example) of known thickness (say 30 μm) and density resting on a metal substrate (for example iron). It is useful to calibrate an ablative laser instrument by irradiating samples containing successive strata of pure metal films (of pre-determined thickness) with increasing energy pulses. This is a practical method to calibrate the system. Example a multiple thin film arrangement of aluminium resting on copper, iron and lead can provide useful data on energy versus density and thickness This is part of our future work and could serve as a baseline reference for laser ablation studies. Not much deliberation has been given to methods for clean crater formation and this could form part of a future study
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