Efficient mixing of additives in polymer material is essential for uniform distribution. This paper explores the capability of using a high-resolution ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) laser ablation technique to assess the uniformity of distribution of selected metal dopants in polypropylene samples. This study simulates the distribution of residual metal catalysts in polymer material and could provide an insight into investigating the bulk dispersion of catalytic residues. Ablative laser technology is relatively unexplored for this purpose, and is one of the few contemporary instrumental methods available for consummate evaluation of material homogeneity. Polypropylene samples were especially loaded with appropriate levels of metal oxides [ZnO, TiO 2 , Mg(OH) 2 ] to achieve ppm levels on distribution in the bulk material. The study subsequently examined the distribution of the metal dopant in the solid phase-both spatially and depth-wise-using a 100 µm-diameter laser beam coupled to an ICP-MS instrument. The laser (213 nm) was programmed to ablate a total depth of 25 µm at each point at 5-µm intervals. Prior to each run, the instrument underwent appropriate calibration and correction for background. The study did not necessitate standardization, and signal intensities were compared with surface metals and those occurring internally. Spectra were generated to observe fluctuations in characteristic metal intensity with penetration depth. In some cases, wide variations were observed by more than two orders of magnitude. The experimentally determined results indicated that metal intensity consistently showed a steady decline with depth, suggesting that most of the spike was concentrated on the surface. In some cases 'hotspots' were observed at certain points, where the dopant accumulated, indicating imperfect mixing of the additive. Uneven levels of surface metals are particularly significant, as these metals would play a major role in abrading and subsequently migrating into the surrounding environment. Irregular distribution of sub-surface metals is equally important as it could provide useful information of dispersion in the bulk of the polymer material. Our work would be of definite interest to environmental science and materials research, and could be useful in modeling studies.

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.