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- LIBS (2)
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The use of laser-induced breakdown spectroscopy (LIBS) for analysis of micro- and nanoparticles is explored, including a brief review of the recent research, both fundamentals and applications, along with new experimental work regarding aerosol particle sampling statistics, analysis of laser ablation particles via aerosol LIBS for matrix effect minimization for bulk solids analysis, and a novel aerosol particle concentration scheme that is suited for near real-time analysis of aerosol nanoparticles. The statistical analysis reveals that the LIBS particle sampling physics are well modeled using Poisson sampling statistics, as based on analysis of calcium-rich ambient air particles. The laser-ablation LIBS (LA-LIBS) methodology was explored for a range of disparate metallic and non-metallic bulk samples, revealing a linear calibration curve for all six samples over the range of relative Mn/Fe mass concentrations. Finally, the microneedle concentration technique for aerosol nanoparticle analysis was successfully demonstrated with linear mass calibration curves for copper-rich nanoparticles. Overall, a fundamental understanding of the plasma–particle physics has enabled the formulation of robust LIBS-based nanoparticle schemes.
Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.
Plasmaparticle interactions are explored through the introduction of single microdroplets into laser-induced plasmas. Both spectroscopic analysis and direct imaging of analyte atomic emission are used to provide insight into the various fundamental processes, namely desolvation, atomization, and atomic diffusion. By doping the 50 µm droplets with Lu, Mg or Ca, the analyte excitation temperature and the ion-to-neutral emission ratio are explored as a function of plasma residence time following breakdown. The data suggest a change in the local plasma conditions about the analyte atoms around 1520 µs following breakdown, which may be interpreted as an overall transition from localized (i.e. perturbed) plasma conditions to the global (i.e. bulk) plasma conditions. A direct assessment of the hydrogen atomic diffusion coefficient following analyte desolvation reveals a value of 1.7 m² s-1 in the first 250500 ns. This value is in good overall agreement with a theoretical analysis and with an analytical treatment of a surface source of hydrogen atoms. In contrast, calcium emission is only observed beyond about 1 µs, with a diffusion coefficient at least an order of magnitude below the hydrogen value. The temporal H and Ca emission data suggest that water vaporizes first, leaving an ever increasing Ca analyte concentration until finally, with nearly all water desorbed, the Ca fraction is vaporized. Overall, the data support the conclusion that finite time-scales of heat and mass transfer play an important role in localized plasma perturbations in the vicinity of the analyte, which has important implications for the LIBS analyte signal.
Quantitative laser-induced breakdown spectroscopy (LIBS) analysis operates on the assumption that the sample is completely dissociated and diffused within the highly energetic plasma on time-scales of analyte analysis, resulting in analyte emission ideally at the bulk plasma temperature and a signal that is linear with analyte mass concentration. However, recent studies focusing on aerosol analysis have found the heat and mass diffusion rates within laser-induced plasmas to be finite, resulting in particle-rich, locally perturbed areas within the hot bulk plasma. The goal of this study is to observe any related plasma differences, by calculating the bulk and local (i.e. analyte rich regions) plasma temperatures and electron density, to better understand the time frame of equilibrium between the local and bulk plasma properties. This study also seeks to determine whether the presence of large quantities of a matrix element can significantly alter the local plasma conditions, thereby generating matrix effects. We report the temporal profiles of particle-derived species, adding additional insight into the effect of local perturbation of plasma properties, with the conclusion that significant plasma residence (tens of microseconds) is necessary to minimize such effects.