Double pulse laser ablation and laser induced breakdown spectroscopy: A modeling investigation

A numerical model, describing laser–solid interaction (i.e., metal target heating, melting and vaporization), vapor plume expansion, plasma formation and laser–plasma interaction, is applied to describe the effects of double pulse (DP) laser ablation and laser induced breakdown spectroscopy (LIBS). Because the model is limited to plume expansion times in the order of (a few) 100 ns in order to produce realistic results, the interpulse delay times are varied between 10 and 100 ns, and the results are compared to the behavior of a single pulse (SP) with the same total energy. It is found that the surface temperature at the maximum is a bit lower in the DP configuration, because of the lower irradiance of one laser pulse, but it remains high during a longer time, because it rises again upon the second laser pulse. Consequently, the target remains for a longer time in the molten state, which suggests that laser ablation in the DP configuration might be more efficient, through the mechanism of splashing of the molten target. The total laser absorption in the plasma is also calculated to be clearly lower in the DP configuration, so that more laser energy can reach the target and give rise to laser ablation. Finally, it is observed that the plume expansion dynamics is characterized by two separate waves, the first one originating from the first laser pulse, and the second (higher) one as a result of the second laser pulse. Initially, the plasma temperature and electron density are somewhat lower than in the SP case, due to the lower energy of one laser pulse. However, they rise again upon the second laser pulse, and after 200 ns, they are therefore somewhat higher than in the SP case. This is especially true for the longer interpulse delay times, and it is expected that these trends will be continued for longer delay times in the μs-range, which are most typically used in DP LIBS, resulting in more intense emission intensities.     

Time-dependent interaction across two conducting spheres in an applied electrostatic field

Charged particles exist widely in variety of technological areas as well as in nature. Even a weak charge on the particles can significantly influence their electric interaction. We investigated the phenomenon of time-dependent electric interaction between two conducting spheres in an electrostatic field. A mirror-image method was developed to analyze this system, and the fundamental role of the charges on the spheres was studied. We concluded that charges conducted to the lower sphere through the alumina tube used in our system play a main role in determining the time-dependent interaction, whereas the influence from air ions is negligible.     

Dielectric barrier discharge in needle-to-plane configuration: Model of surface charge relaxation

Relaxation process of surface charge owing to dielectric barrier discharge (DBD) in “needle – air gap – polyethylene terephthalate film – plane” configuration is considered. Experimental data of the surface charge relaxation (SCR) are obtained by means of the rotating capacitive probe. Taking into account Gaussian radial distribution of accumulated charge density, effective surface and volume electrical conductivities of a barrier dielectric, phenomenological model of SCR for any dielectric thickness is proposed and exact solutions are obtained. The adequacy of the model is confirmed by the numerical computation. There is a good agreement between the experimental results and the model calculations.     

Conductance‐based profiling of nanopores: Accommodating fabrication irregularities

Solid‐state nanopores are nanoscale channels through otherwise impermeable membranes. Single molecules or particles can be passed through electrolyte‐filled nanopores by, e.g. electrophoresis, and then detected through the resulting physical displacement of ions within the nanopore. Nanopore size, shape, and surface chemistry must be carefully controlled, and on extremely challenging <10 nm‐length scales. We previously developed a framework to characterize nanopores from the time‐dependent changes in their conductance as they are being formed through solution‐phase nanofabrication processes with the appeal of ease and accessibility. We revisited this simulation work, confirmed the suitability of the basic conductance equation using the results of time‐dependent experimental conductance measurements during nanopore fabrication by Yanagi et al., and then deliberately relaxed the model constraints to allow for (i) the presence of defects; and (ii) the formation of two small pores instead of one larger one. Our simulations demonstrated that the time‐dependent conductance formalism supports the detection and characterization of defects, as well as the determination of pore number, but with implementation performance depending on the measurement context and results. In some cases, the ability to discriminate numerically between the correct and incorrect nanopore profiles was slight, but with accompanying differences in candidate nanopore dimensions that could yield to post‐fabrication conductance profiling, or be used as convenient uncertainty bounds. Time‐dependent nanopore conductance thus offers insight into nanopore structure and function, even in the presence of fabrication defects.     

Fragmentation pathways and differentiation of positional isomers of sorafenib and structural analogues by ESI‐IT‐MSn and ESI‐Q‐TOF‐MS/MS coupled with DFT calculations

Sorafenib is an orally active multikinase inhibitor for the treatment of renal cell carcinoma. A series of sorafenib structural analogues were investigated in this work for their gas‐phase fragmentation behaviors using electrospray ionization ion trap mass spectrometry and quadrupole time‐of‐flight mass spectrometry in the positive mode. The possible fragmentation pathways were proposed based on ESI‐MS/MS data and theoretical calculation. Different from the typical α‐cleavage of amide, consecutive reactions that involved elimination of H₂O and CH₃NC were observed for 2‐pyridinecarboxamide derivatives, which were followed by the formation of a stabilized 7‐membered ring carbocation by loss of CO. Two possible protonation sites occurred at carbonyl oxygen atoms for aryl‐urea derivatives and the α‐cleavage of urea was the main fragmentation pathways, which was followed by the formation of stable benzo [d] oxazole ring characteristic to aryl‐urea derivatives. The relative abundance of characteristic fragment ions and the energy‐resolved breakdown curves were used to distinguish the 4 sets of positional isomers of sorafenib and analogues. The methodology and results of the present work would contribute to the chemical structure identification of other structural analogues and the potential impurities presented in active pharmaceutical ingredients and drug formulations.     

Processing optimization: A way to improve the ionic conductivity and dielectric loss of electroactive polymers

Electro‐active polymers (EAPs) such as P(VDF‐TrFE‐CTFE) are greatly promising in the field of flexible sensors and actuators, but their low dielectric strength driven by ionic conductivity is a main concern for achieving high electrostrictive performance. It is well known that there is a quadratic dependence of the strain response and mechanical energy density on the applied electric field. This dependence highlights the importance of improving the electrical breakdown EAPs while reducing the dielectric losses. This article demonstrates that it is possible to dramatically increase the electrical breakdown and decrease the dielectric losses by controlling processing parameters of the polymer synthesis and fabrication procedure. As a result, an enhancement of around 70% is achieved in both the strain and blocking force. The effects on the dielectric losses of the polymer crystallinity, molecular weight, solvent purity, and crystallization temperature are also investigated. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1164–1173     

Examination of Polish Identity Documents by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was applied to the preliminary examination of Polish documents – passports and identity cards – for forensic purposes. Several security features of potentially high discrimination capability were selected at both the passports (Alphagram, serial number, the contour map of Europe, and emblem) and identity cards (kinegram, the date of birth, and emblem). Different elemental compositions were identified after comparing the spectra recorded from various measurement locations. It was possible to identify characteristic atomic emission from several elements (such as Ti, Ca, K, Fe, Cr, Mg, Na, La, Cd, Li, V, Al, Mn, Ni, and Cu) based on the type of document, issue date, and evaluated area. In the case of passports, the potentially good discriminators with unique elemental composition were identified, e.g., the serial number and the contour map of Europe printed with the use of intaglio printing technique. Identity cards with the cover of polycarbonate foil constitute a very difficult sample to analyze because of the foil’s influence on the laser ablation phenomenon. This study presents the potential of LIBS as an effective and useful technique to analyze Polish passports for forensic purposes. Its many advantages provide a good alternative to the analytical methods routinely used for the examination of these objects.     

High breakdown strength and low loss binary polymer blends of poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) and poly(methyl methacrylate)

Dielectric materials with high breakdown strength and low loss are of crucial importance in capacitive energy storage electronics. Herein, a kind of polymer blend composed of poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) ferroelectric terpolymer and linear dielectric poly(methyl methacrylate) (PMMA) is presented. The polymer blend shows a breakdown strength of 733 MV/m and a charge‐discharge efficiency over 90% at 200 MV/m with optimized PMMA content, which are 101% and 28% higher than that of neat terpolymer. Moreover, microsecond discharge time of 2.26 μs, along with a power density that is 3.6 times that of the current commercially available biaxially oriented polypropylene, as well as great cyclic performance, has been achieved under an electric field of 200 MV/m. The findings of this research demonstrate that the incorporation of linear dielectric PMMA into poly(vinylidene fluoride)‐based ferroelectric polymer provides a new strategy in designing high breakdown strength low loss dielectric materials for reliable compact flexible film capacitors.     

Optimization of chemical and instrumental parameters in hydride generation laser-induced breakdown spectrometry for the determination of arsenic,antimony, lead and germanium in aqueous samples

A laser induced breakdown spectrometry hyphenated with on-line continuous flow hydride generation sample introduction system, HG-LIBS, has been used for the determination of arsenic, antimony, lead and germanium in aqueous environments. Optimum chemical and instrumental parameters governing chemical hydride generation, laser plasma formation and detection were investigated for each element under argon and nitrogen atmosphere. Arsenic, antimony and germanium have presented strong enhancement in signal strength under argon atmosphere while lead has shown no sensitivity to ambient gas type. Detection limits of 1.1 mg L⁻¹, 1.0 mg L⁻¹, 1.3 mg L⁻¹ and 0.2 mg L⁻¹ were obtained for As, Sb, Pb and Ge, respectively. Up to 77 times enhancement in detection limit of Pb were obtained, compared to the result obtained from the direct analysis of liquids by LIBS. Applicability of the technique to real water samples was tested through spiking experiments and recoveries higher than 80% were obtained. Results demonstrate that, HG-LIBS approach is suitable for quantitative analysis of toxic elements and sufficiently fast for real time continuous monitoring in aqueous environments.     

Elemental Analysis of Cement by Calibration-Free Laser Induced Breakdown Spectroscopy (CF-LIBS) and Comparison with Laser Ablation – Time-of-Flight – Mass Spectrometry (LA-TOF-MS), Energy Dispersive X-Ray Spectrometry (EDX), X-Ray Fluorescence Spectroscopy (XRF), and Proton Induced X-Ray Emission Spectrometry (PIXE)

The purpose of the present study is to determine the elemental composition of Pakistani cement brands using calibration-free laser induced breakdown spectroscopy (CF-LIBS) and to compare the obtained results with the other analytical techniques such as, laser ablation – time-of-flight – mass spectrometry (LA-TOF-MS), energy dispersive X-ray spectrometry (EDX), X-ray fluorescence spectroscopy (XRF) and proton induced X-ray emission spectrometry (PIXE). Compositional results reveal that all the cement brands are mainly composed of calcium, silicon, iron, aluminum, magnesium, potassium, sodium, titanium, lithium and strontium with varying concentrations. The compositions obtained by LIBS and LA-TOF-MS are in good agreement with results obtained by the other standard techniques and demonstrate the potential use of LIBS for the online monitoring of industrial cement production.