Near-IR bromine Laser Induced Breakdown Spectroscopy detection and ambient gas effects on emission line asymmetric Stark broadening and shift

Investigation via Laser-Induced Breakdown Spectroscopy of a near-infrared bromine emission line (827.2 nm) with an UltraViolet ablation laser and a gated detector is reported. The effects of ambient pressure and gas species (air, O₂, N₂ and He) on the atomic emission line strength and spectral profile were systematically investigated. Substantially improved signal strength and reduced background radiation are demonstrated near 100 mbar ambient pressure with all gases. Optimal results were achieved when helium was used. Asymmetric broadening and shift of the 827.2 nm bromine line, attributed to pressure-dependent Stark effect has been revealed. This effect is prominent when air, oxygen or nitrogen are present and is much less manifested when helium is used. Possible interpretations of this effect are presented.     

Influence of ambient gas pressure on laser-induced breakdown spectroscopy technique in the parallel double-pulse configuration

Single-pulse and double-pulse laser-induced breakdown spectroscopy experiments have been performed using two Nd:YAG lasers in the fundamental mode on a brass sample at different air pressures, ranging from 0.1 Torr to atmospheric conditions, in order to obtain information about the different ablation and plasma evolution processes in the different configurations. Neutral and ionized lines originated both by species deriving from the target and from the air environment were analysed. The temperature and electron density values were estimated in all the experimental conditions. A different behavior of the plasma emission versus the air pressure, in the case of lines deriving from the target, was observed in the single-pulse and double-pulse configurations, suggesting that the different environmental conditions in the first and the second laser ablation may be responsible in determining the plasma emission in the two cases. An interpretative model based on the cavity produced in air by the laser-induced shock wave, according to the Sedov theory of the blast wave expansion, was able to qualitatively describe the effects observed in single-pulse and double-pulse experiments.

Besides, the influence of the interpulse delay time between the two laser pulses was explored in the range between 0 and 20 μs. The results, according to the model proposed, provide information on the plume evolution in the single-pulse and double-pulse configurations at different air pressures. In particular, different optimum interpulse delays were found for the observation of neutral lines and ionic lines.     

The effect of multi-component aerosol particles on quantitative laser-induced breakdown spectroscopy: Consideration of localized matrix effects

Spectral measurements were performed in a laser-induced plasma to assess the changes in sodium or magnesium analyte emission response from particle-derived sources with the addition of concomitant mass to the aerosol particles. Temporally resolved measurements revealed up to a 50% enhancement in analyte emission with the addition of the elements copper, zinc or tungsten at mass ratios from 1:9 to 1:19, although the enhancement generally diminished by delay times of 60 μs. Additional measurements in magnesium–cadmium aerosol particles were performed to assess the temporal profile of plasma temperature in the spatial vicinity of the aerosol particles using the ion-to-neutral emission ratios. These measurements revealed a general increase in localized plasma temperature with increasing delay time, which is attributed with an initial suppression of plasma temperature about the aerosol particles as plasma energy is required to vaporize and ionize the aerosol particle mass. These measurements provide direct evidence of a matrix effect for aerosol particles, which is attributed primarily to perturbations in the localized plasma properties. These perturbations are minimized at longer plasma delay times; hence quantitative LIBS analysis of aerosol particles should be performed with careful attention given to the temporal plasma evolution. The data further elucidate the complex interactions between the plasma gas and the aerosol particles, during which the finite time-scales of particle dissociation, and heat and mass transfer are equally important.     

Determination of lead in water using laser ablation–laser induced fluorescence

The detection sensitivity of laser-induced breakdown spectroscopy (LIBS) is improved by coupling it with a laser-induced fluorescence method. A waterjet sample containing 500 ppm of Pb as an analyte was ablated by a 266 nm, frequency-quadrupled Q-switchedNd:YAG laser at an energy of ~ 260 μJ. After a short delay the resulting plume was re-excited with a 283.306 nm, nanosecond pulse dye laser at energies ranging from 45 to 100 nJ. The limit of detection (LOD) of lead in water was determined both by the single-pulse LIBS technique and Laser Ablation coupled with Laser-Induced Fluorecence (LA–LIF) method. It was found to be 75 ppm in the case of single-pulse LIBS and 4.3 ppm for LA–LIF. When the resonant pulse was detuned from the transition wavelength the LA–LIF signal disappeared demonstrating the resonant selectivity of this technique.     

Hydrogen analysis in solid samples by utilizing He metastable atoms induced by TEA CO2 laser plasma in He gas at 1 atm

A TEA CO₂ laser (350 mJ–1.5 J, 10.6 μm, 200 ns, 10 Hz) was focused onto a metal sub-target under He as host gas at 1 atmospheric pressure with a small amount of impurity gas, such as water and ethanol vapors. It was found that the TEA CO₂ laser with the help of the metal sub-target is favorable for generating a strong, large volume helium gas breakdown plasma at 1 atmospheric pressure, in which the helium metastable-excited state was then produced overwhelmingly. While the metal sub-target itself was never ablated. The helium metastable-excited state produced after the strong helium gas breakdown plasma was considered to play an important role in exciting the atoms. This was confirmed by the specific characteristics of the detected H emission, namely the strong intensity with low background, narrow spectral width, and the long lifetime. This technique can be used for gas and solid samples analysis. For nonmetal solid analysis, a metal mesh was introduced in front of the nonmetal sample surface to help initiation of the helium gas breakdown plasma. For metal sample, analysis can be carried out by combining the TEA CO₂ laser and an Nd–YAG laser where the Nd–YAG laser is used to ablate the metal sample. The ablated atoms from the metal sample are then sent into the region of helium gas breakdown plasma induced by the TEA CO₂ laser to be excited through the helium metastable-excited state. This technique can be extended to the analysis of other elements, not limited only to hydrogen, such as halogens.     

Emission characteristics of nickel ionic lines excited by reduced-pressure laser-induced plasmas using argon, krypton, nitrogen, and air as the plasma gas

The emission characteristics of nickel ionic lines in low-pressure laser-induced plasmas are investigated when argon, krypton, nitrogen, or air gas was employed as the plasma gas. The spectrum patterns and the relative intensities of the ionic lines are measured with and without a blind cylinder surrounding the sample surface to separate the detected emission area into two portions roughly: an initial breakdown zone and an expansion zone of the plasma. Their emission intensities are strongly dependent on both the kind and the pressure of the plasma gas. Different major ionic lines are observed in the argon and the krypton plasmas: for example, the Ni II 230.010-nm line (8.25 eV) for argon and the Ni II 231.604-nm line (6.39 eV) for krypton. The excitation mechanism of these ionic lines is considered to be a resonance charge-transfer collision with argon or krypton ion due to good energy matching to the corresponding energy levels of nickel ion. These ionic lines measured with the blind cylinder at reduced pressures of around 1300 Pa give the largest signal-to-background ratios; therefore, the analytical application under such optimum plasma conditions is recommended.     

Study of sub-mJ-excited laser-induced plasma combined with Raman spectroscopy under Mars atmosphere-simulated conditions

Laser-Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are complimentary techniques. LIBS yields elemental information while Raman spectroscopy yields molecular information about a sample, and both share similar instrumentation configurations. The combination of LIBS and Raman spectroscopy in a single instrument for planetary surface exploration has been proposed, however challenges exist for developing a combined instrument. We present LIBS and Raman spectroscopy results obtained using a diode pumped, intracavity doubled, Q-switched, Nd:YLF laser operating at 523 nm, which overcomes some of the difficulties associated with a combined instrument. LIBS spectra were obtained with 170 μJ per pulse at 4 Hz repetition rate in a low pressure Mars-simulated atmosphere and Raman spectra produced with 200 mW at 100 kHz. The Nd:YLF laser is switchable between LIBS and Raman spectroscopy modes only by a change in Q-switch repetition rate. Emissions from Ca, Ca II, Fe, Fe II, Mg, Na, and O atom were identified in the μ-LIBS spectrum of oolithic hematite. Evidence was found for a change in plasma dynamics between 7 and 5 Torr that could be explained as a decrease in plasma temperature and electron density below 5 Torr. This is relevant to future Mars exploration using LIBS as the mean surface pressure on Mars varies from 3.75 to 6 Torr. LIBS plasma dynamics should be carefully evaluated at the pressures that will be encountered at the specific Mars landing site.     

Laser-induced breakdown spectroscopy of γ-Fe2O3 nanoparticles in a biocompatible alginate matrix

An intensive multi-disciplinary research effort is underway at Wayne State University to synthesize and characterize magnetic nanoparticles in a biocompatible matrix for biomedical applications. The particular system being studied consists of 3–10 nm γ-Fe₂O₃ nanoparticles in an alginate matrix, which is being studied for applications in targeted drug delivery, as a magnetic-resonance imaging (MRI) contrast agent, and for hyperthermic treatments of malignant tumors. In the present work we report on our efforts to determine if laser-induced breakdown spectroscopy (LIBS) can offer a more accurate and substantially faster determination of iron content in such nanoparticle-containing materials than competing technologies such as inductively-coupled plasma (ICP). Standardized samples of -Fe₂O₃ nanoparticles (5–25 nm diameter) and silver micropowder (2–3.5 μm diameter) were created with thirteen precisely known concentrations and pressed hydraulically to create solid “pellets” for LIBS analysis. The ratio of the intensity of an Fe(I) emission line at 371.994 nm to that of an Ag(I) line at 328.069 nm was used to create a calibration curve exhibiting an exponential dependence on Fe mass fraction. Using this curve, an “unknown” γ-Fe₂O₃/alginate/silver pellet was tested, leading to a measurement of the mass fraction of Fe in the nanoparticle/alginate matrix of 51 ± 3 wt.%, which is in very good agreement with expectations and previous determinations of its iron concentration.     

Time-resolved ultraviolet laser-induced breakdown spectroscopy for organic material analysis

Ultraviolet pulses (266 nm) delivered by a quadrupled Nd:YAG laser were used to analyze organic samples with laser-induced breakdown spectroscopy (LIBS). We present characteristics of the spectra obtained from organic samples with special attentions on the emissions of organic elements, O and N, and molecular bonds CN. The choice of these atomic or molecular species is justified on one hand, by the importance of these species to specify organic or biological materials; and on the other hand by the possible interferences with ambient air when laser ablation takes place in the atmosphere. Time-resolved LIBS was used to determine the time-evolution of line intensity emitted from these species. We demonstrate different kinetic behaviors corresponding to different origins of emitters: native atomic or molecular species directly vaporized from the sample or those generated through dissociation or recombination due to interaction between laser-induced plasma and air molecules. Our results show the ability of time-resolved UV-LIBS for detection and identification of native atomic or molecular species from an organic sample.     

Rapid dissolution of metal in aqueous media using a modified commercial spark source unit

In this preliminary study the feasibility of a modified spark source was investigated as a method of rapidly dissolving solid conductive samples in aqueous media prior to analysis by atomic spectroscopy. The spark source, originally designed for spark emission spectroscopy in air, was modified by the installation of a spark ablation vessel. This spark ablation vessel was designed for spark ablating samples in aqueous solution, such as deionised water. Samples such as mild steel and brass were ablated in 5–10 ml of deionised water for 2–30 s producing a colloidal suspension. The suspension was readily dissolved by adding 100 μl of concentrated HCl or HNO₃. In this paper the spark ablation vessel is described as well as some of the properties of spark ablation in aqueous solutions. Spark ablation rates on mild steel were measured with respect to spark ablation parameters such as applied current (power), polarity and spark time. Using mild steel as a test sample, spark ablation rates varied from 40 μg s⁻¹, with 2.5 A of applied current, to 70 μg s⁻¹ with 10 A of applied current to the electrodes. The feasibility of using this technique for analysing trace levels (μg g⁻¹) of elements in solid samples was also demonstrated for elements such as Ni in brass with inductively coupled plasma mass spectrometry (ICP-MS). Quantification of selected elements (Cr, Ni, Mn and Cu) in a certified alloy (SRM 663) and a non-certified stainless steel showed good agreement between the measured values using spark ablation and the accepted values.     

As, Cd, Cr, Ni and Pb pressurized liquid extraction with acetic acid from marine sediment and soil samples

Rapid leaching procedures by Pressurized Liquid Extraction (PLE) have been developed for As, Cd, Cr, Ni and Pb leaching from environmental matrices (marine sediment and soil samples). The Pressurized Liquid Extraction is completed after 16 min. The released elements by acetic acid Pressurized Liquid Extraction have been evaluated by inductively coupled plasma-optical emission spectrometry. The optimum multi-element leaching conditions when using 5.0 ml stainless steel extraction cells, were: acetic acid concentration 8.0 M, extraction temperature 100 °C, pressure 1500 psi, static time 5 min, flush solvent 60%, two extraction steps and 0.50 g of diatomaceous earth as dispersing agent (diatomaceous earth mass/sample mass ratio of 2). Results have showed that high acetic acid concentrations and high extraction temperatures increase the metal leaching efficiency. Limits of detection (between 0.12 and 0.5 μg g^− 1) and repeatability of the over-all procedure (around 6.0%) were assessed. Finally, accuracy was studied by analyzing PACS-2 (marine sediment), GBW-07409 (soil), IRANT-12-1-07 (cambisol soil) and IRANT-12-1-08 (luvisol soil) certified reference materials (CRMs). These certified reference materials offer certified concentrations ranges between 2.9 and 26.2 μg g^− 1 for As, from 0.068 to 2.85 μg g^− 1 for Cd, between 26.4 and 90.7 μg g^− 1 for Cr, from 9.3 to 40.0 μg g^− 1 for Ni and between 16.3 and 183.0 μg g^− 1 for Pb. Recoveries after analysis were between 95.7 and 105.1% for As, 96.2% for Cd, 95.2 and 100.6% for Cr, 95.7 and 103% for Ni and 94.2 and 105.5% for Pb.     

Membrane desolvation for the analysis of organic solutions and liquid chromatographic samples with low power helium microwave induced plasma atomic emission detection

A flat sheet membrane desolvator (FSMD) was used to extend the applicability of a 120 W helium microwave induced plasma (He-MIP) to elemental analysis of organic-solvent-based samples and element selective liquid chromatographic detection. With the FSMD on-line, methanol could be nebulized with a sample flow rate of 1.5 ml/min and a carrier gas flow rate of 1.2 l/min without extinguishing the plasma. Under these conditions, applying desolvator countercurrent gas flows in the range 0–8 l/min restored of the original pink color of the pure helium MIP from the bluish-green caused by methanol. Significant reductions in the emission intensities of C₂ species at 436.5, 473.7, 512.9, and 563.6 nm were observed with the application of the FSMD. The intensities of chlorine analyte emission lines at 479.5, 481.0 and 481.9 nm increased with increasing countercurrent gas flow rates and reached a maximum intensity with a flow rate of 5.0 l/min. Detection limits for Cl and Pb were 2.1 and 0.1 ppm using a 1 m focal length monochromator. Other elements and solvent combinations were also examined. Element selective liquid chromatographic detection was preliminarily examined by monitoring 2,6-dichlorobenzene and 5,7-dichlorohydroxyquinoline at the 479.5 nm Cl atomic emission line. Chlorine detection limits in the 3–7 μg range (70–190 ng/s) were obtained.     

Efficient plasma and bubble generation underwater by an optimized laser excitation and its application for liquid analyses by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) measurements were performed on bulk water solutions by applying a double-pulse excitation from a Q-Switched (QS) Nd:YAG laser emitting at 1064 nm. In order to optimize the LIBS signal, laser pulse energies were varied through changing of the QS trigger delays with respect to the flash-lamp trigger. We had noted that reduction of the first pulse energy from 92 mJ to 72 mJ drastically improves the signal, although the second pulse energy was also lowered from 214 mJ to 144 mJ. With lower pulse energies, limit of detection (LOD) for Mg in pure water was reduced for one order of magnitude (34 ppb instead of 210 ppb). In order to explain such a phenomenon, we studied the dynamics of the gas bubble generated after the first laser pulse through measurements of the HeNe laser light scattered on the bubble. The influence of laser energy on underwater bubble and plasma formation and corresponding plasma emission intensity were also studied by photographic technique. From the results obtained, we conclude that the optimal first pulse energy should be kept close to the plasma elongation threshold, in our case about 65 mJ, where the gas bubble has its maximum lateral expansion and the secondary plasma is still well-localized. The importance of a multi-pulse sequence on the LIBS signal was also analyzed, where the pulse sequence after the first QS aperture was produced by operating the laser close to the lasing threshold, with the consequent generation of relaxation oscillations. Low-energy multi-pulses might keep the bubble expansion large prior to the probing pulse, but preventing the formation of secondary weak plasmas in multiple sites, which reduces the LIBS signal. The short interval between the pre-pulses and the probing pulse is another reason for the observed LIBS signal enhancement.     

Effects of atmosphere on laser vaporization and excitation processes of solid samples

Effects of atmosphere on the laser vaporization and excitation processes were investigated with spectral measurements and with the direct measurement of vaporized weight of samples. The samples, metals and ceramics, were positioned in three different atmospheres, i.e. air, argon and helium, from atmospheric pressure to a pressure reduced to a few torr. The time-resolved emission intensities of the Fe I lines and the Al I lines of the Al alloy and Al metal samples were measured in two time windows, i.e. 0–1 μs and 1 μs-1 ms. The excitation temperatures and the electron number densities of the plasmas were also estimated. The emission spectra, the excitation temperatures, and the electron densities were shown to be appreciably influenced by the ambient atmosphere. The amount of sample vaporized which was measured directly with an electric micro-balance after irradiation by 500 laser shots changed considerably with the atmosphere, e.g. from 12 ng/pulse at atmospheric pressure to 330 ng/pulse at 10 torr in argon.

The results are discussed in the scope of the possibility of ambient gas breakdown before sample vaporization and a change in the laser radiation coupling to the solid surface. It is revealed that the control of the interaction between laser radiation and plasmas and the prevention of the preceding gas breakdown are important for effective laser vaporization and for the emission measurement of the plasmas. The sample characteristics also influence the initiation stage of the plasmas and the effects from the atmosphere.     

Particle size limits for quantitative aerosol analysis using laser-induced breakdown spectroscopy: Temporal considerations

The temporal evolution of the Si atomic emission signal produced from individual silica microspheres in an aerosolized air stream was investigated using laser-induced breakdown spectroscopy (LIBS). Specifically, the temporal evolution of Si emission from 2.47 and 4.09-micrometer-sized particles is evaluated over discrete delay times ranging from 15 to 70 µs following plasma initiation. The analyte signal profile from the microspheres, taken as the silicon atomic emission peak-to-continuum ratio, was observed to follow the same profile of silicon-rich nanoparticles over the range of delay times. The ratio of analyte signals for the 2.47 and 4.09-micrometer particles was observed to be approximately constant with plasma decay time and less than the expected mass ratio, leading to the conclusion that further vaporization and enhanced analyte response do not continue with increasing delay times for these microsphere sizes. While recent research suggests that the temporal component of analyte response is important for quantitative LIBS analysis, the current study does confirm earlier research demonstrating an upper size limit for quantitative aerosol particle analysis in the diameter range of 2 to 2.5 µm for silica microspheres.     

Spectroscopic and shadowgraphic analysis of laser induced plasmas in the orthogonal double pulse pre-ablation configuration

This work focuses on the study of the plumes obtained in the double pulse orthogonal Laser Induced Breakdown Spectroscopy (LIBS) in the pre-ablation configuration using both spectroscopic and shadowgraphic approaches. Single and double pulse LIBS experiments were carried out on a brass sample in air. Both the distance of the air plasma from the target surface and the interpulse delay were varied (respectively in the range 0.1–4.2 mm and up to 50 μs) revealing a significant variation of the plasma emission and of the plume-shock wave dynamical expansion in different cases. The intensity of both atomic and ionized zinc lines was measured in all the cases, allowing the calculation of the spatially averaged temperature and electron density and an estimation of the ablated mass. The line intensities and the thermodynamic parameters obtained by the spectroscopic measurements were discussed bearing in mind the dynamical expansion characteristics obtained from the shadowgraphic approach. All the data seem to be consistent with the model previously proposed for the double pulse collinear configuration where the line enhancement is mainly attributed to the ambient gas rarefaction produced by the first laser pulse, which causes a less effective shielding of the second laser pulse.     

Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions.: Comparison of local and spatially integrated measurements

The local values of the parameters that characterize a laser-induced plasma (temperature, electron density, relative number densities of neutral atoms and ions) have been obtained by spatially resolved emission spectroscopy, including the deconvolution of the measured intensity spectra. The plasma has been generated using a Nd:YAG laser with a Fe–Ni alloy in air at atmospheric pressure, and the emission in the time window 3.0–3.5 μs has been detected. The temperature values obtained from neutral atom and ion emissions have been compared in the cases of local and spatially-integrated measurements. Local Boltzmann and Saha–Boltzmann plots with high correlation to linear fittings have been obtained using two broad sets of optically thin neutral atom and ion lines (21 Fe I lines and 15 Fe II lines), resulting in local values of the electronic temperature that coincide within the error. These results of local measurements contrast with those of spatially integrated measurements, for which two different temperatures are obtained from the Boltzmann plots of neutral atoms (9100±150 K) and ions (13 700±300 K). This difference is explained according to the measured distributions of the electronic temperature and the neutral atom and ion number densities, that result in separated emissivity (or population) distributions of neutral atom and ion lines, leading to different neutral atom and ion apparent temperatures (population-averages of the local electronic temperature). Local values of the plasma parameters have been obtained at all the positions with significant emission, including the determination of the electronic temperature from Saha–Boltzmann or Boltzmann plots. The ionization degree is high- and low-varying at the inner part of the plasma, decaying only near the plasma front. The maximum of the ion density does not coincide with the temperature maximum; on the contrary, the axial variation of both the neutral atom and ion densities (that decrease towards the sample surface) is opposite to that of the temperature, a behaviour that is interpreted to result from the plasma expansion process.     

Cloud point extraction–thermospray flame quartz furnace atomic absorption spectrometry for determination of ultratrace cadmium in water and urine

A simple, low cost and highly sensitive method based on cloud point extraction (CPE) for separation/preconcentration and thermospray flame quartz furnace atomic absorption spectrometry was proposed for the determination of ultratrace cadmium in water and urine samples. The analytical procedure involved the formation of analyte-entrapped surfactant micelles by mixing the analyte solution with an ammonium pyrrolidinedithiocarbamate (APDC) solution and a Triton X-114 solution. When the temperature of the system was higher than the cloud point of Triton X-114, the complex of cadmium-PDC entered the surfactant-rich phase and thus separation of the analyte from the matrix was achieved. Under optimal chemical and instrumental conditions, the limit of detection was 0.04 μg/L for cadmium with a sample volume of 10 mL. The analytical results of cadmium in water and urine samples agreed well with those by ICP-MS.     

Electroless Ni-plating on PTFE fine particles

A Ni electroless plating process was used with polytetrafluoroethylene (PTFE) fine particles (25–500 μm). Using nonionic hydrocarbon surfactant, PTFE particles were dispersed in the plating bath. The PTFE hydrophobicity was sufficiently high that Ni was deposited partly on the PTFE surface in the initial step. The Ni-PTFE particles were formed into the Ni-PTFE plate by heat treatment at 350 °C after pressing. The Ni-PTFE plate had electrical conductivity and gas permeability, which were influenced by the pore distribution in the plate. Pores with 1 μm diameter might be especially important to impart high gas permeability to the Ni-PTFE plate.     

Kinetic modeling study of the laser-induced plasma plume of cyclotrimethylenetrinitramine (RDX)

A kinetic model of a laser-induced breakdown spectroscopy (LIBS) plume of cyclotrimethylenetrinitramine (RDX) was developed for the analysis of processes responsible for the LIBS signature of explosives. Air and argon were considered as buffer gases. The model includes a set of processes involving ion chemistry, as well as excitation, ionization, and other processes affecting neutral and ion species. Modeling results show that the overall reaction process can be considered a two-stage process. The first stage corresponds to a fast approach to a quasi-stationary state, while the second stage corresponds to the change of quasi-stationary species concentrations due to the change in temperature. As a result of the two-stage process, the initial mechanism of explosive decomposition is not important in determining its signature in the LIBS measurement time window (1–30 μs). The main processes responsible for generation of excited states for the LIBS emission are electron-excitation impact processes. A mechanism for the appearance of a double peak of the C₂ species concentration in the RDX plasma plume was suggested. Double-peak behavior of the C₂ species was previously experimentally observed during laser ablation of graphite.