The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

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.     

Low pressure laser ablation coupled to inductively coupled plasma mass spectrometry

The particle size distribution in laser ablation inductively coupled plasma mass spectrometry is known to be a critical parameter for complete vaporization of particles. Any strategy to reduce the particle size distribution of laser generated aerosols has the potential to increase the ion signal intensity and to reduce fractionation effects. Due to the fact that vapor generation, nucleation, condensation, and agglomeration take place within an extremely short period of time, ablation under atmospheric pressure might not allow influencing these processes while under reduced pressure condition the cooling of the aerosol and therefore the condensation is expected to be slower. In this study, a low pressure laser ablation cell for the generation of laser aerosols was coupled to an ICP-MS. In contrast to the previously developed trapped ablation mode, the newly designed cell allows the adjustment of the pressure in the ablation cell between 20 and 1400 mbar prior to the ablation.Ablation experiments carried out using this configuration showed a dependence of the aerosol properties (size distribution and particle structure) on the ablation cell pressure. The intensity ratio U/Th measured as a figure of merit for complete vaporization within the ICP indicated a change in the aerosol structure at approximately 500 mbar toward smaller particle size. A significant difference between low pressure and at ambient pressure ablated aerosol was observed. The intensity ratios (U/Th) of the ablated sample moves closer to the bulk composition at lower pressures at the expense of sensitivity. Therefore the decrease in the ICP-MS signal intensity in the low pressure cell can be attributed to vapor deposition within the ablation cell walls.Moreover, scanning electron microscope images of aerosols collected on filters after the low pressure ablation cell suggest the possibility of a slower cooling velocity of the aerosol, which was observed in the condensed material on the surface of ejected spherical particles. The expansion of the laser aerosol was also investigated using polished brass substrates in the expansion path-way for particle collection.     

Elemental composition of laser ablation aerosol particles deposited in the transport tube to an ICP

The element ratios in aerosol particles produced by laser ablation (λ=266 nm, pulse length: 5 ns) of brass and steel in Ar and He and deposited in different segments of the transport tube to an ICP have been measured by ICP-MS. The data are compared with the ratios obtained by a corresponding bulk analysis. For brass, the element compositions of the aerosol particles deposited in different parts of the tube deviated from the bulk and varied along the tube. For steel, moderate agreement of the element ratios in the aerosols and the bulk was found. It is also shown that elemental ratios measured on-line by laser ablation ICP-MS with the laser-produced aerosol should not be calibrated by elemental ratios obtained with wet aerosols from aqueous solutions of the bulk.     

Transport efficiency in femtosecond laser ablation inductively coupled plasma mass spectrometry applying ablation cells with short and long washout times

Relative mass transport efficiencies of near infrared (λ = 795 nm) femtosecond laser generated brass aerosols in helium were measured by ICP-MS applying different ablation cells with short and long washout times. It was found that the transport efficiencies are independent of the cell used within the mutual experimental uncertainties. This finding was confirmed by additional measurements providing the absolute particle mass transport efficiencies of femtosecond laser ablation in He. Here, the transport efficiencies were determined by weighing the samples before and after ablation with a micro-balance, collecting the particles by low-pressure impaction, and evaluating the impacted masses quantitatively by total reflection X-ray fluorescence. Within the experimental uncertainties (± 9–19%) the same absolute transport efficiency (about 77%) was found for all cells applied. This efficiency value can be regarded as a lower limit of the absolute mass transport efficiency since mass losses in the impactor are difficult to quantify.     

The agglomeration state of nanosecond laser-generated aerosol particles entering the ICP

Fundamental understanding of aerosol formation and particle transport are important aspects of understanding and improving laser-ablation ICP–MS. To obtain more information about particles entering the ICP, laser aerosols generated under different ablation conditions were collected on membrane filters. The particles and agglomerates were then visualised using scanning electron microscope (SEM) imaging. To determine variations between different sample matrices, opaque (USGS BCR-2G) and transparent (NIST SRM 610) glass, CaF₂, and brass (MBH B26) samples were ablated using two different laser wavelengths, 193 and 266 nm. This study showed that the condensed nano-particles (∼10 nm in diameter) formed by laser ablation reach the ICP as micron-sized agglomerates; this is apparent from filters which contain only a few well-separated particles and particle agglomerates. Ablation experiments on different metals and non-metals show that the structure of the agglomerates is matrix-dependent. Laser aerosols generated from silicates and metals form linear agglomerates whereas particle-agglomerates of ablated CaF₂ have cotton-like structures. Amongst other conditions, this study shows that the absorption characteristics of the sample and the laser wavelength determine the production of micron-sized spherical particles formed by liquid droplet ejection.     

Characterization of the aerosol produced by infrared femtosecond laser ablation of polyacrylamide gels for the sensitive inductively coupled plasma mass spectrometry detection of selenoproteins

A 2D high repetition rate femtosecond laser ablation strategy (2-mm wide lane) previously developed for the detection of selenoproteins in gel electrophoresis by inductively coupled plasma mass spectrometry was found to increase signal sensitivity by a factor of 40 compared to conventional nanosecond ablation (0.12-mm wide lane) [G. Ballihaut, F. Claverie, C. Pécheyran, S. Mounicou, R. Grimaud and R. Lobinski, Sensitive Detection of Selenoproteins in Gel Electrophoresis by High Repetition Rate Femtosecond Laser Ablation-Inductively Coupled Plasma Mass Spectrometry, Anal. Chem. 79 (2007) 6874–6880]. Such improvement couldn't be explained solely by the difference of amount of material ablated, and then, was attributed to the aerosol properties. In order to validate this hypothesis, the characterization of the aerosol produced by nanosecond and high repetition rate femtosecond laser ablation of polyacrylamide gels was investigated. Our 2D high repetition rate femtosecond laser ablation strategy of 2-mm wide lane was found to produce aerosols of similar particle size distribution compared to nanosecond laser ablation of 0.12-mm wide lane, with 38% mass of particles < 1 µm. However, at high repetition rate, when the ablated surface was reduced, the particle size distribution was shifted toward thinner particle diameter (up to 77% for a 0.12-mm wide lane at 285 µm depth). Meanwhile, scanning electron microscopy was employed to visualize the morphology of the aerosol. In the case of larger ablation, the fine particles ejected from the sample were found to form agglomerates due to higher ablation rate and then higher collision probability. Additionally, investigations of the plasma temperature changes during the ablation demonstrated that the introduction of such amount of polyacrylamide gel particles had very limited impact on the ICP source (ΔT~ 25 ± 5 K). This suggests that the cohesion forces between the thin particles composing these large aggregates were weak enough to have negligible impact on the ICPMS detection.     

Phenomenological studies on structure and elemental composition of nanosecond and femtosecond laser-generated aerosols with implications on laser ablation inductively coupled plasma mass spectrometry

In laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), the properties of laser-generated aerosols, such as size and composition, are crucial for matrix-independent quantification. In this study, the aerosol particle morphology and elemental composition generated by two state-of-the-art laser systems (ArF excimer nanosecond-UV laser and Ti:sapphire femtosecond-IR laser) were investigated by electron microscopic techniques. Electrostatic sampling of the aerosols directly onto transmission electron microscopy (TEM) grids allowed us to study the morphology and elemental composition of the aerosols using TEM and TEM–EDX (energy dispersive X-ray spectroscopy) analyses, respectively. The results of the electron microscopic studies were finally compared to the LA-ICPMS signals of the main matrix components. The investigations were carried out for non-conducting materials (glass and zircon), metallic samples (steel and brass) and semiconductors (sulfides). The studies confirm that ns-LA-generated aerosols dominantly consist of nanoparticle agglomerates while conducting samples additionally contain larger spherical particles (diameter typically 50 to 500 nm). In contrast to ns-laser ablation, fs-LA-generated aerosols consist of a mixture of spherical particles and nanoparticle agglomerates for all investigated samples. Surprisingly, the differences in elemental composition between nanoparticle agglomerates and spherical particles produced with fs-LA were much more pronounced than in the case of ns-LA, especially for zircon (Si/Zr fractionation) and brass (Cu/Zn fractionation). These observations indicate different ablation and particle formation mechanisms for ns- and fs-LA. The particle growth mechanism for ns-LA is most likely a gas-to-particle conversion followed by agglomeration and additional hydrodynamic sputtering for conducting samples. On the other hand, phase explosion is assumed to be responsible for the mixture of large spherical particles and nanoparticle agglomerates as found for fs-LA-generated aerosols. Based on these mechanisms, the overall temporal elemental fractionation effects in ns-LA-ICPMS seem to occur mainly during the ablation. This effect was not observed for fs-LA-ICPMS despite the element separation into different particle fractions, which, on the other hand, could induce severe ICP-induced fractionation.     

Femtosecond laser ablation inductively coupled plasma mass spectrometry: Transport efficiencies of aerosols released under argon atmosphere and the importance of the focus position

Although the utilization of helium as aerosol carrier has been shown to improve both accuracy and sensitivity of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), occasionally, argon is being used due to practical and economic reasons. In order to provide more insight into the mechanisms underlying these performance differences, in this study, transport efficiencies of aerosols released by NIR- and UV-femtosecond laser ablation (LA) of brass applying laminar or turbulent in-cell flow conditions and argon as carrier gas were measured. Aerosol particles were collected by low-pressure impaction or filtered by fine porous membranes. On the basis of aerosol masses collected and mass differences derived from target weighing prior to and after LA, transport efficiencies approximately varied in between 75% and 95%. In addition, LA of a thin Cr layer was performed which allowed to release a well-defined amount of material and, thus, to correct mass balances for debris accumulating around the crater rim. The total aerosol mass released during LA was found to be strongly dependent on the relative focus position, i.e. surface area irradiated, even if the laser pulse energy delivered to the target was kept constant. Furthermore, a physical model only making use of input parameters such as laser spot size and pulse energy was implemented to qualitatively describe the correlation between aerosol mass and laser focus position.     

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.     

Orthogonal Double-pulse versus Single-pulse laser ablation at different air pressures: A comparison of the mass removal mechanisms

The mass removal mechanisms occurring during the ablation of an aluminum target, induced by an Nd:YAG laser at λ = 1064 nm in air at different laser fluences, were investigated at different pressures and in the orthogonal double pulse configuration. Both the spectroscopic analysis of the plasma emission and the microscopic analysis of the craters, providing complementary information on the laser ablation process, were performed. The first technique allowed the calculation of the plasma thermodynamic parameters and an estimation of its atomized mass, while the latter led to the calculation of their volume, as well as a qualitative inspection of the craters profile and appearance. The results obtained at different fluences suggest a complex picture where the air pressure strongly drives the laser shielding effect, which in turn affects the relevance of melt displacement, melt expulsion and phase explosion mechanisms. The measurements performed in double pulse configuration suggest that in this case the ablation process is very similar to that induced at low air pressure. Phase explosion seems to occur in double pulse laser ablation while it seems inhibited in single pulse ablation at atmospheric pressure. Differently, melt splashing is much more efficient in single pulse ablation at atmospheric pressure than in double pulse ablation.     

Optical emission studies of plasma induced by single and double femtosecond laser pulses

Double-pulse femtosecond laser ablation has been shown to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to single-pulse ablation particularly when an appropriate interpulse delay is selected, that is typically in the range of 50–1000 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy analysis of materials. A detailed comparative study of collinear double- over single-pulse femtosecond laser-induced breakdown spectroscopy has been carried out, based on measurements of emission lifetime, temperature and electronic density of plasmas, produced during laser ablation of brass with 450 fs laser pulses at 248 nm. The results obtained show a distinct increase of plasma temperature and electronic density as well as a longer decay time in the double-pulse case. The plasma temperature increase is in agreement with the observed dependence of the emission intensity enhancement on the upper energy level of the corresponding spectral line. Namely, intensity enhancement of emission lines originating from higher lying levels is more profound compared to that of lines arising from lower energy levels. Finally, a substantial decrease of the plasma threshold fluence was observed in the double-pulse arrangement; this enables sensitive analysis with minimal damage on the sample surface.     

Dual-pulse Laser Induced Breakdown Spectroscopy for analysis of gaseous and aerosol systems: Plasma-analyte interactions

Dual-pulse LIBS has been previously investigated to a large extent on solid and liquid phase analytes, where it has been demonstrated to significantly enhance atomic emission signal intensity, and more importantly, to enhance the analyte peak-to-base and signal-to-noise ratios. This study focuses on the effects of an orthogonal dual-pulse laser configuration on the atomic emission response for both purely gaseous and calcium-based aerosol samples. The gaseous sample consisted of purified (i.e. aerosol free) air, from which nitrogen and oxygen spectral emission lines were analyzed. Measurements for the gaseous system resulted in no notable improvements with the dual-pulse configuration as compared to the single-pulse LIBS. Experiments were also conducted in purified air seeded with calcium-rich particles, which revealed a marked improvement in calcium atomic emission peak-to-base (∼ 2-fold increase) and signal-to-noise ratios (∼ 4-fold increase) with the dual-pulse configuration. In addition to increased analyte response, dual-pulse LIBS yielded an enhanced single-particle sampling rate when compared to conventional LIBS. Transmission measurements with respect to the plasma-creating laser pulse were recorded for both single and dual-pulse methods over a range of temporal delays. In consideration of the spectroscopic and transmission data, the plasma-analyte interactions realized with a dual-pulse methodology are explained in terms of the interaction with the initially expanding plasma shock wave, which differs between gaseous and particulate phase analytes, as reported in a recent study [V. Hohreiter, D.W. Hahn, Calibration effects for laser-induced breakdown spectroscopy of gaseous sample streams: analyte response of gas-phase species versus solid-phase species, Anal. Chem. 77 (2005) 1118–1124].     

Visualization of aerosol particles generated by near infrared nano- and femtosecond laser ablation

The expansion of aerosols generated by near infrared (NIR) nanosecond (ns) and femtosecond (fs) laser ablation (LA) of metals at atmospheric pressures was explored by laser-induced scattering. In order to achieve adequate temporal and spatial resolution a pulsed laser source was utilized for illuminating a 0.5 mm-wide cross section of the expanding aerosol. It could, for instance, be shown that NIR-ns-LA under quiescent argon atmosphere provokes the formation of a dense aerosol confined within a radially propagating vortex ring. The expansion dynamics achieved under these conditions were found to be fairly slow whereas the degree of aerosol dispersion for NIR-ns-LA using helium drastically increased due to its lower viscosity. As a consequence, the maximum diameter of expansion differed by a factor of approximately four. The trajectories of aerosol particles generated by NIR-ns-LA using argon could, furthermore, be simulated on the basis of computational fluid dynamics (CFD). For this purpose, a model inspired by the thermal character of NIR-ns-LA taking into account a sudden temperature build-up of 10,000 K at the position of the laser focus was implemented.     

Size-related vaporisation and ionisation of laser-induced glass particles in the inductively coupled plasma

Ongoing discussions about the origin of elemental fractionation occurring during LA-ICP-MS analysis show that this problem is still far from being well understood. It is becoming accepted that all three possible sources (ablation, transport, excitation) contribute to elemental fractionation. However, experimental data about the vaporisation size limit of different particles in the ICP, as produced in laser ablation, have not been available until now. This information should allow one to determine the signal contributing mass within the ICP and would further clarify demands on suitable laser ablation systems and gas atmospheres in terms of their particle size distribution.The results presented here show a vaporisation size limit of laser induced particles, which was found at particle sizes between 90 nm and 150 nm using an Elan 6000 ICP-MS. Due to the fact that the ICP-MS response was used as evaluation parameter, vaporisation and ionisation limits are not distinguishable.The upper limit was determined by successively removing the larger particles from the aerosol, which was created by ablation of a NIST 610 glass standard at a wavelength of 266 nm, using a recently developed particle separation device. Various particle fractions were separated from the aerosol entering the ICP. The decrease in signal intensity is not proportional to the decrease in volume, indicating that particles above 150 nm in diameter are not completely ionised in the ICP. Due to the limited removal range of the particle separation device, which cannot remove particles smaller than 150 nm, single hole ablations were used to determine the lower vaporisation limit. This is based on measurements showing that larger particles occur dominantly during the first 100 laser pulses only. After this period, the ratio of ICP-MS counts and total particle volume was found to be constant while most of the particles are smaller than 90 nm, indicating complete vaporisation and ionisation of these particles.To describe the influence of different plasma forward powers on the vaporisation limit, the range 1000–1600 W was studied. Results indicate that optimum vaporisation and ionisation occurs at 1300 W. However, an increase of the particle ionisation limit towards larger particles was not observed within the accuracy of this study using the full range of parameters available for optimisation on commonly used ICP-MS instruments.     

Direct analysis of trace elements in crude oils by high-repetition-rate femtosecond laser ablation coupled to ICPMS detection

IR-femtosecond pulses were used at high repetition rates (up to 10 kHz) to ablate viscous crude oils for the determination of trace elements by ICPMS. A special internal glass cap was fitted into the ablation cell to minimise oil splashes and remove big particles that would be otherwise spread into the cell. Laser ablation in static and dynamic conditions (i.e. the laser beam being moved rapidly at the surface of the sample) was studied together with some fundamental parameters like repetition rate and fluence. Signal sensitivity and stability were found to be strongly affected by repetition rate and fluence, though not in linear manner, and in some circumstances by the laser beam velocity. Sample transport efficiency was found to decrease with increasing repetition rate, probably due to stronger particle agglomeration when increasing the density of primary particles. ICPMS plasma atomisation/ionisation efficiency was also found to be affected to some extent at the highest repetition rates. Moderate repetition rate (1 kHz), high fluence (24 J cm⁻²) and fast scanning velocity (100 mm s⁻¹) were preferred taking into account signal intensity and stability. Sample transport elemental fractionation was also evidenced, particularly as regards to carbon due to volatilisation of volatile organic species. Matrix effect occurring when comparing the ablation of transparent (base oil) and opaque (crude oil) samples could not be completely suppressed by the use of IR femtosecond pulses, requiring a matrix matching or a standard addition calibration approach. This approach provided good accuracy and very low detection limits in the crude oil, in the range of ng g⁻¹.     

Study of near infra red femtosecond laser induced particles using transmission electron microscopy and low pressure impaction: Implications for laser ablation–inductively coupled plasma-mass spectrometry analysis of natural monazite

The characteristics of infra red femtosecond laser-induced aerosols are studied for monazite (LREE, Th(PO₄)) ablation and correlations are established with inductively coupled plasma-mass spectrometry (ICP-MS) signals. Critical parameters are tested within wide ranges of values in order to cover the usual laser ablation -ICP-MS analysis conditions: pulse energy (0.15 < E₀ < 1 mJ/pulse), pulse width (60 < τ < 3000 fs), ablation time (t ≤ 10 min) and transport length (l ≤ 6.3 m). Transmission electron microscopy reveals that aerosols are made of agglomerates of ~ 10 nm particles and 20–300 nm phosphorus depleted condensed spherical particles. These structures are not affected by any laser ablation parameter. Particle counting is performed using electronic low pressure impaction. Small changes on particle size distribution are noticed. They may be induced either by a peak of ablation rate in the first 15 s at high fluence (larger particles) or the loss of small particles during transport. We found a positive correlation between I (ICP-MS mean signal intensity in cps) and N (particle density in cm^? 3) when varying E₀ and t, suggesting that N is controlled by the irradiance (P₀ in W·cm^? 2). Elemental ratio measurements show a steady state signal after the initial high ablation rate (mass load effect in the plasma torch) and before a late chemical fractionation, induced by poor extraction of bigger, early condensed spherical particles from the deepening crater. Such chemical fractionation effects remain within uncertainties, however. These effects can be limited by monitoring E₀ to shorten the initial transient state and delay the attainment of an unfavorable crater aspect ratio. Most adopted settings are for the first time deduced from aerosol characteristics, for infra red femtosecond laser ablation. A short transport (l < 4.0 m) limits the agglomeration of particles by collision process along the tube. Short τ is preferred because of higher P₀, yet no benefit is found on ICP-MS signal intensity under 200 fs. Under such pulse widths the increased particle production induces more agglomeration during transport, thereby resulting in higher mass load effects that reduce the ionization efficiency of the plasma torch. Thus, pulse energy must be set to get an optimal balance between the need for a high signal/background ratio and limitation of mass load effects in the plasma torch.     

On the stoichiometry of mass transfer from solid to plasma during pulsed laser ablation of brass

We have performed spectroscopic analysis of the plasma produced by pulsed laser ablation of brass in a low pressure argon atmosphere. The intensities of several spectral lines of copper, zinc and lead were measured for succeeding laser pulses applied to the same irradiation site. The intensities and spectral shapes of the observed transitions were compared to the spectral radiance computed for plasma in local thermal equilibrium. At a delay of 600 ns after the laser pulse, the plasma is characterized by typical values of temperature and electron density of 1.1 × 10⁴ K and 1.2 × 10¹⁷ cm^− 3, respectively, and an elemental composition equal to that of the sample. Small changes of spectral line intensities were observed with increasing number of applied laser pulses. They were attributed to the alteration of the plume expansion dynamics as a consequence of crater formation on the sample surface. The results indicate that the mass transfer from the solid to the plasma is stoichiometric.     

Influence of physical properties and chemical composition of sample on formation of aerosol particles generated by nanosecond laser ablation at 213 nm

The influence of sample properties and composition on the size and concentration of aerosol particles generated by nanosecond Nd:YAG laser ablation at 213 nm was investigated for three sets of different materials, each containing five specimens with a similar matrix (Co-cemented carbides with a variable content of W and Co, steel samples with minor differences in elemental content and silica glasses with various colors). The concentration of ablated particles (particle number concentration, PNC) was measured in two size ranges (10–250 nm and 0.25–17 µm) using an optical aerosol spectrometer. The shapes and volumes of the ablation craters were obtained by Scanning Electron Microscopy (SEM) and by an optical profilometer, respectively. Additionally, the structure of the laser-generated particles was studied after their collection on a filter using SEM.     

A study of non-linear calibration graphs for brass with femtosecond laser-induced breakdown spectroscopy

The ablation of brass samples with 170 fs laser pulses was studied by time resolved emission spectroscopy. The intensities of Cu as well as of Zn lines were found to be non-linearly dependent on their concentrations in the samples. This behaviour is explained by changes in the mass ablation rate. Thus, linear dependencies were obtained when the Zn line intensities were normalized to the line intensities of Cu. A simple theoretical model based on compositional differences is presented. The differences in ablation are accounted for using the standard two-temperature diffusion model for short laser pulse ablation and the steady current resistivity as a fundamental parameter describing the compositional changes in a set of brass samples. In principle, it is possible to construct linear calibration curves from the measured Zn and Cu line intensities without internal standardization if the model and energy dependent ablation measurements are taken into account. Inductively coupled plasma mass spectrometry measurements are used to demonstrate this.     

Influence of wavelength,irradiance, and the buffer gas pressure on high irradiance laser ablation and ionization source coupled with an orthogonal Time of Flight Mass Spectrometer

Influence of laser wavelength, laser irradiance and the buffer gas pressure were studied in high irradiance laser ablation and ionization source coupled with an orthogonal time-of-flight mass spectrometer. Collisional cooling effects of energetic plasma ions were proved to vary significantly with the elemental mass number. Effective dissociation of interferential polyatomic ions in the ion source, resulting from collision and from high laser irradiance, was verified. Investigation of relative sensitivity coefficients (RSC) of different elements performed on a steel standard GBW01396, which was ablated at 1064 nm, 532 nm, 355 nm, and 266 nm, has demonstrated that the thermal ablation mechanism could play a critical role with the first three wavelengths, while 266 nm induces non-thermal ablation principally. Experimental results also indicated that there is no evident discrepancy for most metal elements on RSCs and LODs among four wavelengths at high irradiance, except that high boiling point elements like Nb, Mo, and W have higher RSCs at higher irradiance regions of 1064 nm, 532 nm, and 355 nm due to thermal ablation. A geological standard and a garnet stone were also used in the experiment subsequently, and their RSCs and LODs for metal elements show nonsignificant dependence on wavelength at designated irradiances. All results reveal that relatively uniform sensitivity can be achieved at any wavelength for metal elements in the solids used in our experiments at an appropriate irradiance for the low pressure high irradiance laser ablation and ionization source.