Mathematical modelling of laser-induced particulate formation in direct solid microanalysis

The mechanism of laser-assisted particulate sampling was investigated by means of theoretical modelling. Bimodal particle size distribution was described as nanosized nuclei condensation in an expanding laser plume, and microsized droplet ejection in a hydrodynamically instable melt pool. Characteristics of the ambient gas turned out to be important in the formation of both size classes. Microparticle formation was found to be a process sensitive to local conditions (deterministic chaos). The model was applied to the case of ablation of silicon. Implications for the use of lasers as microsampling probes will be discussed throughout the article, as well as the impact on different solid sample classes.     

Study of the melt pelletization process focusing on the micromeritic property of pellets

Melt pelletization of lactose 450 M was carried out in an 8-l high shear mixer using PEG 3000 as the meltable binder. The pore size and size distribution of the melt pellets were determined using mercury intrusion porosimetry. The pore size distribution of melt pellets was found to be bimodal. With a higher binder concentration, post-melt impeller speed or longer post-melt processing time, the fraction of large pores in the agglomerates was reduced but the tendency of the agglomerates to develop sub-micron pores increased. The extent of formation of large pores was dependent on the interplay between the inter-particle distance of lactose particles and the contraction property of molten binder. High process temperature was associated with a greater amount of water loss from the melt agglomerates. The water vapor liberated from the lactose particles, was trapped in the molten PEG during the pelletization process. The formation of sub-micron pores was a result of escape of this water vapor on solidification of the molten PEG as well as agglomerate densification. The quantity of sub-micron pores produced was found to be related to the level of water loss. The melt agglomeration gave rise to large agglomerates when long post-melt processing time, high post-melt impeller speed or binder concentration was used.     

Dynamics of solvent-free grafted nanoparticles

The diffusivity and structural relaxation characteristics of oligomer-grafted nanoparticles have been investigated with simulations of a previously proposed coarse-grained model at atmospheric pressure. Solvent-free, polymer-grafted nanoparticles as well as grafted nanoparticles in a melt were compared to a reference system of bare (ungrafted) particles in a melt. Whereas longer chains lead to a larger hydrodynamic radius and lower relative diffusivity for grafted particles in a melt, bulk solvent-free nanoparticles with longer chains have higher relative diffusivities than their short chain counterparts. Solvent-free nanoparticles with short chains undergo a glass transition as indicated by a vanishing diffusivity, diverging structural relaxation time and the formation of body-centered-cubic-like order. Nanoparticles with longer chains exhibit a more gradual increase in the structural relaxation time with decreasing temperature and concomitantly increasing particle volume fraction. The diffusivity of the long chain nanoparticles exhibits a minimum at an intermediate temperature and volume fraction where the polymer brushes of neighboring particles overlap, but must stretch to fill the interparticle space.     

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.     

Adsorption of paraffin vapor on oxidized molybdenum substrates at nano- and micro-scales

Sputtered oxidized molybdenum surfaces were exposed at room temperature for different times to paraffin vapors obtained at 150 degrees C. Scanning polarization force microscopy (SPFM), optical and confocal microscopy were used to characterize the surfaces. The condensed morphologies are complex and strongly dependent upon the quantity of vapor molecules deposited on the substrate surface. A thin paraffin film is initially formed and quite uniform nano-height drops are nucleated randomly over it within 10-20 s time exposures. Their average contact angle ranged between 1 degrees -2.5 degrees . Further vapor deposition led to a more complex regime where nano-height drops do not show a clear interface with the film, while micro-sized drops do. The tangent approximation method adopted by Salmeron and Xu for the nano-drop regimes was extended to the micro-sized drop regime obtaining an averaged effective contact angle equal to 4 degrees -5 degrees . Both nano-height and micro-sized drops shape and effective contact angles have been discussed taking into account their interactions between the film and the drops.     

Speciation of copper and zinc in size-fractionated atmospheric particulate matter using total reflection mode X-ray absorption near-edge structure spectrometry

The health effects of aerosol depend on the size distribution and the chemical composition of the particles. Heavy metals of anthropogenic origin are bound to the fine aerosol fraction (PM_2.5). The composition and speciation of aerosol particles can be variable in time, due to the time-dependence of anthropogenic sources as well as meteorological conditions. Synchrotron-radiation total reflection X-ray fluorescence (SR-TXRF) provides very high sensitivity for characterization of atmospheric particulate matter. X-ray absorption near-edge structure (XANES) spectrometry in conjunction with TXRF detection can deliver speciation information on heavy metals in aerosol particles collected directly on the reflector surface. The suitability of TXRF-XANES for copper and zinc speciation in size-fractionated atmospheric particulate matter from a short sampling period is presented. For high size resolution analysis, atmospheric aerosol particles were collected at different urban and rural locations using a 7-stage May cascade impactor having adapted for sampling on Si wafers. The thin stripe geometry formed by the particulate matter deposited on the May-impactor plates is ideally suited to SR-TXRF. Capabilities of the combination of the May-impactor sampling and TXRF-XANES measurements at HASYLAB Beamline L to Cu and Zn speciation in size-fractionated atmospheric particulate matter are demonstrated. Information on Cu and Zn speciation could be performed for elemental concentrations as low as 140 pg/m³. The Cu and Zn speciation in the different size fraction was found to be very distinctive for samples of different origin. Zn and Cu chemical state typical for soils was detected only in the largest particles studied (2–4 μm fraction). The fine particles, however, contained the metals of interest in the sulfate and nitrate forms.     

Direct detection of particles formed by laser ablation of matrices during matrix-assisted laser desorption/ionization

We report the detection of nanoparticles formed by irradiating matrix-assisted laser desorption/ionization (MALDI) matrix samples. This is direct evidence for the ejection of large size aggregates in the MALDI process. Nanometer-size particles were generated via a tunable solid-state UV laser, irradiating a sample placed in a nitrogen atmosphere. Size distribution measurements were performed using a differential mobility analyzer and a condensation particle counter. Particles in the 10-1000 nm size range were detected. The dependence of the particle size distribution on the laser fluence, wavelength and matrix was investigated. The observed effects are discussed and related to the MALDI ablation dynamics and gas-phase processes.     

Study of sulfur heterogeneous nucleation from supersaturated vapor on tungsten oxide and sodium chloride seed particles. Determination of contact angle of critical sulfur nuclei

A procedure has been developed for determining the contact angle of a critical nucleus formed on seed particles during the heterogeneous nucleation of a vapor in a flow chamber. The procedure comprises the determination of the fraction of enlarged particles, as well as the selective separation of nanoparticles over sizes to locate the zone of intense nucleation. The concentration and size distribution of aerosol particles have been measured with a diffusion spectrometer of aerosols. Vapor concentration distributions and supersaturation fields have been determined by solving the mass-transfer problem. The calculated supersaturation fields are in good agreement with the location of the intense nucleation zone experimentally found with the help of selective separation. The fractions of enlarged particles have been determined as functions of supersaturation in the chamber. A formula has been derived for calculating the fraction and size distribution function of enlarged particles at known supersaturation and temperature fields and a preset contact angle. The contact angles are selected in a manner such that the calculated fraction of enlarged particles coincides with that measured experimentally. It has been revealed that the contact angle of critical sulfur nuclei formed on tungsten oxide seed particles with average radii 〈R _p〉 ≈ 5.8?4.4 nm is in a range of 21.2?20.5°, while, in the case of sodium chloride seed particles with 〈R _p〉 ≈ 6.0?4.4 nm, the contact angle is 20.4?17.4°. The size of a critical nucleus has been found to be proportional to calculated average radius of a seed particle 〈R _p〉 in both cases.     

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.     

Experimental study of laser-induced plasma: Influence of laser fluence and pulse duration

Influence of laser fluence and pulse duration on the morphology and the internal structure of plasma induced by infrared nanosecond laser pulse on an aluminum target placed in an argon ambient gas of one atmosphere pressure was experimentally studied. Dual-wavelength differential spectroscopic imaging was used in the experiment, which allowed observing the detailed structure inside of the ablation plume with distributions of species evaporated from the target as well as contributed by the ambient gas. Different regimes of post-ablation interaction were investigated using different laser fluences and pulse durations. We demonstrate in particular that plasma shielding due to various species localized in different zones inside of the plume leads to different morphologies and internal structures of the plasma. At moderate fluence, the plasma shielding due to the ablation vapor localized in the central part of the plume leads to its nearly spherical expansion with a layered structure of the distribution of different species. At higher fluence, the plasma shielding becomes strongly contributed by ionized ambient gas localized in the propagation front of the plume. An elongated morphology of the plume is observed with a zone of mixing between different species evaporated from the target or contributed by the ambient gas. Finally with extremely strong plasma shielding by ionized ambient gas in the case of a long duration pulse at high fluence, a delayed evaporation from the target is observed due to the ejection of melted material by splashing.     

Method of measuring charge distribution of nanosized aerosols

In this paper, we present the development of a method to accurately measure the positive and negative charge distribution of nanosized aerosols using a tandem differential mobility analyzer (TDMA) system. From the series of TDMA measurements, the charge fraction of nanosized aerosol particles was obtained as a function of equivalent mobility particle diameter ranging from 50 to 200 nm. The capability of this new approach was implemented by sampling from a laminar diffusion flame which provides a source of highly charged particles due to naturally occurring flame ionization process. The results from the TDMA measurement provide the charge distribution of nanosized aerosols which we found to be in reasonable agreement with Boltzmann equilibrium charge distribution theory and a theory based upon charge population balance equation (PBE) combined with Fuchs theory (N.A. Fuchs, Geofis. Pura Appl. 56 (1963) 185). The theoretically estimated charge distribution of aerosol particles based on the PBE provides insight into the charging processes of nanosized aerosols surrounded by bipolar ions and electrons, and agree well with the TDMA results.     

Laser-induced Fourier transform ion cyclotron resonance mass spectrometry of organic and inorganic compounds: methodologies and applications

The combination of a laser with a Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS) enables a variety of MS experiments to be conducted. The laser can be used either as an intense photonic source for the photoionization of neutral species introduced in a variety of ways into the FTICR cell, or it can be made to directly interact with a solid, generating gas-phase ions. Depending on the experimental conditions used, various laser-matter interactions can occur. When high laser energy (also referred to as power density or irradiance) is used, laser ablation (LA) processes lead to the release of species into the gas phase, a significant fraction of which are ionic. The number of ions decreases with the irradiance. For low irradiance values, the so-called laser desorption (LD) regime applies, where the expelled species are mainly neutrals. LA–FTICRMS and LD–FTICRMS can be used to study a wide range of materials, including mineral, organic, hybrid and biological compounds (although matrix-assisted laser desorption ionization, MALDI, which is not reviewed in this paper, is more commonly applied to biological compounds). This paper will review a selection of methodological developments and applications in the field of laser ionization FTICRMS, LD–FTICRMS, and LA–FTICRMS for the analysis of organics and inorganics in complex mixtures, emphasizing insoluble materials. Specifically, silicate- and carbon-based complex materials as well as organic compounds will be examined due to their relevance to natural environmental and anthropogenic matrices.     

Effect of laser parameters on laser ablation and laser-induced plasma formation: A numerical modeling investigation

A comprehensive numerical model has recently been developed for nanosecond (ns) laser ablation of metallic targets, describing the processes of target heating, melting and vaporization, the resulting plume expansion in 1 atm helium gas, as well as plasma formation in the plume. In the present paper, we investigate the influence of laser parameters, i.e., laser irradiance, pulse duration and wavelength, on typical calculation results, such as the target temperature, melt and evaporation characteristics, the plume expansion velocity, plume (plasma) temperature and ionization degree, densities of neutrals, ions and electrons in the plume, as well as the laser absorption characteristics in the plume (plasma shielding). Comparison is made with experimental data from literature, whenever available, and in general, good agreement is reached between our model predictions and experimental results. Therefore, the model can be useful to predict trends in target and plume (plasma) characteristics, which are difficult to obtain experimentally.     

酞菁钴/纳米铁复合颗粒的结构与微波电磁特性

一些铁磁性金属及其合金超细粉末,具有比饱和磁化强度高、易于合成与粒径可控等优点,因此被广泛应用于磁流体、电磁流变液、微波吸收材料等领域,但由抗氧化性差、密度较大等主要缺点而使其应用受到限制,采用有机／无机复合技术可制备具有有机包覆层的有机／无机复合颗粒,不仅可以增强铁磁性颗粒的抗氧化性,减小了密度,而且可以提高与有机基体的亲和性,为该类材料向更高层次发展提供了可能。     

激光剥蚀-多接收电感耦合等离子体质谱法 测定含钚颗粒物中240 Pu/239 Pu比值

为高精度、准确地获取含钚颗粒物中具有核保障监督意义和核取证价值的钚同位素比值,建立了激光剥蚀-多接收电感耦合等离子体质谱(LA-MC-ICP-MS)测定含钚颗粒物中240 Pu/239 Pu的分析方法.采用检漏、安装排风罩和擦拭剥蚀池内壁等方式有效降低激光剥蚀产物沾污实验室和危及人身安全的潜在风险.联用扫描电迁移率粒径谱仪(SMPS)与激光剥蚀-多接收器等离子体质谱(LA-MC-ICP-MS)研究了激光剥蚀玻璃基体标样产生气溶胶的分布特性,结果表明,剥蚀产物的主要粒径是40~500 nm,应尽量采用水平管道连接激光剥蚀进样系统与MC-ICP-MS,含钚颗粒物分析后剥蚀池持续吹扫时间应大于15 min.采用外标归一化法离线校正质量分馏效应和离子计数器检测效率,建立了含钚颗粒物中240 Pu/239 Pu的LA-MC-ICP-MS分析方法,固定束斑直径30μm、脉冲重复率5 Hz、剥蚀时间5 s,调节能量密度使含钚颗粒物模拟样品中239 Pu的信号强度分别达2×104 cps和2×105 cps,本方法对240 Pu/239 Pu测量的相对实验标准不确定度小于1.4%(n=6),测量结果与参考值的相对偏差小于4.7%,仪器调试时间和单个样品测量时间分别为9.0和0.5 h.含钚颗粒物模拟样品分析结果表明,本方法精度高、结果准确、分析速度快,可满足核保障监督、禁产核查和核取证中含钚颗粒物直接分析的需求.     

A molecular dynamics study of the effects of the inclusion of dopants on ablation in polymethyl methacrylate

Molecular dynamics simulations are used to elucidate mechanisms of ablation in dopant-polymer systems. In one set of simulations, a uniform distribution of thermal absorbers are added to a polymethyl methacrylate substrate and are excited. Chemical decomposition occurs in the regions near the absorbers. Ejection of large pieces of substrate then follows the thermo-chemical breakdown of material. In another set of simulations, an absorbing cluster is embedded in the polymethyl methacrylate substrate at a depth of 50 or 250 A. Only the particles comprising the cluster are excited during the laser pulse. Ejection of material is initiated upon the fracture of the cluster and the cleavage of the surrounding polymer bonds with little chemical damage during the process. These two mechanisms of ejection suggest different pathways of ablation in doped polymer materials.     

Studies on Fundamental Processes During Laser Ablation using Time-of-Flight Mass-Spectrometry (TOF-MS)

When a short pulsed, high power laser is focused on any solid target,a portion of the material is instantaneously exploded into its vapor. Laser ablation is a term to describe this explosive laser material interaction. Various processes like ejection of ions,atoms and clusters, thermal evaporation, plasma initiation and expansion, interaction between the plasma and the target, may result.     

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.     

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.     

Pickering emulsion polymerization of graphene oxide-stabilized styrene

Polystyrene (PS) particles were prepared via Pickering emulsion polymerization using graphene oxide (GO) as the stabilizer. The results show that pH is an important factor in the stability of Pickering emulsions. The effects of two different phase initiators, the water phase initiator potassium persulfate and the oil phase initiator azobisisobutyronitrile, on the morphology of PS particles in Pickering emulsion polymerization had been investigated in detail. Wrinkled particles were prepared using the water phase initiator, and spherical particles were prepared using the oil phase initiator. In addition, hexadecane was used as the auxiliary stabilizer in the polymerization, which narrowed the diameter distribution of the PS spheres, and the hollow PS spheres were fabricated. The size of the GO particles also influenced the final morphology of the particles. Nano-sized polymer particles were grafted onto the surface of micro-sized GO. Small GO particles were suitable for Pickering emulsion polymerization to prepare the composite particles. The thermogravimetric analysis of the prepared particles confirmed that they were PS/GO composite particles, which could have a wide range of potential applications, such as in catalysts, sensors, environmental remediation, and energy storage.