Joachim Koch

Particle size distributions and compositions of aerosols produced by near-IR femto- and nanosecond laser ablation of brass

Particle size distributions and compositions of primary aerosols produced by means of near-IR femtosecond laser ablation (λ = 775 nm) of brass in He or Ar at atmospheric pressure have been measured. Aerosols were characterized using a 13-stage low-pressure impactor covering a size range from 5 nm up to 5 μm and subsequently analyzed applying total reflection X-ray fluorescence spectrometry. The results indicate, that for femtosecond laser ablation in the low-fluence regime (<5 J cm−2) ultra-fine aerosols (mean diameter dp ≈ 10 nm/peak width wp ≈ 35 nm) are produced. Furthermore, the total Cu/Zn ratio of these aerosols corresponds to the composition of the bulk material. In contrast, ablation above 10 J cm−2 results in the formation of polydisperse, bimodal aerosols, which are distributed around dp1 ≈ 20 nm (wp1 ≈ 50 nm) and dp2 ≈ 1 μm (wp2 ≈ 5 µm), respectively, and whose total Cu/Zn ratio slightly deviates from the bulk composition. In order to examine the influence of pulse duration on particle size distribution and aerosol composition, comparative measurements by means of near-IR nanosecond ablation were also performed. The data show that nanosecond ablation generally leads to an intensified formation of particles in the micrometer range. Moreover, the composition of these aerosols strongly departs from the stoichiometry of the bulk. Aspects concerning the formation of particles during ablation as well as implications for the element-selective analysis by inductively coupled plasma spectrometry are discussed.

Review of the State-of-the-Art of Laser Ablation Inductively Coupled Plasma Mass Spectrometry

This paper is a review of the basic principles and recent developments of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a method for the element- and isotope-selective trace analysis of solid materials. In the course of this review, the aerosol formation/transportation process, quantification issues, as well as technical aspects concerning the system configuration and ICP operating conditions are outlined. Furthermore, the performance of femtosecond (fs) LA-based analyses as one of the most important advancements made over the past years is discussed. The benefits offered by fs-LA in comparison to LA using nanosecond (ns) laser sources are demonstrated on the basis of oxide layer and silicate glass analyses with different applied calibration strategies.

Performance characteristics of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry at ∼265 and ∼200 nm

The analytical figures of merit of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) using the 3rd and 4th harmonics of Ti:Sapphire (∼265 and ∼200 nm, respectively) were explored. For this purpose, elemental ratios of aerosols produced by LA of silicate glass (SRM NIST 610) were studied under varying fluence conditions ranging from moderate values of 2 J cm−2 up to 30 J cm−2, taking into account e.g. laser-induced (66Zn/65Cu) and particle size-related (238U/232Th) phenomena. It could, for instance, be shown that signal ratios were less dependent on the wavelength or laser repetition rate chosen. Furthermore, fractionation indices defined using the temporal drift of elemental ratios over two equal parts of the acquired signal were subject to systematic changes for threshold-close fluences. As a consequence, corresponding 42Ca-normalized values were found to deviate by more than 20% from unity. In contrast, LA at higher fluences resulted in less pronounced discrepancies, falling below 5% even for the most critical elements such as 66Zn, 111Cd, and 208Pb. The complete suppression of particle size-related fractionation quantified on the basis of the 238U/232Th-system turned out to be highly consistent with the absence of μ-sized particles which were measured by optical particle counting (OPC). The relative fraction of particles >0.5 μm was determined to be less than 5%, independent on the wavelength, fluence, or laser repetition rate chosen. Moreover, our results indicate the occurrence of ICP-induced elemental fractionation during analysis due to an increased mass loading of the plasma source if medium or high fluences are applied. Using the 66Zn/65Cu-ratio as a thermometric probe, the change in plasma ionization temperature among low and high mass loading conditions was estimated to be −900 K. Nevertheless, UV-fs-LA-ICP-MS analysis of different matrices (silicate glass SRM NIST 610 and brass (Zn ∼20%)) performed within the high fluence range was found to enhance the accuracy for non-matrix-matched calibration. Evidence is given that the enhancement observed mainly depends upon the suppression of laser- and/or transport-induced fractionation.

Capabilities of inductively coupled plasma mass spectrometry for the detection of nanoparticles carried by monodisperse microdroplets

Recently, first analyses of single sub-micrometre particles, embedded in liquid droplets, by inductively coupled plasma optical emission spectrometry (ICP-OES) with a size-equivalent detection limit of several hundred nanometres were reported. To achieve lower detection limits which might allow for the analysis of particles in the nanometre size range a more sensitive technique such as mass spectrometry (MS) is required. Various modifications of particle delivery and data acquisition systems commonly used were carried out to install a setup adequate for ICP-MS detection. These modifications enabled us to supply droplets generated by a commercial microdroplet generator (droplet size: 30–40 µm) with nearly 100% efficiency and high uniformity to the ICP. Analyses were performed using both standard solutions of dissolved metals at concentrations of 1 (Ag), 2 (Au), 5 (Au), or 10 (Cu) mg L−1 and highly diluted suspensions of gold and silver nanoparticles with sizes below 110 nm. In doing so, detection efficiencies of 10−6 counts per atom could be achieved while size-related limits of quantification were found to be 21 nm and 33 nm for gold and silver, respectively. Furthermore, the advantages of utilizing microdroplet generators vs. conventional nebulizers for nanoparticle analyses by ICP-MS are discussed.

Non-matrix matched calibration of major and minor concentrations of Zn and Cu in brass, aluminium and silicate glass using NIR femtosecond laser ablation inductively coupled plasma mass spectrometry

The feasibility of using near-infrared (NIR) femtosecond laser ablation (fs-LA) inductively coupled plasma mass spectrometry (ICP-MS) for the analysis of solid samples with non-matrix matched standard reference materials was studied. Major and minor concentrations of Zn and Cu (433 μg g−1–0.95 g g−1) were measured in three sets of metallic and dielectric standards (brass, aluminium, silicate glass) using He as the ablation cell gas and, with admixed Ar, for aerosol transportation from the cell into an Ar-ICP-MS instrument. Not surprisingly for ICP-MS detection, the experimental Zn/Cu ion ratios were found to be dependent on the sampler cone position in the plasma. However, at a fixed sampler position the experimental Zn/Cu ratios of the brass and Al samples were found to be proportional to the certified ratios independent of the laser fluence applied (range: 2–42 J cm−2). In contrast, the Zn/Cu ratio of an optical transparent glass sample (NIST 610) was found to be strongly fluence dependent. However, with increasing fluence the measured ratio asymptotically approached the experimentally expected ratio taking into account the results obtained from the brass and aluminium measurements.

Elemental fractionation of dielectric aerosols produced by near-infrared femtosecond laser ablation of silicate glasses

The composition and fractionation properties of dielectric aerosols generated by near-infrared femtosecond laser ablation of silicate glass (carrier gas: helium, 1 atm) have been examined. Aerosols were classified using low-pressure impaction of particles with diameters from 7 nm up to 7 μm. The element-selective analyses of impacted material has been restricted to minor matrix constituents (nominal concentration ∼4.5%) applying total reflection X-ray fluorescence. It has been found that for fluences larger than 5 J cm−2 the total Zn-, Ca-, Sr-, Ba-, and Pb-specific composition of these aerosols corresponds to that of the bulk material even though the size-dependent particle composition strongly altered. Typical deviations were of the order of 5–10%. In contrast, fluences below 5 J cm−2 usually resulted in stronger differences from the bulk composition indicating intensified fractionation during the ablation process. Our results furthermore demonstrate, that the major fraction of the aerosol mass is located within the mesoscopic size range, i.e. from 10 up to 100 nm, fairly independent on the fluence applied. However, the relative percentage of micrometer particles has been found to significantly decrease for higher fluences. Scanning electron microscopy of impacted brass particles moreover revealed a fractal-like structure of deposits. Implications for the classification of such structures using low-pressure impaction are discussed.

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

Abstract 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.

Non-matrix matched calibration using near-IR femtosecond laser ablation inductively coupled plasma optical emission spectrometry

The facilities of near-IR (NIR) femtosecond laser ablation (fs LA) inductively-coupled plasma optical emission spectrometry (ICP-OES) for the analysis of solid materials applying non-matrix matched standards have been examined. Three sets of metallic and dielectric standards (brass, aluminium, silicate glass) have been analyzed for Zn and Cu using He as buffer gas. It has been found that the measured Zn/Cu ratio of these standards remains nearly unchanged in the fluence range considered (0.8 J cm−2 ≤ F ≤ 11.0 J cm−2). At fluences close to the threshold, however, the Zn/Cu ratio for brass and aluminium tends to slightly increase, suggesting preferential evaporation of Zn during the ablation process. Furthermore, LA above a fluence of 5 J cm−2 resulted in linear, matrix-independent calibration graphs. According to the results achieved, NIR fs LA-ICP-OES carried out in the medium and high fluence regime permits efficient suppression of fractionation and performance of accurate analyses even if non-matrix matched standard materials have to be used.

Evaluation of different techniques for particle size distribution measurements on laser-generated aerosols

Knowledge about aerosol particle sizes in laser-generated aerosols is required to better understand non-stoichiometric effects as they occur during laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). However, published particle size distributions measured by different techniques provide an inconclusive picture, since measurements base on different physical properties of a particle.This is especially the case for the size determination of agglomerated nano-particles, which contribute significantly to the total particle mass transported to the ICP. While their aerodynamic diameters measured by particle impaction are always smaller than the volume equivalent ones (da dve). This leads to large discrepancies between measured da and dm values, which is shown within this work on brass aerosols generated by laser ablation. Results of light scattering measurement, which is a quick and simultaneous method, show particle diameters in between impaction and DMA data. However, it is important to know that the different methods are not in contradiction to each other, but measure specific physical properties of a particle and complement each other. Results of this study demonstrate the advantages and limitations of the individual particle size measurement techniques and should be seen as guidance to select the adequate technique for laser generated aerosol particle size determinations.

Laser ablation characteristics of yttria-doped zirconia in the nanosecond and femtosecond regimes

The laser ablation characteristics of yttria-stabilized zirconia (YSZ) have been investigated as a function of the target microstructure and dopant level for different nanosecond- [ArF, KrF, and XeCl excimers; Nd:YAG (yttrium aluminum garnet) (fourth harmonic)] and femtosecond-laser sources [Ti:sapphire (fundamental and third harmonic)]. Particle ejection, which compromises the quality of coatings prepared by pulsed laser deposition (PLD), was analyzed in detail. Nanosecond-laser pulses cause a severe thermomechanical surface cracking and exfoliation of micron-sized fragments on a microsecond to millisecond time scale in the case of 8–9.5 mol % Y2O3-doped, fully stabilized zirconia (8YSZ and 9.5YSZ) targets. As a consequence of the intrinsic material brittleness, fully stabilized YSZ coatings deposited by PLD contained particles for all tested conditions. Lower doped partially stabilized zirconia (3YSZ) exhibits a superior fracture toughness attributed to a laser-induced partial transition to the monoclinic phase, detected by Raman spectroscopy, which enables the deposition of particle-free dense thin films by conventional PLD using nanosecond-UV laser radiation at moderate fluences of 1.2–1.5 J/cm2. The ablation dynamics of ultrashort laser pulses differ fundamentally from the nanosecond regime as evidenced, e.g., by time-resolved shadowgraphy and light scattering experiments. Femtosecond pulses prevent the exfoliation of micron-sized fragments but result invariably in a pronounced ejection of submicron particles. The resulting PLD coatings are porous and reveal a large surface roughness as they consist of an agglomeration of nanoparticles. Femtosecond-NIR pulses provide a factor of 2.5–10 higher material removal rates compared to nanosecond- and femtoseco- nd-UV pulses. The ablation metrics, i.e., threshold fluence and effective absorptivity, mainly depend on the laser wavelength while the pulse duration, target microstructure, and dopant level are of minor importance. Evidence is presented that incubation effects play a significant role in nanosecond- and femtosecond-laser ablations of YSZ enabling material removal at comparatively low fluences for sub-bandgap photon energies.