Kay Niemax

A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples

The ablation of brass samples in argon shield gas by 170 fs and 6 ns laser pulses has been studied by optical emission spectroscopy of the evolving plasmas. Differences observed in the temporal behavior of the spectral line intensities are explained by the shielding effect of the Ar plasma for ns-pulses and the free expansion of the plasma of the ablated material in case of fs-pulses. Brass with different ZnrCu ratios were used as samples. Different types of crater formation mechanisms in the case of ns- and fs-pulses were observed. At 40 mbar argon pressure the thresholds of ablation were found to be ; 0.1 and ; 1.5 J cm y2 for fs- and ns-pulses, respectively. With an internal standardization of zinc to copper it is possible to correct for differences in the ablation rates and to obtain linear calibration curves. For optimum experimental conditions, narrower confidence intervals for the determination of unknown concentrations were found in case of fs-pulses. Within the range of the laser intensities used, no dependence of the ZnrCu line intensity ratio on the number of laser pulses applied to the same ablation spot was observed, neither for fs- nor for ns-pulses, which is interpreted as the absence of fractional vaporization. Q 2000 Elsevier Science B.V. All rights reserved.

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.

Reheating of a Laser-Produced Plasma by a Second Pulse Laser

Focused radiation of a Nd:YAG laser was used for ablation and production of free sample atoms from electrically conducting and nonconducting solids for analytical purposes. The microplasmas were generated in an argon buffer gas atmosphere at reduced pressure. In order to increase the intensities of analyte lines the microplasmas were reheated by a second Nd:YAG laser after the process of atomization was completed. It is demonstrated for aluminum and manganese in glass and steel samples and for magnesium and manganese in glass, copper, and aluminum that by internal standardization a common calibration curve can be obtained.

Basic investigations for laser microanalysis: III. Application of different buffer gases for laser-produced sample plumes

We have measured the diameters and depths of craters in a copper sample and the amount of material ablated by the 1.06-μm radiation of a pulsed Nd: YAG laser in the buffer gases argon, neon, helium, air and nitrogen as well as the emission intensities of analyte atoms in dependence on laser power and buffer gas pressure. The results are correlated with corresponding data of the plasma temperatures and the relative electron densities in the plasma. Criteria for the choice of the buffer gas, the buffer gas pressure and the laser power for optical emission spectrometry of microplasmas are given.

The dielectric barrier discharge — a powerful microchip plasma for diode laser spectrometry

The dielectric barrier discharge plasma is presented as a powerful microchip source for analytical spectrometry. The dielectric barrier discharge is characterized by small size, low electric power (<1 W), low gas temperature (approx. 600 K) and excellent dissociation capability for molecular species, such as CCl2F2, CClF3 and CHClF2. It has been used here in the plasma modulation diode laser absorption spectrometry of excited chlorine and fluorine in noble gases as well as in air/noble gas mixtures. The analytical figures of merit of diode laser absorption spectrometry obtained with the dielectric barrier discharge are comparable with the results found earlier with dc and microwave induced plasmas of larger size with much higher plasma powers. Detection limits of 400 ppt and 2 ppb for CCl2F2 in He were found using the Cl 837 nm and the F 685 nm line, respectively.

Basic investigations for laser microanalysis: I. Optical emission spectrometry of laser-produced sample plumes

In order to find optimum conditions for laser ablation and atomization for analytical purposes, time resolved emission spectra of Nd:YAG laser-produced plasma plumes propagating into noble gas atmospheres of different pressures were measured with an optical multichannel analyzer. The time evolution of the plasma temperature was determined. Strong temperature changes were observed depending on the matrix composition. The analytical figures of merit of optical emission spectrometry (OES) of laser-produced sample plasmas were determined using, as examples, measurements of two analytes (silicon and chromium) in homogeneous and low-alloyed standard steel samples.

Laser ablation inductively coupled plasma mass spectrometry—current shortcomings, practical suggestions for improving performance, and experiments to guide future development

Experimental parameters and processes important in laser ablation such as the wavelength, fluence and pulse width of the laser, the form of the ablation cell, the cell gas, the particle transport and atomization in the ICP have an impact on the analytical performance of laser ablation inductively coupled mass spectrometry. The paper discusses the influence of the aforementioned parameters and processes on LA-ICP-MS and presents proposals to map out strategies for optimum LA-ICP-MS analyses. Additionally, experiments are identified which still need to be done for better understanding of the relevant processes and further improvement of the analytical performance of the technique.

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.

Microplasmas for analytical spectrometry

Recent developments of miniaturized powerful and robust plasmas for analytical applications are reviewed. The plasmas described and discussed may be used for analyte detection in chromatographic or electrophoretic “lab-on-a-chip” systems.

Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry

Abstract A diode laser is used for the selective excitation of 235 U and 238 U in a laser-induced plasma applying Nd:YAG laser pulses to UO 2 samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ (880 MW cm −2 ). Three samples with 0.204, 0.407 and 0.714% 235 U were measured. For example, for the UO 2 pellet with the natural uranium isotopic composition (99.281% 238 U and 0.714% 235 U), the accuracy and precision were 7% and 5% (460 shots), respectively, limited by the continuum emission background from the laser-induced plasma.