N. L. LaHaye

Experimental and computational study of complex shockwave dynamics in laser ablation plumes in argon atmosphere

We investigated spatio-temporal evolution of ns laser ablation plumes at atmospheric pressure, a favored condition for laser-induced breakdown spectroscopy and laser-ablation inductively coupled plasma mass-spectrometry. The 1064 nm, 6 ns pulses from a Nd:YAG laser were focused on to an Al target and the generated plasma was allowed to expand in 1 atm Ar. The hydrodynamic expansion features were studied using focused shadowgraphy and gated 2 ns self-emission visible imaging. Shadowgram images showed material ejection and generation of shock fronts. A secondary shock is observed behind the primary shock during the time window of 100-500 ns with instabilities near the laser cone angle. By comparing the self-emission images obtained using fast photography, it is concluded that the secondary shocks observed in the shadowgraphy were generated by fast moving target material. The plume front estimates using fast photography exhibited reasonable agreement with data obtained from shadowgraphy at early times ≤400 n...

The effect of ultrafast laser wavelength on ablation properties and implications on sample introduction in inductively coupled plasma mass spectrometry.

We investigated the role of femtosecond (fs) laser wavelength on laser ablation (LA) and its relation to laser generated aerosol counts and particle distribution, inductively coupled plasma-mass spectrometry (ICP-MS) signal intensity, detection limits, and elemental fractionation. Four different NIST standard reference materials (610, 613, 615, and 616) were ablated using 400 nm and 800 nm fs laser pulses to study the effect of wavelength on laser ablation rate, accuracy, precision, and fractionation. Our results show that the detection limits are lower for 400 nm laser excitation than 800 nm laser excitation at lower laser energies but approximately equal at higher energies. Ablation threshold was also found to be lower for 400 nm than 800 nm laser excitation. Particle size distributions are very similar for 400 nm and 800 nm wavelengths; however, they differ significantly in counts at similar laser fluence levels. This study concludes that 400 nm LA is more beneficial for sample introduction in ICP-MS, particularly when lower laser energies are to be used for ablation.

The effect of laser pulse duration on ICP-MS signal intensity, elemental fractionation, and detection limits in fs-LA-ICP-MS

We investigated the role that pulse width has on the analytical capabilities of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), specifically in the range of 40 fs to 300 ps. Three main results of LA-ICP-MS were examined: signal intensity or sensitivity, elemental fractionation or that the ablated aerosol is representative of the bulk, and detection limits of various elements in the samples. The samples used for this experiment were NIST glass standards, dielectrics that exhibit different ablation properties compared to metal targets. Our results demonstrate that in the range of 40 fs to 1 ps, negligible differences occur in signal intensity, elemental ratios, and detection limits. The U/Pb and U/Th ratios, which were examined to ensure limited fractionation, give comparable results at all pulse widths investigated.

The influence of ns- and fs-LA plume local conditions on the performance of a combined LIBS/LA-ICP-MS sensor

Both laser-induced breakdown spectroscopy (LIBS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are well-established analytical techniques with their own unique advantages and disadvantages. The combination of the two analytical methods is a very promising way to overcome the challenges faced by each method individually. We made a comprehensive comparison of local plasma conditions between nanosecond (ns) and femtosecond (fs) laser ablation (LA) sources in a combined LIBS and LA-ICP-MS system. The optical emission spectra and ICP-MS signal were recorded simultaneously for both ns- and fs-LA and figures of merit of the system were analyzed. Characterization of the plasma was conducted by evaluating excitation temperature and electron density of the plume under various irradiation conditions using optical emission spectroscopy, and correlations to ns- and fs-LIBS and LA-ICP-MS signal were made. The present study is very useful for providing conditions for a multimodal system as well as giving insight into how laser ablation plume parameters are related to LA-ICP-MS and LIBS results for both ns- and fs-LA.

The influence of laser pulse duration and energy on ICP-MS signal intensity, elemental fractionation, and particle size distribution in NIR fs-LA-ICP-MS

Laser parameters, typically wavelength, pulse width, irradiance, repetition rate, and pulse energy, are critical parameters which influence the laser ablation process and thereby influence the LA-ICP-MS signal. In recent times, femtosecond laser ablation has gained popularity owing to the reduction in fractionation related issues and improved analytical performance which can provide matrix-independent sampling. The advantage offered by fs-LA is due to shorter pulse duration of the laser as compared to the phonon relaxation time and heat diffusion time. Hence the thermal effects are minimized in fs-LA. Recently, fs-LA-ICP-MS demonstrated improved analytical performance as compared to ns-LA-ICP-MS, but detailed mechanisms and processes are still not clearly understood. Improvement of fs-LA-ICP-MS over ns-LA-ICP-MS elucidates the importance of laser pulse duration and related effects on the ablation process. In this study, we have investigated the influence of laser pulse width (40 fs to 0.3 ns) and energy on LA-ICP-MS signal intensity and repeatability using a brass sample. Experiments were performed in single spot ablation mode as well as rastering ablation mode to monitor the Cu/Zn ratio. The recorded ICP-MS signal was correlated with total particle counts generated during laser ablation as well as particle size distribution. Our results show the importance of pulse width effects in the fs regime that becomes more pronounced when moving from femtosecond to picosecond and nanosecond regimes.

Femtosecond laser ablation-based mass spectrometry: An ideal tool for stoichiometric analysis of thin films

An accurate and routinely available method for stoichiometric analysis of thin films is a desideratum of modern materials science where a material’s properties depend sensitively on elemental composition. We thoroughly investigated femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs-LA-ICP-MS) as an analytical technique for determination of the stoichiometry of thin films down to the nanometer scale. The use of femtosecond laser ablation allows for precise removal of material with high spatial and depth resolution that can be coupled to an ICP-MS to obtain elemental and isotopic information. We used molecular beam epitaxy-grown thin films of LaPd(x)Sb2 and T′-La2CuO4 to demonstrate the capacity of fs-LA-ICP-MS for stoichiometric analysis and the spatial and depth resolution of the technique. Here we demonstrate that the stoichiometric information of thin films with a thickness of ~10 nm or lower can be determined. Furthermore, our results indicate that fs-LA-ICP-MS provides precise information on the thin film-substrate interface and is able to detect the interdiffusion of cations.

Persistence of uranium emission in laser-produced plasmas

Detection of uranium and other nuclear materials is of the utmost importance for nuclear safeguards and security. Optical emission spectroscopy of laser-ablated U plasmas has been presented as a stand-off, portable analytical method that can yield accurate qualitative and quantitative elemental analysis of a variety of samples. In this study, optimal laser ablation and ambient conditions are explored, as well as the spatio-temporal evolution of the plasma for spectral analysis of excited U species in a glass matrix. Various Ar pressures were explored to investigate the role that plasma collisional effects and confinement have on spectral line emission enhancement and persistence. The plasma-ambient gas interaction was also investigated using spatially resolved spectra and optical time-of-flight measurements. The results indicate that ambient conditions play a very important role in spectral emission intensity as well as the persistence of excited neutral U emission lines, influencing the appropriate spectral acquisition conditions.

Characterization of laser ablation sample introduction plasma plumes in fs-LA-ICP-MS

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a well-established elemental analysis technique which has been applied to a variety of fields. However, LA-ICP-MS still faces substantial challenges in the deviation of detected elemental concentrations from the known composition of the sample, or elemental fractionation. Applying optical emission spectroscopy (OES) during the LA process for LA-ICP-MS can help to characterize the emitted plasma plume and potentially identify sources of elemental fractionation during LA. We characterize LA sample introduction plume using OES during fs-LA-ICP-MS analysis of a brass sample. Fundamental plume parameters, i.e., electron temperature and density are estimated and correlated with ICP-MS signal characteristics, which give insight into how laser ablation plume parameters are related to LA-ICP-MS results. The hydrodynamic expansion features of fs-LA sample introduction plumes are evaluated using shadowgraphy and time- and spectrally-resolved fast-gated photography. Results showed dissimilarities in Cu I and Zn I expansion features. There is a very good correlation between the ICP-MS signal intensities and characteristic parameters of the sample introduction plumes. However, the crater formation during single spot ablation caused significant changes in line emission intensities though the estimated temperature and density showed constant values.

Correction: Characterization of laser ablation sample introduction plasma plumes in fs-LA-ICP-MS

Correction for ‘Characterization of laser ablation sample introduction plasma plumes in fs-LA-ICP-MS’ by N. L. LaHaye et al., J. Anal. At. Spectrom., 2014, DOI: 10.1039/c4ja00200h.

Crater formation and signal intensity in nano- and femto-second laser ablation inductively coupled plasma mass spectrometry

Inductively coupled plasma-mass spectrometry (ICP-MS) is a widely used analytical technique and produces highly accurate results. One of the main disadvantages of the technique, however, is the necessity of solid sample preparation into a solution; this is remedied by the use of laser ablation (LA) for direct solid sampling. LA is the process of delivering energy to a sample via a laser and, consequently, removing part of the sample and forming a small crater on the surface of the sample. Currently there exist several issues in LA sample introduction to ICP-MS commonly called ‘elemental fractionation’. A better understanding of fundamental laser ablation mechanisms and particle generation during LA process are necessary in order to efficiently couple the laser beam into the sample, ablate a reproducible quantity of mass, minimize the plasma shielding and fractionation, and control and optimize ablated particle transport.