Louis St-Onge

Enhanced laser-induced breakdown spectroscopy using the combination of fourth-harmonic and fundamental Nd:YAG laser pulses

Abstract We have studied the combination of fourth-harmonic (266 nm) and fundamental (1064 nm) Nd:YAG laser pulses of the same irradiance. On a metallic target (Al), a sequence of ultraviolet (UV) and near-infrared (NIR) pulses produces deeper craters and can lead under certain conditions to analyte signal enhancements larger than those obtained with a NIR–NIR sequence. Compared to a single NIR pulse, signal enhancements by factors of approximately 30 for the Si I 288.16-nm line and 100 for the Al II 281.62-nm line were observed with double pulses of the same total energy. This effect correlates with a substantial increase in plasma temperature, with ionic lines and lines having a higher excitation energy experiencing a larger enhancement. Moreover, the optimal pulse separation is found to be larger for ionic than for neutral lines (∼3 compared to ∼0.1 μs). Another finding of this study concerns the combination of two different wavelengths (266 and 1064 nm) in a single ‘mixed-wavelength’ pulse, a scheme that also leads to an enhanced laser-induced breakdown spectroscopy (LIBS) sensitivity. It is proposed that the double-pulse and mixed-wavelength approaches are both capable of temperature and signal enhancement for the same reason: a larger portion of laser energy is absorbed in the plasma region containing the analyte atoms, instead of being absorbed at the sample surface or in the atmosphere.

Quantitative analysis of pharmaceutical products by laser-induced breakdown spectroscopy

Abstract In this paper, the capabilities of laser-induced breakdown spectroscopy (LIBS) for rapid analysis of multi-component pharmaceutical tablets are illustrated using several examples. The atomic line emission from an element present only in a particular component of the tablet (for instance, emission of phosphorus from the drug, or of magnesium from the lubricant) enables the quantitative analysis of that component. It is also demonstrated that simple schemes can significantly improve the analytical performance of LIBS in this context. In particular, internal standardization with a carbon line was found to enable the correction of a matrix effect, apart from improving the precision of measurement. Furthermore, an improvement in the linearity of calibration was observed when the plasma continuum emission was used as internal standard. Finally, in the case of drugs containing halogen species (e.g. F or Cl), producing the plasma in a helium atmosphere caused a seven to eight-fold increase of the signal-to-background ratio, thus improving sensitivity. These data illustrate the strengths of LIBS for fast at-line assessment of the reliability of pharmaceutical manufacturing processes.

Analysis of solids using laser-induced plasma spectroscopy in double-pulse mode

Abstract We investigate plasmas formed from solid aluminium alloys in air using a Nd:YAG laser in double-pulse mode, in view of the possibility of enhancing analyte line emissions. In particular, time-resolved studies of emission characteristics and plasma properties are carried out. It is found that the Al II 281.6 nm line is considerably enhanced when using a double-pulse burst instead of a single pulse of equal energy. However, the electron density is found to be approximately the same in both cases, and the plasma temperature is less than 10% higher with the double-pulse burst. The line enhancement is rather explained by the formation of a larger volume of emitting gas. This, in turn, can be linked to a greater ablated mass, as well as to the presence of a preplasma into which the second laser pulse is absorbed. The influence of the interpulse interval on the peak intensity of the Al II line and of several neutral lines of different elements is also studied. For neutral lines, a maximum enhancement factor of 3–4 is attained with an interval in the range 0.5–1 μs. Finally, it is found that the relative standard deviation of 20 consecutive intensity measurements is reduced by a factor of 2–3 when going from single- to double-pulse mode.

Towards quantitative depth-profile analysis using laser-induced plasma spectroscopy: investigation of galvannealed coatings on steel

Abstract Laser-induced plasma spectroscopy (LIPS) is applied to depth-profile analysis, with the particular goal of examining how LIPS depth profiles can be fully calibrated. For this purpose, we concentrate on the representative case of galvannealed coatings on steel (i.e. annealed zinc-coated steel). In particular, a method is proposed wherein the second derivative of the zinc intensity profile enables the determination of the coating/substrate interface position. Calibration for the major elements (iron and zinc) is based on a non-linear relationship between the iron-to-zinc line intensity ratio and the iron-to-zinc concentration ratio. Quantitative depth profiles of three elements (Al, Fe and Zn) are obtained for two galvannealed samples. The iron profiles are found to be in broad agreement with those obtained by transmission electron microscopy/energy dispersive X-ray spectrometry.

An evaluation of a commercial Échelle spectrometer with intensified charge-coupled device detector for materials analysis by laser-induced plasma spectroscopy ☆

Abstract In this work we evaluate the performance of a commercial Echelle spectrometer coupled with an intensified charge-coupled device (ICCD) detector for the analysis of solid samples by laser-induced plasma spectroscopy (LIPS) in air at atmospheric pressure. We compare results obtained in aluminum alloy samples with this system and with a ‘conventional’ Czerny-Turner spectrometer coupled to an intensified photodiode array (IPDA). We used both systems to generate calibration curves and to determine the detection limit of minor elements, such as Mg, Cu, Si, etc. Our results indicate that no significant differences in terms of analytical figures of merit exist between the Echelle/ICCD system and a conventional Czerny-Turner spectrometer with IPDA. Moreover, measurements of plasma temperature and electron density using the two assemblies give, in general, very similar results. In the second part of this work, we aim to present a critical view of the Echelle spectrometer for LIPS applications, by drawing up the balance sheet of the advantages and limitations of the apparatus. The limitations are either inherent to the dispersion method, or result from the dynamic range of the detector. Moreover, the minimum ICCD readout time does not allow a fast data acquisition rate. On the other hand, the Echelle spectrometer allows complete elemental analysis in a single shot, as spectral lines of major, minor and trace constituents, as well as plasma parameters, are measured simultaneously. This enables a real-time identification of unknown matrices and an improvement in the analytical precision by selecting several lines for the same element.

Quantitative analysis of additives in solid zinc alloys by laser-induced plasma spectrometry

In this paper the use of laser-induced plasma spectrometry (LIPS) for the quantitative analysis of Al, Cu, Fe, Pb and Sn components in solid zinc alloys is evaluated. Laser-induced plasmas are characterized using spectroscopic diagnostic techniques that yield the excitation temperature and electron density. Optimal experimental conditions for analysis are evaluated, including time gating parameters and distance from focusing lens to target where it is found that the focus of the laser beam should be positioned behind the target in order to prevent secondary air plasmas from forming in front of the target. Calibration curves are produced for several analytical lines, and the analytical performance of the technique is assessed. While low detection limits (<60 ppm) are found, the precision of measurement could be improved.

Influence of Er:YAG and Nd:YAG wavelengths on laser-induced breakdown spectroscopy measurements under air or helium atmosphere

Laser-induced breakdown spectroscopy (LIBS) is widely dependent on the conditions of its implementation in terms of laser characteristics (wavelength, energy, and pulse duration), focusing conditions, and surrounding gas. In this study two wavelengths, 1.06 and 2.94 microm, obtained with Nd:YAG and Er:YAG lasers, respectively, were used for LIBS analysis of aluminum alloy samples in two conditions of surrounding gas. The influence of the laser wavelength on the laser-produced plasma was studied for the same irradiance by use of air or helium as a buffer gas at atmospheric pressure. We used measurements of light emission to determine the temporally resolved space-averaged electron density and plasma temperature in the laser-induced plasma. We also examined the effect of laser wavelength in two different ambient conditions in terms of spectrochemical analysis by LIBS. The results indicate that the effect of the surrounding gas depends on the laser wavelength and the use of an Er:YAG laser could increase linearity by limiting the leveling in the calibration curve for some elements in aluminum alloys. There is also a significant difference between the plasma induced by the two lasers in terms of electron density and plasma temperature.

A mathematical framework for modeling the compositional depth profiles obtained by pulsed laser ablationPresented at the 2002 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, USA, January 6???12, 2002.

This paper presents a simple mathematical model relevant to studies of depth-profile analysis utilizing pulsed laser ablation. The main model predictions (the amount of analyte ablated by a given laser shot, and the resulting crater profile) are independent of the method used to detect the analyte, so that the results are relevant to several techniques: laser-induced breakdown spectrometry (LIBS), laser-ablation inductively coupled plasma optical emission spectrometry or mass spectrometry (LA-ICP-OES or LA-ICP-MS) or laser-ablation time-of-flight mass spectrometry (LA-TOF-MS). The main focus of the model is on the influence of the laser beam radial energy distribution on the depth profiles. Accordingly, super-Gaussian (top-hat) as well as Gaussian laser beams can be modeled. Although the model can be used to simulate depth profiles in samples where the analyte concentration varies continuously as a function of depth, a particular emphasis of this paper is on multilayer samples. The notion of fluence threshold for ablation is also introduced, and the influence of crater aspect ratio is studied. The model is tested in relation to LIBS studies of galvannealed coatings on steel that use a Gaussian Nd∶YAG laser beam at 1064 nm. The simulated depth profiles are found to correctly reproduce the shape of an experimental depth profile of the zinc signal, and the influence of specific model parameters is investigated.

Laser-induced breakdown spectroscopy: A new tool for materials analysis

The laser-induced breakdown spectroscopy (LIBS) technique has shown in recent years its great potential for rapid quantitative analysis of materials. In this paper, we will present work carried out in our laboratory that has addressed the influence of the main experimental parameters on the quality of LIBS measurements. In particular, we will discuss new approaches (double-pulse mode, ultrashort laser pulse, mixed–wavelength pulse, Echelle spectrometer, etc.) and their impact on the sensitivity and the accuracy of the LIBS technique. Finally, we will describe our application of LIBS to the analysis of liquid samples.The laser-induced breakdown spectroscopy (LIBS) technique has shown in recent years its great potential for rapid quantitative analysis of materials. In this paper, we will present work carried out in our laboratory that has addressed the influence of the main experimental parameters on the quality of LIBS measurements. In particular, we will discuss new approaches (double-pulse mode, ultrashort laser pulse, mixed–wavelength pulse, Echelle spectrometer, etc.) and their impact on the sensitivity and the accuracy of the LIBS technique. Finally, we will describe our application of LIBS to the analysis of liquid samples.

On-line analysis of liquid samples by laser induced plasma spectroscopy (LIPS)

A LIPS monitor for real-time analysis of aqueous and other liquids has been developed. This system enables the laser to sample a fresh surface while preventing problems associated with the laser liquid interaction. We outline its successful laboratory and on-line industrial operation.