Isotopic analysis and plasma diagnostics for lithium detection using combined laser ablation–tuneable diode laser absorption spectroscopy and laser-induced breakdown spectroscopy

https://doi.org/10.1016/j.sab.2020.106051Get rights and content

Highlights

  • Measurement of lithium isotope ratio by LA-TDLAS.

  • Promising new approach for determining isotope ratios without sample preparation.

  • Dual pulse techniques applied to laser absorption of ablation plasmas.

  • Determination of electron density of laser ablation plasma from LIBS.

  • Direct comparison between LIBS and LA-TDLAS.

Abstract

Laser-Induced Breakdown Spectroscopy (LIBS) enables rapid stand-off measurements of potentially hazardous material which is particularly favourable for applications in the nuclear industry. Isotopic characterisation with LIBS remains challenging because of the narrow separation of isotopic emission lines, which demands large, high resolution spectrometers. To address this, laser ablation (LA)-Tuneable Diode Laser Absorption Spectroscopy (TDLAS) has been combined with LIBS to analyse laser-produced plasma of lithium. Isotope ratio calculations were performed by fitting multiple peaks to the Li absorption spectra, achieving a relative error of 13% at later delay times after line broadening had reduced. Double-pulsed laser ablation resulted in narrower absorption peaks but more noise in the absorption spectra. LIBS and LA-TDLAS spectra were used simultaneously to calculate plasma temperature and electron density for both single- and double-pulsed laser ablation. The Doppler broadening of absorption peaks from LA-TDLAS was used to calculate the plasma temperature after long time delays of 200 μs and the temperature followed an exponential decay which was extrapolated back to early delay times in order to predict the temperature throughout the plasma lifecycle. The electron density was calculated using the Stark broadening of emission peaks from the LIBS spectra, which were temperature-corrected by accounting for the Doppler broadening. Both single- and double-pulsed-laser ablation plasmas exhibited maximum electron densities of around 1016 to 1017 cm−3, although the decay rate was reduced by around a factor of 2 using DP-laser ablation. We have shown that the combination of emission and absorption spectroscopy with LIBS and LA-TDLAS is useful for isotopic analysis and calculating laser ablation plasma properties. We have demonstrated that double pulsed laser ablation has the potential to enable more isotopic pairs to be analysed due to the narrower absorption linewidth.

Introduction

Laser ablation (LA) -based analytical techniques offer rapid and robust atomic information, often irrespective of a samples' surface finish or physical state. LA-Inductively Coupled Plasma Mass Spectroscopy (LA-ICP-MS) and Laser-Induced Breakdown Spectroscopy (LIBS) are two examples of how commercially available pulsed lasers have rapidly advanced the field of atomic spectroscopy. Of these two techniques, LIBS is more adaptable to field-deployment and portability, as it does not require a special sample chamber or vacuum, allows standoff capabilities which can shield the user from harmful material [[1], [2], [3]], requires zero or minimal sample preparation, can extract multi-elemental information from any sample material and enables rapid measurements to be performed, thereby allowing in-line analysis to be achieved [[4], [5], [6]]. Problems associated with other elemental analysis techniques which can be used for in-line analysis, such as X-Ray Fluorescence (XRF) or Spark Discharge Optical Emission Spectroscopy (SD-OES), can be overcome by LIBS and the technique is amenable to the detection of a broader range of analytes than these methods [7].

Line broadening mechanisms, particularly Stark broadening, and bremsstrahlung background emission reduce the spectral quality of LIBS [8]. These drawbacks can be managed with control of the time delay and width of the acquisition window, but ultimately limit the potential for quantitative analytical data and isotopic information to be obtained from a measurement. Numerous attempts have been made to increase the spectral resolution and improve the detection limits which can be achieved by LIBS: these include the use of multiple laser pulses for the ablation process [[9], [10], [11], [12]]; hardware additions such as spectrometer enhancements [13,14], magnetic fields [15], and electric discharges [16]; chemometric methods [[17], [18], [19], [20], [21], [22]]. Many of chemometric techniques have become commonplace, particularly multivariate data analyses such as Partial Least Squares (PLS) and Principal Component Analysis (PCA).

Double-Pulsed-LIBS (DP-LIBS) has been shown to have an improved analytical performance over Single-Pulsed-LIBS (SP-LIBS) in terms of Limit of Detection (LoD) and shot-to-shot variation [9,23,24]. In DP-LIBS the first laser pulse ablates the sample and creates the plasma whilst the second pulse re-heats the already-vaporised analyte. Many different adaptations of DP-LIBS have been reported. A collinear design is often used, with both laser pulses focussed from vertically above the sample surface. This has been demonstrated with multiple-pulses from the same laser [25] and with separate lasers aligned coaxially [11,26,27]. Orthogonal orientations, in which a second laser pulse interacts with the ablation plume from a position parallel to the sample surface, have also been realised [[28], [29], [30]]. Firing the orthogonal laser as a pre-pulse to rarefy the sheath gas also changes the dynamics of plume expansion and can improve the emission intensity (i.e. reduce bremsstrahlung effects) [28,31]. Additionally, combinations of various laser wavelengths [11], spot sizes [32], pulse energies [12] and pulse lengths [10,33] have been studied. Many of these have demonstrated significant improvements in terms of Signal to Noise Ratio (SNR) and LoDs compared to SP-LIBS.

Characteristics of LIBS such as standoff capability and immediate in situ quantification suggest the technique would be particularly suited to applications within the nuclear fuel cycle. However, a challenge limiting implementation of this approach in the nuclear industry is the inability of LIBS to offer isotopic characterisation of materials without the use of expensive, bulky, low light throughput spectrometers. Isotopic LIBS continues to be difficult to achieve because of the narrow separation of isotopic emission lines combined with line-broadening mechanisms within the laser ablation-produced plasma (LPP). For example, the largest separation of emission lines between the 235U and 238U isotopes is 25 pm [34], and for 239Pu and 240Pu just 13 pm [35]. Isotope shifts are caused by the mass shift and the volume (or ‘field’) shift. For elements of lower atomic mass such as H and Li, a change in mass caused by the addition of one neutron has a large effect in relative terms, whereas for the heavier elements such as U and Pu it is negligible. However, the volume shift is greater for heavier elements. As such, isotope shifts are greatest for elements at the extremities of the periodic table. The elements towards the centre of the periodic table exhibit very small isotope shifts (less than 1 pm for most elements) [36].

Tuneable semiconductor diode lasers have enhanced atomic spectroscopy because of their narrow spectral linewidth, ever increasing spectral coverage, high stability, low cost, small size and low power requirements [[37], [38], [39]]. Detection limits of the order of pg/g have been achieved in elemental analyses performed using Tuneable Diode Laser Absorption Spectroscopy (TDLAS) [40] under appropriate conditions. Isotope-selective measurements can be performed using TDLAS: the required high resolution provided by the narrow laser linewidth [41]. Absorption spectroscopy is generally carried out on gas and liquid samples, as light must be detected beyond a sample and compared to a reference beam. As such, an integral part of TDLAS is atomisation of solid samples. This has been accomplished in the past using graphite furnaces [[42], [43], [44]], flames [[45], [46], [47]] and plasma [48,49] atomisers.

LA-TDLAS facilitates atomisation of solid samples with minimal sample preparation, whilst maintaining the benefits of standoff, rapid and in situ sampling associated with laser spectroscopy. Additionally, for certain elements, LA-TDLAS could determine isotopic distributions which are not easily achievable by other field-deployable techniques. As such, LA-TDLAS has been proposed as a method to analyse uranium and plutonium isotopics in the nuclear industry [[48], [49], [50], [51], [52], [53], [54]]. Most literature to date has focussed solely on absorption spectroscopy and not utilised the emissions from the LPP in an analytical manner (i.e. using LIBS). Additionally, there are few studies [55] conducted at atmospheric pressure due to the challenges associated with the complex LA process. Hyphenating the elemental analysis of LIBS with the isotopic analysis of LA-TDLAS would create a combined method which could find use throughout the nuclear fuel cycle. The benefits of laser-based spectroscopy of high sample throughput, minimal sample preparation, portable experimental hardware and the opportunity for stand-off analysis would be maintained with the two complimentary analyses. The combined strategy would also be appealing to the field of nuclear forensics [56,57]. The combined analysis could also use complimentary information to improve overall accuracy. For example, in most laser ablation experiments the temperature is estimated from prior knowledge of the equipment parameters and the sample material, whereas in these experiments the LA-TDLAS results were used to calculate the temperature more accurately. This led to more accurate electron density calculations as the line broadening caused by temperature (Doppler broadening) could be accounted for.

In this study, a combined LA-TDLAS with LIBS setup has been constructed and tested for analytical performance under atmospheric conditions. Lithium was chosen as a sample material because the Li isotope shift is comparable to that found in nuclear materials (6Li ↔ 7Li = 15 pm; 235U ↔ 238U = 25 pm; 239Pu ↔ 240Pu = 13 pm). It was important to understand the laser ablation plasma's physical conditions of temperature and electron density to improve the accuracy of the isotope predictions. As such, temporal changes of temperature and electron density were monitored by recording simultaneous absorbance (LA-TDLAS) and emission (LIBS) measurements. Due to the aforementioned benefits of DP-LIBS on the emission spectra, combined LA-TDLAS and LIBS measurements were also performed with DP-LA. It was hoped that the longer plasma lifetime and better shot-to-shot consistency achieved with DP-LA should lead to a longer time period for LA-TDLAS measurements to take place and improve the signal to noise ratio. Increasing the plasma lifetime could enable rapid wavelength ramping across several atomic or isotopic transitions as the plasma remains in a quasi-static state for a longer duration.

Section snippets

Laser ablation – tuneable diode laser absorption spectroscopy

The ablation laser was a Q-switched Nd:YAG laser (Innolas Spitlight 600) operating at the fundamental wavelength of 1064 nm with a 6 ns pulse duration and a pulse energy of 85 mJ/pulse. The sample chamber (with xyz mobile stage) and beam focussing optics were designed and manufactured by Applied Photonics ltd. (Skipton, UK).

In brief, the TDLAS setup consisted of a collimated laser diode with a wavelength centred around 671 nm (Roithner Lasertechnik QL67F6SA) with a beam diameter of 1 mm, two

Plasma temperature and isotope ratio calculation using LA-TDLAS of the 670.78 nm Li D lines

Fig. 2 displays the temporal changes in emission and absorbance. The emission profile shows the area of a Voigt fit of the 670 nm Li I emission line. The 670 nm emission line showed large self-absorption and so could not be accurately quantified, and the resulting area of fit serves only as a qualitative guide to show the temporal differences between absorption and emission. The absorbance lines show two laser diode wavelengths: on peak centre of 7Li D1 and away from the absorption peak at

Conclusion

We have demonstrated an experimental setup of combined LA-TDLAS with LIBS which is capable of rapid evaluation of isotopic and elemental information from a sample, and demonstrated the feasibility of the technique with lithium samples. This combination of analytical techniques maintains the benefits of traditional LIBS (rapid, standoff, no sample preparation, non-destructive) and additionally allows isotopic measurements which are normally impossible due to the small isotope shift of electronic

Author statement

GH: Methodology, Investigation, Software, Formal analysis, Writing; EM: Conceptualisation, Writing – Reviewing and Editing; PC: Methodology, Resources; PM: Conceptualisation, Methodology, Supervision, Writing – Reviewing and Editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The Authors would like to thank the Atomic Weapons Establishment (AWE plc) and the EPSRC Materials for Demanding Environments Centre for Doctoral Training (M4DE CDT) for funding of the PhD research project, and AWE for the loan of the LIBS apparatus. Also, we would like to thank Andrew Murray from the University of Manchester for his help with the construction of the external cavity diode laser.

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