Laser Ablation Molecular Isotopic Spectrometry for ...

30 Spectroscopy 29(6)

June 2014

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Laser Ablation Molecular Isotopic Spectrometry for Rare Isotopes of the Light Elements Laser ablation molecular isotopic spectrometry (LAMIS) involves measuring isotope-resolved molecular emission. Measurements of several key isotopes (hydrogen, boron, carbon, nitrogen, oxygen, and chlorine) in laser ablation plumes were demonstrated. Requirements for spectral resolution of the optical detection system could be significantly relaxed when the isotopic ratio was determined using chemometric regression models. Multiple applications of LAMIS are anticipated in the nuclear power industry, medical diagnostics and therapies, forensics, carbon sequestration, and agronomy studies. Alexander A. Bol’shakov, Xianglei Mao, Dale L. Perry, and Richard E. Russo

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aser ablation molecular isotopic spectrometry (LAMIS) is a technique that uses optical spectra of transient molecular species produced in ablation plumes in air or buffer gases (Ar, Ne, He, and N2) for rapid isotopic analysis of solid samples (1–8). This technique is similar to laser-induced breakdown spectroscopy (LIBS), but optical emission spectra in LAMIS are measured at longer delays after an ablation pulse than what is used in LIBS. Molecular emissions yield relatively easily detectable isotopic information. Therefore, LAMIS adds a supplementary function of isotopic measurements to the well-established benefits of LIBS (real-time elemental analysis at atmospheric pressure, minimal sample preparation, chemical mapping and depth profiling at high spatial definition, and laboratory and field operation possible at a standoff distance to the sample). The LIBS and LAMIS techniques can be accomplished on the same instrument. Molecular radicals are generated effectively when the ablation plume cools down, resulting in an increase of the molecular emission in the plasma afterglow. Several mechanisms contribute to the formation of molecules in the plume such as radiative association of neutral atoms, associative excitation or ionization, recombination of molecular ions, fragmentation of polyatomic clusters or nanoparticles, and vaporization of intact molecules from the ablated surface. Molecular spectra are advantageous for isotopic analysis because the isotopic shifts in molecular emission are considerably larger than in atomic spectra. The difference in isotopic masses has only a small effect on electronic transitions in atoms, but significantly affects the vibrational and rotational

energy levels in molecules (1). A compact spectrometer can be sufficient to resolve molecular isotopic spectra, therefore LAMIS measurements can be performed in the field (4,5). The ability to measure isotope abundance using a portable spectrometer with modest spectral resolution is a significant merit of LAMIS, along with no sample preparation and data collection at ambient pressure. The qualitative and quantitative studies of isotopes of the light elements are both very important for their use in many fields and for their very extensive scope, with many applications in different fields of their chemistries and materials science. This article describes measurements of isotopes of hydrogen, boron, carbon, nitrogen, oxygen, and chlorine. All of these elements are ubiquitous in nature as well as in practice, with carbon being the basis of many millions of compounds. Variation of the natural isotopic ratio 13C/12C in different materials ranges from 0.96% to 1.15% (9). These variations can be measured using LAMIS. Fractionation in stable isotopes of C, N, and O can be particularly indicative of a range of diverse biotic and abiotic processes relevant to the ecology, biosphere, and geochemistry, both organic (fossil) and inorganic. Fertilizers enriched typically above 40% in 15N relative to 14N are broadly used to track the efficiency of plant uptakes, fertilizer losses, and nitrogen turnover in soil. High neutron absorbing capacity of the 10B isotope led to the development of multiple boron-loaded materials for neutron shielding in nuclear reactors and spent fuel storage pools, as well as for screens and curtains in nuclear medicine centers. Enrichment from 50% up to 99% in 10B relative to



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Figure 1: Isotope-specific molecular spectra of BO (A→X; 0-3) emission formed during ablation of two aluminum-based composite materials containing different 11B/10 B ratios. The upper spectrum (BorAluminum) is shifted up for clarity. Lower spectrum: Boral.

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Wavelength (nm) Figure 2: Emission spectra of CN vibrational band progression (B→X; Δν = +1) formed during ablation of benzamide pellets enriched in 13C and 15N isotopes. Spectra were averaged over 100 laser pulses. 11B

is often used. Many neutron detection devices are also based on highly borated (10B) and deuterated (2H) scin-

tillator materials. Heavy water nuclear reactors require deuterium enrichment to 99.85%. Water-18O is used as a pre-

cursor in the radiopharmaceutical industry. Deuterated drugs and stable isotopic markers are increasingly used in the fields of medicine, pharmacology, nutrition, and physiology for tracing biochemical processes. Medical multinuclear magnetic resonance imaging (MRI) scanners require compounds enriched in nuclear magnetic resonance (NMR)-active isotopes (13C, 15N, and 17O). To assess quality, specifications, or aging of these materials, their isotopic homogeneity or distribution, and degree of isotopic enrichment can be rapidly tested by LAMIS in open air or inert gas flush without using laborious and expensive mass spectrometry (MS). The feasibility of standoff LAMIS analysis is particularly important for the nuclear industry. Natural geochemical heterogeneity in isotopic composition reflects the complex history of our planet. Isotopic ratios of B, C, O, and Cl are particularly variable in nature because of chemical reactivity, high solubility, and volatility associated with the compounds consisting of these elements. Boron and chlorine isotopic fractionation is used to trace the records of evolution and weathering reactions because of interactions of rocks, soil, and sediment materials with water. Biomediated alteration of the 37Cl/35Cl ratio is distinctive in microbial reduction of anthropogenic perchlorates, biphenyls, freons, and other chlorinated compounds. Simultaneous measurement of multiple stable isotopes is also increasingly used to constrain the carbon cycling in ecosystems better. LAMIS can facilitate the isotopic analysis directly in the field. Numerous applications of LAMIS are anticipated in the nuclear power industry, medical diagnostics and therapies, forensics, carbon sequestration, and agronomy studies.

Experimental

A f lash-lamp pumped Nd:YAG laser operating at 1064 nm with a pulse energy of 50–150 mJ and a pulse duration of 4 ns was used to ablate the samples. The laser beam was focused onto the sample with a fused-silica lens to a spot diameter of ~100 μm, either in open air or in a cylindrical quartz


chamber flushed with inert gas. Inert gas flow removed ambient air when the rare isotopes of nitrogen and oxygen were to be measured. A second lens was used to collect the laser-induced plasma emission in the direction perpendicular to the ablating laser beam. Two spectrometric configurations were interchangeably used. In one of them, collected light was focused onto a fiberoptic cable coupled to a compact echelle spectrograph (EMU-65, Catalina Scientific) fitted with an electron multiplying charge-coupled device (EMCCD). In the other configuration, collected light was focused onto the entrance slit of a Czerny-Turner spectrograph fitted with an intensified charge-coupled device (ICCD). The spectrum acquisition gate widths and delays were optimized to detect molecular emission, while minimizing intensity of atomic and continuum radiation. The collected spectra were averaged over several tens or hundreds of laser shots (see further text and notes in the figure captions). The spectral averaging, background correction, normalization, and final chemometric regression analysis of spectral data were performed using proprietary software designed at Applied Spectra for multivariate calibration and quantification of LIBS spectra. Substances with known isotopic content were obtained from commercial sources in the form of powder, then mixed in appropriate proportions and pressed by a 7-ton press into 1-cmdiameter pellets. Isotopically enriched water was frozen into ice tables, which were kept cold by a Peltier thermoelectric plate during ablation for the LAMIS analysis. Samples of borated aluminum materials were provided by Ceradyne, Inc.

Results and Discussion

Our previous measurements of boron isotopes 10B and 11B were performed using three chemically pure substances, isotope-enriched 10B2O3, 11B2O3, and natural BN (2,7). The results indicated that LAMIS can be calibrated using multivariate partial least squares regression (PLSR) for quantitative measurements of isotopic ratio with high precision represented by relative standard

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deviation of only ±0.45% (2σ = 9‰), and this can be further improved (7). Abundances of individual isotopes in material composition can be measured at least down to ~1% (13C and 15N can be detected in subnatural concentrations, below 0.3%). High spectral resolution was found unnecessary for isotopic determination (2). However, an ambiguity remained about whether similar results could be obtained on practical samples with possible spectral interferences from other chemical elements.

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The pertinence of LAMIS measurements to industrial applications was tested on neutron absorber materials. Two samples of aluminum alloyed composites with different boron isotope ratios were ablated and analyzed using a 320-mm IsoPlane spectrograph fitted with a PI-MAX4 ICCD detector (Acton/ Princeton Instruments). One of these samples was a Boral core (Ceradyne) that consolidated aluminum alloy with 56% boron carbide of natural boron isotope abundance. The other sample was


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Wavelength (nm) Figure 3: CN vibrational bands (B→X; Δν = –1) formed during ablation of natural graphite and enriched benzamide pellets. Spectra were averaged over 100 laser pulses.

Predicted 13C Linear fit Confidence

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Figure 4: Calibration plot with a linear fit and 95% confidence interval for the 13C isotopic content calculated using the PLSR model.

BorAluminum (Ceradyne), a regular unalloyed aluminum (grade 1100) with 4.5% added elemental boron, enriched in the lighter isotope 10B up to 95%. The ablation spectra of these samples recorded in the 591–608 nm spectral in-

terval are presented in Figure 1 (continuum background subtracted). They are different because the isotopic composition is different. These spectra belong to moderately resolved rotational lines of boron monoxide in the vibrational band

(0-3) of the A 2Πi→X 2Σ+ emission system. The spectra were acquired using a 1800-grooves/mm (gr/mm) grating with the ICCD delay of 10 µs and a gate width of 100 µs. Spectral intensity was accumulated over 1000 laser pulses (analysis time 100 s at 10 Hz). In previous work on LAMIS, the BO A2Πi→X 2Σ+ emission was used at both the (0-2) band (4,7) and the (0-3) band (2,7,10). A choice of the spectral interval was driven by quality of the spectra for multivariate quantification with different chemometric procedures. The data in Figure 1 illustrate a significant isotopic shift between spectra of Boral (80% 11B) and BorAluminum (95% 10B), requiring only modest spectral resolution. Another important factor is that this spectral interval does not have any visible interferences from atomic aluminum or chemical impurities in the samples. When the ablation spectrum was recorded at the usual acquisition delays for LIBS (