Laser Ablation Molecular Isotopic Spectrometry for Rare Isotopes of the Light Elements

Jun 01, 2014
Volume 29, Issue 6

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.

Laser 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 11B is often used. Many neutron detection devices are also based on highly borated (10B) and deuterated (2H) scintillator materials. Heavy water nuclear reactors require deuterium enrichment to 99.85%. Water-18O is used as a precursor 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, halocarbons, 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.

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