Examples of laser ablation spectra of diatomic radicals SrO generated from isotope-enriched SrCO3 pellets and C2 molecules from samples of graphite and 13C powder are presented in Figure 3. These spectra were collected using a compact echelle spectrograph EMU-65 (Catalina Scientific, Tucson, AZ) fitted with an Electron Multiplying Charge- Coupled Device (EMCCD). Such an instrument would be suitable for space operations. The results demonstrate that isotopes are easily resolvable (shifts of 0.15nm in SrO and 0.7nm in C2). Even partially resolved spectra can be sufficient for the quantitative isotopic detection especially if a range of multiple spectral features, such as rotational lines, is measured at once. Requirements for spectral resolution can be further relaxed when the isotopic ratio is determined using chemometric analysis of spectra. The ability to measure isotope abundance with a low-resolution spectrometer is a significant attribute of LAMIS.
In terrestrial applications LAMIS is poised to speed up, to simplify, and to make isotopic analysis more affordable than at present, although it will remain generally less sensitive than traditional mass spectrometry. Multiple applications of LAMIS are anticipated in the nuclear power industry, medical diagnostics and therapies, forensics, carbon sequestration, ecological and agronomical studies. Several examples of the possible uses are described below to illustrate the breadth of LAMIS applicability.
Owing to its unique nuclear properties, the isotope-enriched 10B is often used in medical technology, particularly in boron neutron capture therapy (BNCT). The neutron capture cross section of 10B is six orders of magnitude higher than that of 11B and is significantly higher than that of any other material. In BNCT, neutron capture by 10B-loaded targeted drugs generates lethal radiation that damages DNA within individual malignant cells. To raise an effective dose, the tumor cells are loaded with 10B at 30—170 μg/g in tissue. Three-dimensional isotopic mapping with fine spatial resolution is necessary in boron radiochemotherapeutic cancer research. Presently, isotopic mapping at the scale of tens of microns can be obtained with laser ablation—inductively coupled plasma— mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS), both involving large and expensive instrumentation that requires a vacuum. In contrast, LAMIS can be implemented as a fieldable device. The nominal sample quantity in LAMIS can be less than 1μg (<103 human cells, i.e. less than a cube of 10×10×10 cells), which defines the micro-sampling spatial resolving ability of LAMIS.
Natural and isotope-enriched boron is often utilized as neutron shielding and neutron absorbing materials in nuclear reactors and spent fuel storage pools. Whether borated metals are used as neutron capturers or whether boric acid is added to cooling water, the ability to measure the 10B and 11B content by LAMIS will be particularly useful for monitoring. Such monitoring can be achieved remotely, delivering a laser beam through a small window and collecting optical emission either by a telescope or a cable of optical fiber.
The application of LAMIS for localized boron isotope determination would enable the development of handheld semiconductor-based neutron sensors in which a layer of 10B-enriched boron carbide with thickness of 1.5 to 100μm is deposited on top of a regular silicon-based diode. Other neutron detection devices lined, coated, or loaded with 10B can be rapidly tested by LAMIS for isotopic and elemental composition, hetero- or homogeneity, and possible defects for the purpose of quality assessment and quality control. The same is true for the production of 10Benriched borated steel, aluminum and borobond ceramics.
Other considerations: Carbon isotopes are indicative of primary bio-productivity and energy cycling and are, therefore, important for the understanding of biochemistry. The biological enhancement of 12C over 13C can be up to 5%, and is measurable by LAMIS. The stable isotope 15N is often used as a marker, particularly to track the efficiency of fertilizers in agronomy. Measurement of the D/H isotopic ratio is essential in paleoclimatology, material sciences, biological and medical research — among many other areas. Deuterated scintillators are used in neutron detectors, and their analysis can be another area of LAMIS applications. Measurement of oxygen and chlorine isotopes is also feasible — which isotopes are very important in geochemical studies.
Applied Spectra has developed a prototype LIBS instrument for standoff measurements at a distance of 30 meters, and also has participated in 50-meter standoff measurements using LIBS in the field. NASA’s ChemCam can measure LIBS spectra from 7 meters away. Similar standoff distances should be possible for isotopic analysis using LAMIS.
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