Dependence of Laser-induced Breakdown Spectroscopy Results on Pulse Energies and Timing Parameters Using Soil Simulants

The  dependence of some LIBS detection capabilities on lower pulse energies (&lt;100  mJ) and timing parameters were examined using synthetic silicate ...

Applied Research Associates (ARA), Inc.

The use of laser-induced breakdown spectroscopy (LIBS) to detect a variety of elements in soils has been demonstrated and instruments have been developed to facilitate these measurements. The ability to determine nitrogen in soil is also important for applications ranging from precision farming to space exploration. For terrestrial use, the ideal situation is for measurements to be conducted in the ambient air, thereby simplifying equipment requirements and speeding the analysis. The high concentration of nitrogen in air, however, is a complicating factor for soil nitrogen measurements. Here we present the results of a study of LIBS detection of nitrogen in sand at atmospheric and reduced pressures to evaluate the method for future applications. Results presented include a survey of the nitrogen spectrum to determine strong N emission lines and determination of measurement precision and a detection limit for N in sand (0.8% by weight). Our findings are significantly different from those of a similar study recently published regarding the detection of nitrogen in soil.

Determination of nitrogen in sand using laser-induced breakdown spectroscopy.

Recently, laser-induced breakdown spectroscopy (LIBS) has been developed for the elemental analysis of geological samples for application to space exploration. There is also interest in using the technique for the analysis of water ice and ice/dust mixtures located at the Mars polar regions. The application is a compact instrument for a lander or rover to the Martian poles to interrogate stratified layers of ice and dusts that contain a record of past geologic history, believed to date back several million years. Here we present results of a study of the use of LIBS for the analysis of water ice and ice/dust mixtures in situ and at short stand-off distances (< 6.5 m) using experimental parameters appropriate for a compact instrument. Characteristics of LIBS spectra of water ice, ice/soil mixtures, element detection limits, and the ability to ablate through ice samples to monitor subsurface dust deposits are discussed.

Analysis of water ice and water ice/soil mixtures using laser-induced breakdown spectroscopy: application to Mars polar exploration.

Laser-induced breakdown spectroscopy (LIBS) measurements are typically carried out using pulses (<20 ns, >50 mJ) from a flashlamp-pumped electro-optically Q-switched Nd:YAG laser (EO-laser) or excimer laser. Here we report LIBS analyses of solids using an acousto-optically Q-switched Nd:YAG laser (AO-laser) producing 150 ns pulses of lower energy (10 mJ) at repetition rates up to 6 kHz. The high repetition rate allows increased spatial or depth sampling over a given time period compared to the EO-laser. Results of AO-laser based LIBS analysis of (1) steels, (2) soils, and (3) surface stains and dusts are described. Detection limits for Cr, Cu, Mn, Ni, and Si in steel ranged from 0.11 to 0.24% using a commercial polychromator-based detection system with limits 4--30 times lower achieved using a laboratory-based detection system. The minimum detectable masses of Ba, Cr, Mn, and Sr on a metal surface were estimated with 1.2 pg/shot achieved for Sr. Detection limits for Ba and Sr in soil were 296 and 52 ppm, respectively. The temperatures, spectra, and emission decay curves from plasmas generated by the AO- and EO-lasers are compared and some characteristics of particles ablated by the AO-laser are described.

Laser-induced breakdown spectroscopy analysis of solids using a long-pulse (150 ns) Q-switched Nd:YAG laser.

Emission enhancement mechanisms in dual-pulse LIBS.

An apparatus has been investigated based on laser-induced breakdown spectroscopy (LIBS) for the rapid determination of the spatial distribution of elements on surfaces. Cylindrical optics are used to create a linear spark approximately 1 cm in length. Light emitted by atoms excited along the spark is collected and provides a spatial profile of elemental composition in the sample when analyzed with a spectrometer and gated charge-coupled device (ICCD) detector. Moving the spark across the sample surface as spectral data is recorded at regularly spaced intervals allows for the development of a three-dimensional elemental distribution map (emission intensity versus spatial distribution across an area). An analysis of the spatial resolution of this methodology is presented along with representative data from several sample types. Application of full-image analysis allowing for simultaneous investigations into the spatial distributions of multiple elements is also discussed and results are presented.

Capabilities of surface composition analysis using a long laser-induced breakdown spectroscopy spark.

Emissions from a laser-induced breakdown spectroscopy (LIBS) plasma were examined in the region from 750 nm to 2000 nm. A Nd:YAG laser at 532 nm and 75 mJ per pulse were used to initiate the plasma. The detector was an InGaAs 1024 element diode array cooled to -100 degrees C. An f/4 spectrometer with gratings blazed for this region was used as the dispersive element. Survey spectra of soils, uranium, and other selected samples were taken in air and in a flow cell purged with argon at a local pressure of 0.84 x 10(5) Pa. Strong infrared lines of neutral aluminum, carbon, potassium, silicon, sulfur, and uranium, as well as once ionized lines of calcium, were observed out to 1670 nm. For potassium, the detection limits of the infrared (IR) system were compared with those obtained from a standard intensified charge-coupled device (ICCD) spectrometer arrangement, using the 766-770 nm doublet. Detection limits with the IR system were twice as high as those obtained from the ICCD detector.

Laser-induced breakdown spectra in the infrared region from 750 to 2000 nm using a cooled InGaAs diode array detector.

The goal of this work is a detailed study of uranium detection by laser-induced breakdown spectroscopy (LIBS) for application to activities associated with environmental surveillance and detecting weapons of mass destruction (WMD). The study was used to assist development of LIBS instruments for standoff detection of bulk radiological and nuclear materials and these materials distributed as contaminants on surfaces. Uranium spectra were analyzed under a variety of different conditions at room pressure, reduced pressures, and in an argon atmosphere. All spectra displayed a high apparent background due to the high density of uranium lines. Time decay curves of selected uranium lines were monitored and compared to other elements in an attempt to maximize detection capabilities for each species in the complicated uranium spectrum. A survey of the LIBS uranium spectra was conducted and relative emission line strengths were determined over the range of 260 to 800 nm. These spectra provide a guide for selection of the strongest LIBS analytical lines for uranium detection in different spectral regions. A detection limit for uranium in soil of 0.26% w/w was obtained at close range and 0.5% w/w was achieved at a distance of 30 m. Surface detection limits were substrate dependent and ranged from 13 to 150 microg/cm2. Double-pulse experiments (both collinear and orthogonal arrangements) were shown to enhance the uranium signal in some cases. Based on the results of this work, a short critique is given of the applicability of LIBS for the detection of uranium residues on surfaces for environmental monitoring and WMD surveillance.

Detection of uranium using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) was used in a blind study to successfully differentiate bacterial pathogens, both species and strain. The pathogens used for the study were chosen and prepared by one set of researchers. The LIBS data were collected and analyzed by another set of researchers. The latter researchers had no knowledge of the sample identities other than that (1) the first five of fifteen samples were unique (not replicates) and (2) the remaining ten samples consisted of two replicates of each of the first five samples. Using only chemometric analysis of the LIBS data, the ten replicate bacterial samples were successfully matched to each of the first five samples. The results of this blind study show it is possible to differentiate the bacterial pathogens Escherichia coli, three clonal methicillin-resistant Staphylococcus aureus (MRSA) strains, and one unrelated MRSA strain using LIBS. This is an important finding because it demonstrates that LIBS can be used to determine bacterial pathogen species within a defined sample set and can be used to differentiate between clonal relationships among strains of a single multiple-antibiotic-resistant bacterial species. Such a capability is important for the development of LIBS instruments for use in medical, water, and food safety applications.

The use of laser-induced breakdown spectroscopy for distinguishing between bacterial pathogen species and strains.

We present spectra of depleted uranium metal from laser plasmas generated by nanosecond Nd:YAG (1064 nm) and femtosecond Ti:sapphire (800 nm) laser pulses. The latter pulses produce short-lived and relatively cool plasmas in comparison to the longer pulses, and the spectra of neutral uranium atoms appear immediately after excitation. Evidence for nonequilibrium excitation with femtosecond pulses is found in the dependence of spectral line intensities on the pulse chirp.

Comparative study of femtosecond and nanosecond laser-induced breakdown spectroscopy of depleted uranium.

In this work, the use of laser-induced breakdown spectroscopy (LIBS) to differentiate live pathogens and killed viruses on substrates is investigated. Live pathogens B. anthracis Sterne strain and F. tularensis live vaccine strain were interrogated as lawn and colonies on agar; dilutions on agar; and dilutions on glass slides, and it was found possible to differentiate among all samples. UV killed hantavirusstrains were studied as dilutions on slides and it was also found possible to differentiate among strains. To the best of our knowledge, this is the first study in which LIBS has been used to differentiate virus samples.

Use of laser-induced breakdown spectroscopy for the differentiation of pathogens and viruses on substrates.

The development of field-deployable instruments to monitor radiological, nuclear, and explosive (RNE) threats is of current interest for a number of assessment needs such as the on-site screening of suspect facilities and nuclear forensics. The presence of uranium and plutonium and radiological materials can be determined through monitoring the elemental emission spectrum using relatively low-resolution spectrometers. In addition, uranium compounds, explosives, and chemicals used in nuclear fuel processing (e.g., tributyl-phosphate) can be identified by applying chemometric analysis to the laser-induced breakdown (LIBS) spectrum recorded by these spectrometers. For nuclear forensic applications, however, isotopes of U and Pu and other elements (e.g., H and Li) must also be determined, requiring higher resolution spectrometers given the small magnitude of the isotope shifts for some of these elements (e.g., 25 pm for U and 13 pm for Pu). High-resolution spectrometers will be preferred for several reasons but these must fit into realistic field-based analysis scenarios. To address the need for field instrumentation, we evaluated a previously developed field-deployable hand-held LIBS interrogation probe combined with two relatively new high-resolution spectrometers (Î»/Î”Î» ~75,000 and ~44,000) that have the potential to meet field-based analysis needs. These spectrometers are significantly smaller and lighter in weight than those previously used for isotopic analysis and one unit can provide simultaneous wide spectral coverage and high resolution in a relatively small package. The LIBS interrogation probe was developed initially for use with low resolution compact spectrometers in a person-portable backpack LIBS instrument. Here we present the results of an evaluation of the LIBS probe combined with a high-resolution spectrometer and demonstrate rapid detection of isotopes of uranium and hydrogen and highly enriched samples of (6)Li and (7)Li.

Monitoring uranium, hydrogen, and lithium and their isotopes using a compact laser-induced breakdown spectroscopy (LIBS) probe and high-resolution spectrometer.

In laser-induced breakdown spectroscopy (LIBS), a series of powerful laser pulses are directed at a surface to form microplasmas from which light is collected and spectrally analyzed to identify the surface material. In most cases, no sample preparation is needed, and results can be automated and made available within seconds to minutes. Advances in LIBS spectral data analysis using multivariate regression techniques have led to the ability to detect organic chemicals in complex matrices such as foods. Here, the use of LIBS to differentiate samples contaminated with aldrin, 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin, chlorpyrifos, and dieldrin in the complex matrices of tissue fats and rendering oils is described. The pesticide concentrations in the samples ranged from 0.005 to 0.1 Î¼g/g. All samples were successfully differentiated from each other and from control samples. Sample concentrations could also be differentiated for all of the pesticides and the dioxin included in this study. The results presented here provide first proof-of-principle data for the ability to create LIBS-based instrumentation for the rapid analysis of pesticide and dioxin contamination in tissue fat and rendered oils.

Detection of Pesticides and Dioxins in Tissue Fats and Rendering Oils Using Laser-Induced Breakdown Spectroscopy (LIBS).

The rapid detection of biological contaminants, such as Escherichia coli O157:H7 and Salmonella enterica , on foods and food-processing surfaces is important to ensure food safety and streamline the food-monitoring process. Laser-induced breakdown spectroscopy (LIBS) is an ideal candidate technology for this application because sample preparation is minimal and results are available rapidly (seconds to minutes). Here, multivariate regression analysis of LIBS data is used to differentiate the live bacterial pathogens E. coli O157:H7 and S. enterica on various foods (eggshell, milk, bologna, ground beef, chicken, and lettuce) and surfaces (metal drain strainer and cutting board). The type ( E. coli or S. enterica ) of bacteria could be differentiated in all cases studied along with the metabolic state (viable or heat killed). This study provides data showing the potential of LIBS for the rapid identification of biological contaminants using spectra collected directly from foods and surfaces.

David A. Cremers

Applied Research Associates (ARA), Inc.

Dependence of Laser-induced Breakdown Spectroscopy Results on Pulse Energies and Timing Parameters Using Soil Simulants