Monty J. Ferris

Detection of Metals in the Environment Using a Portable Laser-Induced Breakdown Spectroscopy Instrument

A portable instrument, based on laser-induced breakdown spectroscopy (LIBS), has been developed for the detection of metal contaminants on surfaces. The instrument has a weight of 14.6 kg, fits completely into a small suitcase (46 × 33 × 24 cm), and operates from 115 V ac. The instrument consists of a sampling probe connected to the main analysis unit by electrical and optical cabling. The hand-held probe contains a small laser to generate laser sparks on a surface and a fiber-optic cable to collect the spark light. The collected light is spectrally resolved and detected with the use of a compact spectrograph/CCD detector system. The instrument has been evaluated for the analysis of metals in the environment: Ba, Be, Pb, and Sr in soils; Pb in paint; and Be and Pb particles collected on filters. Detection limits in ppm for metals in soils were 265 (Ba), 9.3 (Be), 298 (Pb), and 42 (Sr). The detection limit for Pb in paint was 0.8% (8000 ppm), corresponding to 0.052 mg/cm2. The higher limit obtained for Pb in paint is attributed to the use of the 220.35-nm Pb(II) line instead of the stronger 405.78-nm Pb(I) line used for soils. Spectral interferences prevented use of the 405.78-nm line to determine Pb in paint. The surface detection limit for Be particles on filters was dependent on particle size and ranged from 21 to 63 ng/cm2. The detection limit for Pb particles on filters was 5.6 μg/cm2.

Characterization of Laser-Induced Breakdown Spectroscopy (LIBS) for Application to Space Exploration

Early in the next century, several space missions are planned with the goal of landing craft on asteroids, comets, the Moon, and Mars. To increase the scientific return of these missions, new methods are needed to provide (1) significantly more analyses per mission lifetime, and (2) expanded analytical capabilities. One method that has the potential to meet both of these needs for the elemental analysis of geological samples is laser-induced breakdown spectroscopy (LIBS). These capabilities are possible because the laser plasma provides rapid analysis and the laser pulse can be focused on a remotely located sample to perform a stand-off measurement. Stand-off is defined as a distance up to 20 m between the target and laser. Here we present the results of a characterization of LIBS for the stand-off analysis of soils at reduced air pressures and in a simulated Martian atmosphere (5–7 torr pressure of CO2) showing the feasibility of LIBS for space exploration. For example, it is demonstrated that an analytically useful laser plasma can be generated at distances up to 19 m by using only 35 mJ/pulse from a compact laser. Some characteristics of the laser plasma at reduced pressure were also investigated. Temporally and spectrally resolved imaging showed significant changes in the plasma as the pressure was reduced and also showed that the analyte signals and mass ablated from a target were strongly dependent on pressure. As the pressure decreased from 590 torr to the 40–100 torr range, the signals increased by a factor of about 3–4, and as the pressure was further reduced the signals decreased. This behavior can be explained by pressure-dependent changes in the mass of material vaporized and the frequency of collisions between species in the plasma. Changes in the temperature and the electron density of the plasmas with pressure were also examined and detection limits for selected elements were determined.

Matrix effects in the detection of Pb and Ba in soils using laser-induced breakdown spectroscopy

With the use of laser-induced breakdown spectroscopy (LIBS), the effects of chemical speciation and matrix composition on Pb and Ba measurements have been investigated by using sand and soil matrices. A cylindrical lens was used to focus the laser pulses on the samples because it yielded higher measurement precision than a spherical lens for the experimental conditions used here. The detection limits for Pb and Ba spiked in a sand matrix were 17 and 76 ppm (w/w), respectively. In spiked soil, the detection limits were 57 and 42 ppm (w/w) for Pb and Ba, respectively. Measurement precision for five replicate measurements was typically 10% RSD or less. Two factors were found to influence emissions from Pb and Ba present in sand and soil matrices as crystalline compounds: (1) compound speciation, where Ba emission intensities varied in the order carbonate > oxide > sulfate > chloride > nitrate, and where Pb emission intensities varied in the order oxide > carbonate > chloride > sulfate > nitrate; and (2) the composition of the bulk sample matrix. Emissions from Ba(II) correlated inversely with the plasma electron density, which in turn was dependent upon the percent sand in a sand/soil mixture. The analytical results obtained here show that a field-screening instrument based on LIBS would be useful for the initial screening of soils contaminated with Pb and Ba.

Effect of Sampling Geometry on Elemental Emissions in Laser-Induced Breakdown Spectroscopy

In laser-induced breakdown spectroscopy (LIBS), a focused laser pulse is used to ablate material from a surface and form a laser plasma that excites the vaporized material. Geometric factors, such as the distance between the sample and the focusing lens and the method of collecting the plasma light, can greatly influence the analytical results. To obtain the best quantitative results, one must consider this geometry. Here we report the results of an investigation of the effect of sampling geometry on LIBS measurements. Diagnostics include time-resolved spectroscopy and temporally and spectrally resolved imaging using an acousto-optic tunable filter (AOTF). Parameters investigated include the type of lens (cylindrical or spherical) used to focus the laser pulse onto the sample, the focal length of the lens (75 or 150 mm), the lens-to-sample distance (LTSD), the angle-of-incidence of the laser pulse onto the sample, and the method used to collect the plasma light (lens or fiber-optic bundle). From these studies, it was found that atomic emission intensities, plasma temperature, and mass of ablated material depend strongly on the LTSD for both types of lenses. For laser pulse energies above the breakdown threshold for air, these quantities exhibit symmetric behavior about an LTSD approximately equal to the back focal length for cylindrical lenses and asymmetric behavior for spherical lenses. For pulse energies below the air breakdown threshold, results obtained for both lenses display symmetric behavior. Detection limits and measurement precision for the elements Be, Cr, Cu, Mn, Pb, and Sr, determined with the use of 14 certified reference soils and stream sediments, were found to be independent of the lens used. Time-resolved images of the laser plasma show that at times >5 μs after plasma formation a cloud of emitting atoms extends significantly beyond the centrally located, visibly white, intense plasma core present at early times (<0.3 μs). It was determined that, by collecting light from the edges of the emitting cloud, one can record spectra using an ungated detector (no time resolution) that resemble closely the spectra obtained from a gated detector providing time-resolved detection. This result has implications in the development of less expensive LIBS detection systems.

Transportable laser-induced breakdown spectroscopy (LIBS) instrument for field-based soil analysis

A field-deployable analyzer for the determination of metals in soils is described. The instrument is based on laser- induced breakdown spectroscopy (LIBS), a form of atomic emission spectroscopy. In the LIBS technique, powerful laser pulses are focused on the soil to form a series of microplasmas that vaporize and excite the elemental constituents of the soil. The plasma light is spectrally analyzed to determine the elemental composition. The results of an initial field test of the instrument are described and a brief critique of the results presented.

Capabilities of LIBS for analysis of geological samples at stand-off distances in a Mars atmosphere

The use of LIBS for stand-off elemental analysis of geological and other samples in a simulated Mars atmosphere is being evaluated. Analytical capabilities, matrix effects, and other factors effecting analysis are being determined. Through funding from NASAs Mars Instrument Development Program (MIDP), we have been evaluating the use of LIBS for future use on landers and rovers to Mars. Of particular interest is the use of LIBS for stand-off measurements of geological samples up to 20 meters from the instrument. Very preliminary work on such remote LIBS measurements based on large laboratory type equipment was carried out about a decade ago. Recent work has characterized the capabilities using more compact instrumentation and some measurements have been conducted with LIBS on a NASA rover testbed.

Remote elemental analysis using laser-induced breakdown spectroscopy

Focusing powerful laser pulses on a material produces microplasmas that vaporize and excite a small amount of the sample. By spectrally resolving the plasma emission, the elemental composition of the material can be determined. This method, termed laser-induced breakdown spectroscopy (LIBS), has many advantages that make it particularly suited for field-based monitoring. These include: simplicity, multielement detection capability, minimal sample preparation, and remote analysis capability. The remote elemental analysis capability of LIBS is unique compared to other conventional analysis methods. Remote analysis can be provided either by direct focusing of laser pulses on a distant object or by fiber optic delivery of the laser energy to the sample. To date, useful spectra of rock samples have been obtained by projecting the laser pulses out to a distance of 24 meters and collecting the plasma light with a simple lens system. Elements at major and minor concentrations were easily detected. Using fiber optic delivery of the laser pulses, LIBS spectra can be obtained from samples in relatively inaccessible locations (e.g. down a borehole, in a reactor). Laser pulses of 80 mJ at 10 Hz repetition rate have been used to remotely generate the laser plasmas and to collect the plasma light using a single fiber optic.