Christopher B. Stipe

Laser-Induced Breakdown Spectroscopy of Steel: A Comparison of Univariate and Multivariate Calibration Methods

Laser-induced breakdown spectroscopy (LIBS) was carried out on twenty-three low to high alloy steel samples to quantify their concentrations of chromium, nickel, and manganese. LIBS spectral data were correlated to known concentrations of the samples and three calibration methods were compared. A standard LIBS calibration technique using peak area integration normalized by an internal standard was compared to peak area integration normalized by total light and the multivariate statistical technique of partial least squares. For the partial least squares analysis, the PLS-1 algorithm was used, where a predictive model is generated for each element separately. Partial least squares regression coefficients show that the algorithm correctly identifies the atomic emission peaks of interest for each of the elements. Predictive capabilities of each calibration approach were quantified by calculating the standard and relative errors of prediction. The performance of partial least squares is on par with using iron as an internal standard but has the key advantage that it can be applied to samples where the concentrations of all elements are unknown.

Observation of Superaggregates from a Reversed Gravity Low-Sooting Flame

Copyright 2012 American Association for Aerosol Research

Evaluation of Laser-Induced Breakdown Spectroscopy (LIBS) for Measurement of Silica on Filter Samples of Coal Dust

Airborne silica dust (quartz) is common in coal mines and represents a respiratory hazard that can lead to silicosis, a potentially fatal lung disease. With an eye toward developing a portable monitoring device for rapid analysis of silica dust, laser-induced breakdown spectroscopy (LIBS) was used to quantify quartz in coal dust samples collected on filter media. Pure silica (Min-U-Sil™ 5), Georgia kaolin, and Pittsburgh-4 and Illinois-6 coal dusts were deposited separately and at multiple mass loadings onto 37-mm polyvinylchloride (PVC) filters. LIBS-generated silicon emission was monitored at 288.16 nm, and non-silica contributions to that signal from kaolinite were removed by simultaneously detecting aluminum. Measurements of the four samples were used to calculate limits of detection (LOD) for silicon and aluminum of approximately 0.08 lg/cm2 and 0.05 μg/cm2, respectively (corresponding to 0.16 μg/cm2 and 0. 20 μg/cm2 for silica and kaolinite, respectively). Relative errors of prediction are around 10%. Results demonstrate that LIBS can dependably quantify silica on filter samples of coal dust and confirm that accurate quantification can be achieved for very lightly loaded samples, which supports the potential application of LIBS for rapid, in-field monitoring.

Soot particle disintegration and detection by two-laser excimer laser fragmentation fluorescence spectroscopy

A two-laser technique is used to study laser-particle interactions and the disintegration of soot by high-power UV light. Two separate 20 ns laser pulses irradiate combustion-generated soot nanoparticles with 193 nm photons. The first laser pulse, from 0 to 14.7 J/cm2, photofragments the soot particles and electronically excites the liberated carbon atoms. The second laser pulse, held constant at 13 J/cm2, irradiates the remaining particle fragments and other products of the first laser pulse. The atomic carbon fluorescence at 248 nm produced by the first laser pulse increases linearly with laser fluence from 1 to 6 J/cm2. At higher fluences the signal from atomic carbon saturates. The carbon fluorescence from the second laser pulse decreases as the fluence from the first laser increases, suggesting that the particles fully disintegrate at high laser fluences. We use an energy balance parameter, called the photon/atom ratio, to aid in understanding laser-particle interactions. These results help define the regimes where photofragmentation fluorescence methods quantitatively measure total soot concentrations.

Comparison of field portable measurements of ultrafine TiO2: X-ray fluorescence, laser-induced breakdown spectroscopy, and Fourier-transform infrared spectroscopy

Laboratory measurements of ultrafine titanium dioxide (TiO2) particulate matter loaded on filters were made using three field portable methods (X-ray fluorescence (XRF), laser-induced breakdown spectroscopy (LIBS), and Fourier-transform infrared (FTIR) spectroscopy) to assess their potential for determining end-of-shift exposure. Ultrafine TiO2 particles were aerosolized and collected onto 37 mm polycarbonate track-etched (PCTE) filters in the range of 3 to 578 μg titanium (Ti). Limit of detection (LOD), limit of quantification (LOQ), and calibration fit were determined for each measurement method. The LODs were 11.8, 0.032, and 108 μg Ti per filter, for XRF, LIBS, and FTIR, respectively and the LOQs were 39.2, 0.11, and 361 μg Ti per filter, respectively. The XRF calibration curve was linear over the widest dynamic range, up to the maximum loading tested (578 μg Ti per filter). LIBS was more sensitive but, due to the sample preparation method, the highest loaded filter measurable was 252 μg Ti per filter. XRF and LIBS had good predictability measured by regressing the predicted mass to the gravimetric mass on the filter. XRF and LIBS produced overestimations of 4% and 2%, respectively, with coefficients of determination (R(2)) of 0.995 and 0.998. FTIR measurements were less dependable due to interference from the PCTE filter media and overestimated mass by 2% with an R(2) of 0.831.

Trapping and aerogelation of nanoparticles in negative gravity hydrocarbon flames

We report the experimental realization of continuous carbon aerogel production using a flame aerosol reactor by operating it in negative gravity (−g; up-side-down configuration). Buoyancy opposes the fuel and air flow forces in −g, which eliminates convectional outflow of nanoparticles from the flame and traps them in a distinctive non-tipping, flicker-free, cylindrical flame body, where they grow to millimeter-size aerogel particles and gravitationally fall out. Computational fluid dynamics simulations show that a closed-loop recirculation zone is set up in −g flames, which reduces the time to gel for nanoparticles by ≈106 s, compared to positive gravity (upward rising) flames. Our results open up new possibilities of one-step gas-phase synthesis of a wide variety of aerogels on an industrial scale.

LIBS in Industry: Sparks Fly

Laser-induced breakdown spectroscopy (LIBS) is emerging as a potent industrial spectroscopy. Uniquely capable for light-element detection, useful for material identiifcation, depth profiling, and elemental surface mapping, LIBS can solve numerous industrial problems in real-time.

Sorting of Aluminum and Scrap Metal using Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy (LIBS) systems can be used for both positive material identification (PMI) and quantification of elements in a variety of aluminum alloys and stainless steels. We compare performance of a compact LIBS system with a full-size desktop LIBS system and assess differences. Spectra were obtained from 18 standard samples and used to build a library for analysis. The calibration library, once built, was tested against both additional measurements on the standard samples and on scrap aluminum obtained from a local scrap yard. Analysis was performed using both individual shots and averaged sets of laser shots on the unknown samples.