T.A. Labutin

Femtosecond laser-induced breakdown spectroscopy

The presented review summarizes nearly two decades of studies on femtosecond laser-induced breakdown spectrometry (fs-LIBS). When an ultra-short (<1 ps) laser pulse is used for ablation, the physics of laser-induced plasma changes dramatically in comparison with ablation by pico or nanosecond pulses. A femtosecond laser pulse interacts only with the electron subsystem, while nanosecond pulses continuously interact with different thermodynamic states of material, starting from solid through liquid into plasma. The properties of ultra-short laser radiation, the timescale of fs-laser ablation and the radiative properties of fs plasma are briefly described. We consider the advantages of fs-LIBS, namely, low ablation thresholds, high-spatial resolution, and rapid analysis of samples, which require minimal invasion and allow high-efficiency transportation of laser radiation in filamentation mode for remote analysis. Moreover, we discussed possible limitations of the technique and different approaches to overcome such constraints while retaining the unique possibilities of fs-LIBS.

Comparison of single- and multivariate calibration for determination of Si, Mn, Cr and Ni in high-alloyed stainless steels by laser-induced breakdown spectrometry

The quantitative analysis of high-alloyed steels by LIBS is usually complicated by overlap of the analytical lines with iron lines due to the complex structure of the emission spectra of each component. To overcome this problem, we compared two calibration strategies in the current research work. Univariate regression analysis was used for a number of analytical lines of Si, Mn, Ni, and Cr with and without strong spectral interference with other lines. Several methods of data pre-processing (for example, by normalization using an internal standard or baseline correction) to compensate for matrix effects or the pulse to pulse deviations of the analytical signal have been compared with the calibration curves constructed with the use of peak intensities. As an alternative to the univariate strategy, multivariate calibration based on principal component regression (PCR) was used in this work. We examined two criteria separately to select the most predictive model. The minimal values of the relative Root Mean Square Error of Cross Validation (RMSECV, %) provided the best prediction accuracy while the use of the well known F-criterion reduced the number of principal components up to 4 or 5 for each analyte without significant worsening of prediction capability. The measurements within four spectral windows (210–230 nm, 280–300 nm, 345–365 nm and 400–420 nm) were carried out on a set of 10 standard samples. Univariate calibration for Cr, Ni and Mn provided the best prediction (R2 = 0.996) if an appropriate reference line could be found and analytical lines were not overlapped with others. The best prediction for Si (R2 = 0.94) was obtained with the use of a peak signal of the Si 212.41 nm line without normalization. Otherwise, PCR provided good predictive capability (RMSECV, % = 3, 4, 5 and 9 of quantification of Mn, Cr, Ni and Si, respectively) in the spectral ranges where numerous matrix lines strongly interfered with analytical lines.

Automatic identification of emission lines in laser-induced plasma by correlation of model and experimental spectra.

We have applied an algorithm to automatically identify emission lines in laser-induced breakdown spectrometry (LIBS). A Q-switched Nd:YAG laser at 355 nm was used to ablate a high-alloy stainless steel sample. The algorithm was implemented by three parts: simulation of the set of spectra corresponding to different temperature (T) and electron density (N(e)), searching the best correlated pair of a model spectrum and an experimental one, and attributing the peaks with certain lines. In order to construct the model spectra, we used the parameters of atomic and ionic lines, levels, the mechanisms of the broadening of spectral lines, and the selected parameters of the spectrograph. The highest correlation coefficient between the model and the experimental spectrum was 0.943 for T = 0.675 eV and lg(N(e)) = 16.7 cm(-3). More than 40 emission lines were labeled automatically in the spectral region 393.34-413.04 nm.

Determination of Ag, Cu, Mo and Pb in soils and ores by laser-induced breakdown spectrometry

We report the results of an analytical assessment of the determination of Ag, Cu, Pb and Mo by laser-induced breakdown spectrometry in certified and natural soils and ores for the purposes of geochemical exploration. Strong matrix effects were observed in the determination of Mo in soils and ores. The intensity of two Mo lines (313.26 and 550.65 nm) in ore samples was three to five times larger than in soil samples. We corrected such sample-to-sample variations by the selection of internal standards. There was no evidence of strong matrix effects for the other elements. As is typical in atomic emission spectrometry, the linear dynamic range of determination by laser-induced breakdown spectrometry was narrow (1–100 ppm) for the resonance lines as a result of self-absorption. The detection limits of Ag, Mo, Cu and Pb were 0.3, 0.3, 0.6 and 8 ppm, respectively. Such sensitivity is sufficient to determine Mo, Cu and Pb at the level of their crustal abundance.

Reduction of the matrix influence on analytical signal in laser-enhanced ionization spectrometry with laser sampling

The novel approach using a slope of correlation line (laser-enhanced ionization of lithium versus laser-induced plasma emission of aluminum) as analytical signal was proposed for reduction of matrix interferences in laser-enhanced ionization spectrometric determination of Li with laser sampling.

Rapid, direct determination of strontium in natural waters by laser-induced breakdown spectroscopy

We report a LIBS technique for Sr determination in different types of natural waters, which provides sufficient sensitivity for strontium quantification in marine studies, and for the safety control of drinking waters. The technique provides rapid measurements, not longer than 1 min per sample, without any preconcentration or dilution of waters. We demonstrated that the ionic line Sr II 407.77 nm was preferable for strontium quantification in natural waters compared to atomic line Sr I 460.73 nm, since the ratio between them equaled to ∼30. One of the obstacles is a variability of the total content of salts in waters from 0.01% to 5%. We found that the salinity had a strong influence on electron density (Ne); Ne increased dramatically in the case of low salinity (0 to 100 mg L−1), and it did not change essentially for the salinity above 150 mg L−1. At the same time, plasma temperature (∼1.1 × 104 K) was independent of salinity. Since an increase of salinity suppressed the ionic signal of strontium, the addition of NaCl as an ionization buffer diminishes considerably the matrix effects on analytical results. LODs of Sr varied from 25 μg L−1 for pure or fresh waters to 200 μg L−1 for salty waters. We have shown that the suggested technique provided the accurate determination of strontium in samples of the Laptev Sea water and four types of mineral waters within the concentration range from 1 to 20 mg L−1.

Carbon determination in carbon-manganese steels under atmospheric conditions by Laser-Induced Breakdown Spectroscopy

The most sensitive lines of carbon, used nowadays for its determination in steels by laser-induced-breakdown spectroscopy (LIBS), are at vacuum UV and, thereby, LIBS potential is significantly reduced. We suggested the use of the C I 833.51 nm line for carbon determination in low-alloy steels (c(C)~0.186-1.33 wt.%) in air. Double-pulse LIBS with the collinear scheme was performed for maximal enhancement of a carbon emission signal without substantial complication of experimental setup. Since this line is strongly broadened in laser plasma, it overlapped with the closest iron lines greatly. We implemented a PCR method for the construction of a multivariate calibration model under spectral interferences. The model provided a RMSECV = 0.045 wt.%. The predicted carbon content in the rail templet was in an agreement with the reference value obtained by a combustion analyzer within the relative error of 6%.

Application of Laser-Induced Breakdown Spectrometry for analysis of environmental and industrial materials

Actual state of affairs, main advantages and problems of Laser-Induced Breakdown Spectroscopy (LIBS) in analysis of industrial materials and environmental samples are discussed. Methods for LIBS sensitivity enhancement such as double-pulse ablation, a combination of LIBS with laser-induced fluorescence, the use of additional sources of excitation (spark) and confinement of plasma by magnetic field or shock wave are compared with respect to figures-of-merit. A set of LIBS approach to qualitative fast classification of materials, especially based on the correlation between parameters of laser plasma and sample properties, are discussed in details. Progress in environmental analysis of soils, sands etc. with the use of LIBS is demonstrated. Detection limits of the most elements in soils and aluminum alloys obtained until now are critically considered.

Analysis of Slightly Volatile Samples by Atomic-Ionization Spectrometry with Laser Ablation into Flame

The potentialities of laser-enhanced atomic-ionization spectrometry with laser ablation for the analysis of slightly volatile and difficultly ionized samples were studied using lithium polyvanadate as an example. The effect of the cathode voltage and the power of the ablation beam on the signal in the analysis of such samples was studied. The optimum measurement range was determined as a function of the number of subsequent ablation pulses acting upon the same surface point. This minimized the effect of the surface contaminants and the change in ablation conditions as the crater deepened.

Selection of an analytical line for determining lithium in aluminum alloys by laser induced breakdown spectrometry

Possibilities for determining lithium in aluminum alloys by laser spark spectrometry are studied. The optimum conditions for registering the emission signal of lithium at which the effect of the continuous background radiation of the laser plasma attains a minimum are found. The possibility of determining lithium by laser spark spectrometry using the spectral line at 610 nm is studied for the first time. A comparison of the detection limits and sensitivities of determining lithium by emission its lines at 610 and 671 nm has indicated the advisability of using the line 610 nm for the studied alloys. The detection limit calculated using the 3σ test was found to be 230 ppm (610 nm) and 870 ppm (671 nm).