S.M. Zaytsev

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.

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%.

Rapid determination of zinc in soils by laser-induced breakdown spectroscopy

The possibility of quantitative detection of trace zinc levels in soils per single laser pulse using laser-induced breakdown spectroscopy is shown. The development of laser plasma and the signal-to-noise ratio are studied when evaporating soils by the second (532 nm) and the third (355 nm) harmonics of an Nd:YAG pulse laser. The use of the third harmonics permits one to reach the zinc detection limit (18 ppm) below Occupational Exposure Limits (OEL) in soil (150 ppm) and below the mean abundance in the earth’s crust of zinc (83 ppm). This allows the use of the suggested technique for the rapid determination of soil pollution with zinc and searching for geochemical anomalies.

A novel approach to sensitivity evaluation of laser-induced breakdown spectroscopy for rare earth elements determination

We report the potential of Laser-Induced Breakdown Spectroscopy (LIBS) for the determination of lanthanum and yttrium in soils and rocks. Since the main problem to quantify rare earth elements by LIBS is their rich spectra and the consequent frequent spectral interferences with the matrix elements, we demonstrated how thermodynamic modeling of the spectra can assist spectroscopists in the estimation of LIBS sensitivity. The theoretical LOD for La was close to the one retrieved from experimental data (6 ppm), while the theoretical LOD for Y was one order of magnitude higher than the experimental LOD (6 ppm vs. 0.4 ppm). The possible reasons for such a discrepancy are discussed.

Experimental Stark parameters of Mn I lines in the y 6 P ° → a 6 S multiplet under conditions of “long” laser plasma

The Stark broadening parameters for the components of the y6P° → a6S Mn I multiplet have been determined under conditions of “long” laser plasma: the electron impact width and line shift due to the quadratic Stark effect have been measured in the range of electron densities of (4.8–45) × 1016 cm–3. The presence of positive Stark shifts of the multiplet components (from +0.1 to +0.15 Å) has been demonstrated for the first time. The Stark widths are found to increase in the multiplet with a decrease in J of the upper level and significantly decrease with an increase in the plasma temperature. The d/w ratio is identical for all multiplet components; it amounts to 0.35–0.65, depending on the observation conditions. The dependences of this ratio on the plasma temperature and electron density have also been considered.

Comment on “Laser produced plasma diagnosis of carcinogenic heavy metals in gallstones” by M. A. Gondal, M. A. Shemis, A. A. I. Khalil, M. M. Nasr and B. Gondal, JAAS, 2016, 31, 506

The selection of analytical lines to determine Pb, Cr, Cd, Hg, and Ni in gallstones in the recently published paper with the above title is criticized. Also, the unreliable calibration of their spectrometer is demonstrated as a reason for incorrect assignment of spectral lines.

Determination of copper content in soils and ores by laser-induced breakdown spectrometry

It is demonstrated that the method of laser-induced breakdown spectrometry can be applied for quantitative determination of the copper content at a level of 500–40000 g/t, typical for copper ores and soil in the outcrop areas. To avoid the self-absorption of the resonance copper lines, we studied the most intense nonresonant lines of copper atoms and ions. It is shown that the Cu I 521.82-nm line is rather intense and provides the linear calibration. The copper detection limit for this line of 280 g/t allows its use for rapid mapping of outcrop areas.