Elemental misinterpretation in automated analysis of LIBS spectra

Abstract

In this work, the Stark effect is shown to be mainly responsible for wrong elemental allocation by automated laser-induced breakdown spectroscopy (LIBS) software solutions. Due to broadening and shift of an elemental emission line affected by the Stark effect, its measured spectral position might interfere with the line position of several other elements. The micro-plasma is generated by focusing a frequency-doubled 200 mJ pulsed Nd/YAG laser on an aluminum target and furthermore on a brass sample in air at atmospheric pressure. After laser pulse excitation, we have measured the temporal evolution of the Al(II) ion line at 281.6 nm (4s(1)S-3p(1)P) during the decay of the laser-induced plasma. Depending on laser pulse power, the center of the measured line is red-shifted by 130 pm (490 GHz) with respect to the exact line position. In this case, the well-known spectral line positions of two moderate and strong lines of other elements coincide with the actual shifted position of the Al(II) line. Consequently, a time-resolving software analysis can lead to an elemental misinterpretation. To avoid a wrong interpretation of LIBS spectra in automated analysis software for a given LIBS system, we recommend using larger gate delays incorporating Stark broadening parameters and using a range of tolerance, which is non-symmetric around the measured line center. These suggestions may help to improve time-resolving LIBS software promising a smaller probability of wrong elemental identification and making LIBS more attractive for industrial applications.

Line Selection and Parameter Optimization for Trace Analysis of Uranium in Glass Matrices by Laser-Induced Breakdown Spectroscopy (LIBS)

“…Significant changes occur in the emission intensity and broadening versus time. Depending on the spectral line, Stark shifts can persist for a few microseconds in a LIBS plasma as demonstrated previously by Bengoechea et al 41 and Hübert et al 42 Neglecting contributions from Stark shift in LIBS analysis can lead to misinterpretation of elemental composition of the sample. 42 The Stark shift will cause an asymmetric line profile that can complicate data analysis and lead to errors when the common symmetrical functions (e.g., Lorentzian, Gaussian, Voigt line profiles) are used in the multi-component fittings.…”

Good practices in LIBS analysis: Review and advices

“…This limit is very dependent on the experimental parameters and consequently one should never conclude that an element is absent, but rather that its concentration is below the limit of detection. In addition, spectral interferences sometimes observed in the LIBS spectra could drive to wrong conclusions [32]. To overcome this problem, it is highly recommended to take advantage of the redundancy by checking the coexistence of a series of different lines related to the same element.…”

Resonance-Enhanced Detection of Metals in Aerosols using Single Particle Mass Spectrometry

“…Minor contributions to the measured width could result from Stark broadening (typically at higher laser power (Hübert and Ankerhold, 2011)) and from interferences with the adjacent absorption lines. 180…”