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

Abstract

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.

Laser Induced Breakdown Spectroscopy

“…45 Despite this approach can simplify the LIBS instrumentation by preclude the use of expensive gated detectors, it has not achieved the same popularity of TRELIBS due the physical instrumental modifications (positioning of the collecting optics) that need to be implemented during signal optimization. On the other hand, the control of the delay time after shooting the laser pulse is, by far, simpler and already included in the software of the gated cameras.…”

“…However, due to the higher throughput of optical filters (there is no slit) in relation to dispersive spectrographs, modern tunable filters, such as acousto-optical tunable filters (AOTF), have been used in LIBS for plasma imaging, at a particular line emission, using bi-dimensional detecting systems. 45,[148][149][150] An AOTF is an optical band pass filter based on a diffraction of the radiation by an acoustic wave, whose frequency determines the selected wavelength, propagating through an isotropic or anisotropic crystal medium (depending on the wavelength range of the AOTF). 151 Plasma imaging using AOTF has been used for investigating the spatial and temporal distribution of atomic and ionic lines, 45 the effect of the focusing lens, the lens to sample distance, the angle of incidence of the laser pulse onto the sample, 45,148 and the effect of Figure 7.…”

Optical emission in magnetically confined laser-induced breakdown spectroscopy

“…3,4 The detection sensitivity of LIBS varies significantly between the different elements and is dependent on the element and excitation properties of sample sources. Some techniques have been developed to improve the LIBS sensitivity, such as oblique incidence of laser upon the surface 5 and dual-pulse excitation. 6,7 In recent years, the use of a magnetic field to confine laser-induced plasmas has attracted more and more interest.…”

Single-shot LIBS spectral quality for waste particles in open air

“…This threshold also provides the criterion for detecting anomalous spectra. There is no observable monotonic trend in the dependence on LIA, as also reported in [13], and not even a common trend between the different materials.…”