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  • Spectroscopy Advance Targets Industrial Applications

Photonics Spectra
Mar 1998
Lawrence Normie

Industry has been slow to accept laser-induced breakdown spectroscopy for chemical analysis because of the technology's inherent signal instability and because it measures relative, rather than absolute, concentrations of trace elements. However, improvements that have been developed at the Israel Institute of Technology (Technion) promise to open up new applications in process control and environmental monitoring.

The process works by measuring the emission spectra from a plasma plume created when a laser pulse hits the material. The industrial potential lies in the high sensitivity (a few nanograms and concentrations in the low parts-per-million range) and the capacity for online remote operation through fiber optic links. It provides rapid in situ elemental analysis of solids without laboratory sample preparation.

A time-integrated image reveals the internal structure of the plasma obtained using laser-induced breakdown spectroscopy.

The stumbling block has been signal fluctuations originating in nonlinear phenomena related to plasma formation and the laser ablation process, which combine to severely limit detection sensitivity. That should change, thanks to research at Technion's department of chemistry. A group led by Israel Schechter has shown that simultaneous spatial and temporal measurement of the plume can partially compensate for the unwanted signal variations, producing better detection performance.

Conventional laser-induced breakdown spectroscopy relies upon laser pulse averaging, which, because of nonlinear effects, does not improve performance. Schechter claims that the only way to boost sensitivity further is to analyze the spectra created during individual breakdown events initiated by single laser pulses. Simultaneous spatial and temporal measurements obtained for each laser shot then provide the information necessary for pulse shaping, temperature mapping and temporal profile characterization, resulting in substantially improved sensitivity.

A 3-D representation shows the integrated signal at each pixel of the image.

Fibers do the work

In Schechter's setup, the plasma is spectrally analyzed via an array of 12 optical fibers, each monitoring a different location in the plume. To furnish the spatial resolution, the spectral signals are recorded simultaneously on the "virtual strips" of a cooled intensified charge-coupled device camera, essentially forming a dozen independent detectors. An image intensifier time-gates the signal, producing the required time-domain resolution.

Employing a proprietary data analysis program, Schechter has obtained order-of-magnitude improvement in sensitivity when measuring the temporal and spatial features of plasma spectra from single laser pulses. The method has been demonstrated in analyzing heavy metals in sand, soils and industrial aerosols.

The Technion technique allows high spatial resolution at full spectral sensitivity. Spatial resolution is important in detecting individual chemical elements, Schechter said, For example, copper can be detected best at the plasma's center, while zinc is detected optimally in the outer region.

The group is working on eliminating the image intensifiers in hopes of developing field-portable instruments. It also is investigating the behavior of the plasma instability with a view to formulating algorithms that will permit reliable chemical analysis regardless of individual plasma idiosyncrasies.

An apparatus for the data acquisition technique is under development. Schechter said its success will be an important step toward widespread industry adoption.

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