Technical report | Preliminary Investigation of Standoff Laser-Induced Breakdown Spectroscopy of Metallic Samples

Abstract

 Analytical chemistry techniques with the ability to analyse objects of interest at significant standoff ranges are of great interest to defence and national security organisations. One analytical technique which has demonstrated standoff capability is Laser-Induced Breakdown Spectroscopy (LIBS). LIBS is performed by using a high-powered laser to ablate a small amount of a sample and to form a plasma. The high temperature of the plasma results in atomic emission of light, which is used to identify the type of atoms present and therefore the nature of the sample. In simplistic terms, it is possible to perform LIBS at standoff distances by remotely irradiating the sample with a laser and collecting the radiation from the laser-induced plasma through a magnifying, large-aperture telescope. This report describes a series of experiments in which initial attempts to perform LIBS over moderate standoff distances were undertaken. Metallic samples were employed, as these materials represent readily available single element samples or simple alloy mixtures which simplify the initial investigation of an analytical technique. The ability of LIBS to identify these samples at a moderate standoff distance (approximately 3.5 m) is investigated, and the e_ects of laser focussing and pulse energy are examined. Finally, specific suggestions are provided for further development of a standoff LIBS system.

Execitive Summary

Analytical chemistry techniques with the ability to analyse objects of interest at significant standoff ranges are of great interest to defence and national security organisations. One analytical technique which has demonstrated standoff capability is Laser Induced Break-down Spectroscopy (LIBS). LIBS is performed by using a laser to ablate a small amount of a sample and to form a plasma from this ablated material. The high temperature of the plasma results in atomic emission of light. The wavelength of this light is characteristic of the emitting atom, and can therefore be used to identify the type of atoms present and therefore the nature of the sample. 

In simplistic terms, it is possible to perform LIBS at standoff distances by remotely irradiating the sample with a laser and collecting the radiation from the laser-induced plasma. However, the introduction of standoff  distance complicates the technique due to laser focussing and diffraction, and laser-induced plasma radiation collection. It is potentially possible to overcome these diffculties by introducing laser focussing elements, a high-powered laser, and a magnifying telescope of large aperture for the collection of the radiation. 

Experiments were conducted to investigate the ability of LIBS to identify metallicsamples over moderate standoff distances (approximately 3.5 m). In order to simplify the experiments, the investigation was limited to metallic samples, as these provide readily available single element samples or simple alloy samples containing a low number of elements. 

A lens was used to focus the high-energy laser pulses onto the sample, while a 6º aperture Newtonian telescope was used to collect the radiation from the laser-induced plasma. Differentiation of a range of metallic samples was trivial under these experimental conditions. Spectra obtained under these conditions clearly exhibit intense peaks which are characteristic of the elements present in the sample. 

Further experiments focussed on characterising the effects of varying the laser pulse energy and removing the laser focussing lens. Both of these variations effectively vary the power density of the laser pulses impinging upon the sample. As expected, reducing the laser pulse energy or removing the laser focussing lens resulted in decreases in the absolute intensity of spectral peaks, and a decrease in the quality and signal to noise ratio of the spectra. This is believed to be due to the decrease in laser power density resulting in reduced sample ablation and plasma formation, and reduced plasma temperature resulting in lower emission intensity. 

The results of these experiments provide clear guidance in further developing the LIBS system in order to improve and enhance the standoff capability.