Remote laser-induced plasma spectrometry for elemental analysis of samples of environmental interest C. Lo´pez-Moreno, S. Palanco and J. J. Laserna* Department of Analytical Chemistry, Faculty of Sciences, University of Ma ´laga, E-29071 Ma ´laga, Spain. E-mail: laserna@uma.es Received 8th June 2004, Accepted 31st August 2004 First published as an Advance Article on the web 23rd September 2004 Remote laser-induced plasma spectrometry has been demonstrated as a valuable analytical tool both for qualitative inspection and quantitative determinations on environmental samples. For this objective, the pulsed radiation of a Q-switched Nd:YAG laser at 1064 nm has been used to produce a plasma in a remote sample, the light emission being collected under a coaxial open-path optical scheme and guided towards a spectrograph and then detected by an intensified CCD. A prospective study has been carried out to assess the suitability of the technique for the remote analysis of samples from a coastal scenario subjected to a high industrial activity. All the measurements have been done in the laboratory. Among the main factors influencing the analytical results, sample moisture and salinity, sample orientation and surface heterogeneity have been identified. The presence and distribution of Fe and Cr as a contaminant on sample surface has been quantified and discussed for samples including soil, rocks, and vegetation. At a stand-off distance of 12 m from the spectrometer to the sample, limits of detection in the order of 0.2% have been obtained for both elements. Introduction Industrial environments are subjected to a highly acoustic, visual and chemical contamination. Continual monitoring of the effects of industrial activity, whether in ground, air or aqueous media is often necessary to keep industrial impact to acceptable levels. The inaccessibility of some locations and their hostile environments are challenges for the development of analytical techniques capable to afford the difficulties of such analytical problems. 1 Real-time, non-preparative, in situ or remote-capable are among the desirable features from candidate elemental characterisation techniques. It has been more than 40 years since it was first noticed that plasma emission of a high-power pulsed laser irradiating a material contains sufficient spectroscopic information for the elemental analysis of that material. In the intervening period there have been numerous investigations, application demon- strations, and instrument variations reported by researchers, technologists, and world-wide instrument manufacturers using the technique that has become known as laser-induced break- down spectroscopy (LIBS) or laser-induced plasma spectro- scopy (LIPS). 2–5 LIPS is an elemental analysis technique that is an alternative to established analysis methods such as X-ray fluorescence, atomic absorption and emission spectroscopy, mass spectrometry and neutron activation analysis. In the present state of development, only under ideal circumstances LIPS can compete with these technologies in terms of analy- tical figures of merit, that is, limits of detection, precision, accuracy and so forth. However a number of appealing characteristics make LIPS of interest for analytical purposes including fast, real-time analysis, in-situ field operation and analysis without sample preparation. 6–10 One of the unique capabilities of LIPS is to remotely monitor the elemental composition of solids. Remote LIPS analysis can be imple- mented using optical fibers for transmitting the light to or from the sample. 11,12 However, it is often more interesting to use an open path approach in which the interrogating laser beam and the returning light are transmitted through the atmosphere. With this configuration, remote, difficult-to-access locations and hostile environments can be analyzed without the physical presence of the operator. 13–19 In previous works, the capabil- ities of remote LIPS have been explored by our group. The technology has been tested and evaluated under laboratory and field conditions for analytical identification of steel grades at 45 m from the instrument 15 and for real-time analysis of hot solid steel and liquid steel at about 1420 1C. 17 For this purpose several instruments have been developed which incorporate the latest advances in ns laser sources and parallel detection systems, and newly developed concepts for laser beam delivery and optical collection of the returned light. In the present work, remote LIPS is demonstrated as a valuable analytical tool for both qualitative inspection and quantitative determinations on environmental samples pro- ceeding from a coastal scenario subjected to high industrial activity. A number of issues influencing the analytical results such as moisture, salinity, sample orientation and heterogene- ity have been evidenced. LIPS capabilities for spatial analysis are exploited to study the presence, concentration and distri- bution of Cr as a contaminant in a widespread of samples including soil, rocks, and vegetation. Experimental The experimental setup is sketched in Fig. 1. The laser source is a Q-switched Nd:YAG laser providing a pulsed output of up to 1500 mJ and 12 ns in a flat profile of 12 mm in diameter. The beam guiding component is an 8.9 beam expander integrated by a couple of 1064 nm antireflection-coated best-form BK7 lenses with effective focal lengths of 22.25 mm and 198.84 mm. Attending to geometry, a coaxial scheme in which the laser beam and the plasma emission share the same optical axis 19 has been employed in the present work. A flat circular mirror 300 mm in diameter tilted 451 is used to bend the return path towards the collecting optics. A central aperture 110 mm in diameter was drilled in this mirror to allow the expanded laser beam to pass through. A commercial Newton telescope with F/4 and a primary mirror of 200 mm in diameter was employed for light collection. The plasma is imaged with a variation in its position of up to 120 mm depending upon the sample distance. Given such a large variation in plasma image position, the use of a fused-silica fiber optic cable (200 mm/250 mm core/cladding) was considered as an alternative to directly ARTICLE www.rsc.org/jaas DOI: 10.1039/b408534e This journal is & The Royal Society of Chemistry 2004 J. Anal. At. Spectrom., 2004, 19 , 1479–1484 1479