Volume 54, Number 10, 2000 APPLIED SPECTROSCOPY 1429 0003-7028 / 00 / 5410-1429$2.00 / 0 q 2000 Society for Applied Spectroscopy Fast Atomic Mapping of Heterogeneous Surfaces Using Microline-Imaging Laser-Induced Breakdown Spectrometry M. P. MATEO, S. PALANCO, J. M. VADILLO, and J. J. LASERNA * Department of Analytical Chemistry, Faculty of Sciences, University of Ma ´laga, E-29071 Ma ´laga, Spain A new approach for quick simultaneous and multielemental char- acterization of heterogeneous solid samples using laser-induced breakdown spectrometry (LIBS) is presented. The basic idea relies on focusing the incident laser beam with a cylindrical lens to pro- duce a long and narrow microline plasma. The emitted light is then projected along the spectrograph slit, where each ablated location on the sample generates a signal at a dened height, and acquired with a charge-coupled device (CCD) detector. The method has been tested for compositional mapping of solar cells, enabling a 25-fold increase of analysis speed as compared to conventional LIBS con- guration. Index Headings: Laser-induced breakdown spectrometry; LIBS; Chemical imaging; Materials analysis. INTRODUCTION Materials characterization is a vast eld where differ- ent scientic disciplines and techniques cohabit in the common effort to provide a complete description of the sample of interest from different points of view. Con- cerning direct chemical elemental analysis, there are many alternatives capable of determining the elements present in a given sample. However, most of them require working under moderate or high vacuum conditions, or they present some restrictions in terms of sample type, shape, or conducting properties. Laser-induced break- down spectrometry (LIBS) is considered a promising technique in terms of the outstanding capabilities of per- forming fast and accurate analysis in air at atmospheric pressure without limitations in sample size or nature. In spite of these unique advantages, LIBS is still affected by several drawbacks derived from the lack of standard- ization and different matrix effects (selectively vapori- zation, sample fractionation), which limit real quantita- tive approaches. Several books and review articles pro- vide a complete description of theoretical fundaments and recent applications in different industrial and analytical situations. 1–5 Recently, LIBS has been explored as an al- ternative technique in surface analysis 6 due to its capa- bilities of furnishing information on the lateral 7,8 and in- depth 9–11 distribution of the constituent elements of a sample at the micrometric level. As a result of major progress in the performance of the technique, depth-re- solved measurement at the nanometric range has also been achieved. 12 A new approach within the capabilities of LIBS for materials characterization is compositional mapping. 13–17 In this instance, a selective image of the sample, depict- ing the distribution of a given element, is obtained by delivering consecutive laser pulses over a xed position Received 2 December 1999; accepted 23 May 2000. * Author to whom correspondence should be sent. and rastering the laser through a dened path over the analyzed area. When a multidimensional detector is used, a complete spectrum covering a selected range of useful wavelengths is obtained for each laser event. After selec- tion of the desired emission lines, the spatial distribution of the element present in the sample may be mapped out. To date, detailed spatial information by the so-called im- aging-mode LIBS has been achieved by point-to-point mapping using a spherical lens focusing system, at the cost of increased measurement time and data storage re- quirements. For instance, with a N 2 laser, 800 sampling points are required to describe a 3 3 3 mm 2 area with 80 and 150 mm resolution (for the X and Y directions, respectively). 18 Many other laser-based imaging tech- niques also share the problems mentioned, in particular Raman microscopy 19,20 where the sample is to be ana- lyzed by rastering the laser or by moving the sample through dened paths. In these cases, different strategies using cylindrical lenses or scanning mirrors have been developed to maintain a compromise between spatial res- olution and analysis time to a workable regime. 21 An alternative approach for obtaining images using LIBS involves exploring the two-dimensional capability of charge-coupled device (CCD) detectors. In these types of systems, one dimension of the detector is reserved for light dispersion (providing spectral information), while emission from different locations on the surface is effec- tively focused onto the second dimension of the CCD (providing spatial information). Full advantage of this ap- proach can be obtained if the laser is focused to a micro- line at an energy exceeding the plasma threshold uence along the full microline, and the plasma formed in a di- rection parallel to the spectrometer entrance slit is col- lected. With the use of this focusing and detection sys- tem, each ablated location in the sample will generate a signal at a dened height in the slit. In the present paper this proposed microline-imaging LIBS is presented, and performance parameters for generation of surface chem- ical images are discussed. EXPERIMENTAL Instrumentation. The diagram of the setup is shown in Fig. 1. A Q-switched Nd:YAG laser (Continuum, Model Surelite SLI-20, pulse width 5 ns) operating at the 1064 nm wavelength was used to irradiate the samples. The energy output was varied in the range between 21 and 88 mJ/pulse by controlling the ash lamp voltage and the Q-switch delay. No appreciable changes in the spatial distribution of the laser beam were observed in the working range. The energy output was monitored dai- ly with a pyroelectric joulemeter (Gentec, Model ED- 200). The beam was delivered with the use of quartz