Comparative study of different methodologies for quantitative rock analysis by Laser-Induced Breakdown Spectroscopy in a simulated Martian atmosphere

Abstract

Laser-Induced Breakdown Spectroscopy was selected by NASA as part of the ChemCam instrument package for the Mars Science Laboratory rover to be launched in 2009. ChemCam's Laser-Induced Breakdown Spectroscopy instrument will ablate surface coatings from materials and measure the elemental composition of underlying rocks and soils at distances from 1 up to 10 m. The purpose of our studies is to develop an analytical methodology enabling identification and quantitative analysis of these geological materials in the context of the ChemCam's Laser-Induced Breakdown Spectroscopy instrument performance. The study presented here focuses on several terrestrial rock samples which were analyzed by Laser-Induced Breakdown Spectroscopy at an intermediate stand-off distance (3 m) and in an atmosphere similar to the Martian one (9 mbar CO₂). The experimental results highlight the matrix effects and the measurement inaccuracies due to the noise accumulated when low signals are collected with a detector system such as an Echelle spectrometer equipped with an Intensified Charge-Coupled Device camera. Three different methods are evaluated to correct the matrix effects and to obtain quantitative results: by using an external reference sample and normalizing to the sum of all elemental concentrations, by using the internal standardization by oxygen, a major element common to all studied matrices, and by applying the Calibration Free Laser-Induced Breakdown Spectroscopy method. The three tested methods clearly demonstrate that the matrix effects can be corrected merely by taking into account the difference in the amount of vaporized atoms between the rocks, no significant variation in plasma excitation temperatures being observed. The encouraging results obtained by the three methods indicate the possibility of meeting ChemCam project objectives for stand-off quantitative analysis on Mars.

Introduction

ChemCam, laser-induced remote sensing for chemistry and micro-imaging, is a new type of instrument under development to investigate details of the Martian geochemistry. It is one of the experiments selected by NASA for the mobile Mars Science Laboratory (MSL) rover, scheduled for launch in 2009. Laser-Induced Breakdown Spectroscopy (LIBS) has been chosen for planetary science applications for several reasons: it can remove dust layers remotely and perform depth profiles [1], [2] through weathering coatings at stand-off distances [3], [4], and thus provide analyses of the pristine rock samples. Compared to many other existing techniques, the LIBS experimental arrangement is simple, giving rise to the possibility of a compact instrument [5], [6], [7], [8], [9], [10], with low weight, size and power consumption. These features make the LIBS technique especially suited for a rover. The LIBS experimental arrangement has to be optimized for space flight, and relevant minerals, rocks and soils must be tested under a simulated Mars atmosphere [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The LIBS technique allows both qualitative and quantitative elemental analysis of a wide range of materials. However, as for any method of solid sample analysis, the accuracy of quantitative results is often compromised by the so-called matrix effects [19], [20], [21], [22], [23], in which the analytical response is influenced by the physical properties of the sample, as well as its overall chemical composition. In a laboratory, matrix effects can be partially overcome by applying one calibration curve for each element contained in any substrate of interest. This approach requires many reference samples and is very time consuming. Consequently, it is not compatible with field analyses of complex and a priori unknown samples, as for the case of soils and sediments that will be encountered on the Martian surface. Hence, it is of prime importance for the ChemCam project to improve the methodology for correcting the matrix effects and for obtaining quantitative results on inhomogeneous and unknown samples on Mars.

The analytical approach commonly used for the correction of matrix effects (referring to the Goddard/Loge approach [24], [25]) is based on normalization of the emission signal to a reference signal, generally an emission line of the major element of the marix. In a previous work performed in our laboratory, Chaléard et al. [20] proposed to normalize the LIBS signal to an experimental temperature coefficient and to the acoustic signal emitted by the shock wave produced by the plasma expansion and supposedly proportional to the vaporized mass. However, this method is not applicable to the Martian environment where the acoustic wave is not detectable due to the low ambient pressure. Panne et al. [21] used a normalization procedure based on the measurement of the electronic excitation temperature and the electronic density of the plasma. Xu et al. [26] proposed to normalize the LIBS signal by the continuum emission which should be proportional to the plasma electron density. Corsi et al. presented in Ref. [27] a critical comparison of the above techniques for compensating the matrix effects in LIBS measurements. They concluded that the Calibration Free (CF) LIBS method was the most powerful. The CF method was developed to determine, without any calibration curve but by using the equations describing the plasma, the concentration of atomic components in solid, liquid and gaseous samples [28]. This approach was tested in Ref. [16] on several samples of terrestrial origin, mostly volcanic rocks, which could be one analogue to expected Martian samples. Results for main constituents were found to be in good agreement with an Energy Dispersive X-ray (EDX) analysis and demonstrated that the Calibration Free LIBS method could be of very great help for any first search in a mineralogical analysis. The main limitation of the CF method for quantitative analysis is the insufficient precision on the tabulated factors such as the level degeneracy and the transition probability, resulting in analytical errors generally higher than the required precision.

The aim of the present work was to identify the matrix effects occurring in LIBS analyses of several types of natural rocks: mafic volcanic rocks (basalt, trachy-andesite, trachyte, obsidian), gabbro and limestone under the simulated Mars atmosphere (9 mbar CO₂) and to develop an analytical procedure enabling correction of these matrix effects for quantitative stand-off analysis (3 m) of a priori unknown samples. For this purpose, we tested and compared different methods: the use of multi-matrix calibration curves, external normalization with correction for the total concentrations, internal normalization by a major element, and the application of the CF LIBS approach.