Correlation between native bonds in a polymeric material and molecular emissions from the laser-induced plasma observed with space and time resolved imaging

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Abstract

Emissions from C2 molecules and CN radicals in laser-induced plasmas on polymeric materials were observed with time-resolved spectroscopic imaging. More precisely, differential imaging with a pair of narrowband filters (one centered on the emission line and another out of the line) was used to extract emission images of interested molecules or radicals. The correlation between the molecular emission image of the plasma and the molecular structure of the polymer to be analyzed was studied for four different types of materials: polyamide (PA) with native CN bonds, polyethylene (PE) with simple CC bonds, polystyrene (PS) with delocalized double CC bonds, and polyoxymethylene (POM) which neither contains CC nor CN bonds. A clear correlation is demonstrated between emission and molecular structure of the material, allowing the identification of several organic compounds by differential spectroscopic imaging.

Highlights

► Plasma imaging method to discriminate different type of polymers. ► Molecular emissions (CN and C2) are spatially and temporally correlated to native bonds. ► Several formation processes of molecular fragments are observed.

Introduction

Detection, identification and analysis either qualitative or quantitative, of organic materials represent often a crucial need in a wide range of applications. In general, classical analytical chemistry provides solutions suitable for the use in laboratory, which is the case of ICP-OES or ICP-MS for example. Recently emerged applications require however more and more analytical abilities in various operation environments and circumstances, which include the ability to provide in situ and/or online analysis and that of remote or standoff operations. One can mention for such applications, automated or manual sorting of waste plastics, detection and identification of residues of explosives, pigments and organic binders on mural paintings or biological agents. The importance of these applications stimulated since several years the research and development of efficient techniques and methods. Laser-induced breakdown spectroscopy (LIBS) has been identified as a promising technique to fill the needs for the above mentioned applications [1], [2], [3], [4], [5], [6], [7], [8] because of the possibility of multimaterial analyzes irrespective of the physical and chemical characteristics of the studied sample. The previous works showed the potential of LIBS for these applications but at the same time, the challenge to make it an efficient technique for analysis of organic materials. Indeed the basic mechanisms involved in LIBS determine its predominant nature of elemental analysis technique, since the temperature in laser-induced plasma is in general high enough to completely atomize the part of the sample irradiated by the laser pulse. The difficulty for the analysis of organic materials arises from the fact that organic compounds are similar from the point of view of elemental composition. The several basic elements, C, H, O, or N, are ubiquitously present in all organic compounds. The detection of these elements or one part of these elements does then not provide specific feature of an organic material, without quantitative analysis.

Two approaches have been investigated for the extension of LIBS to the analysis of organic materials. The first one is based on the use of chemometric procedures for LIBS spectra of organic materials. Such procedures extract the most significant information from the whole spectral range and especially the interrelation between the emission intensities from the different elements or molecules to provide and enhance the specific feature of an organic material for its identification [8], [9], [10]. Another approach consists in the detection of characteristic molecules or radicals in the laser-induced plasma from an organic material. In some experimental conditions, such molecular or radical species can be transferred directly from the sample to the plasma and can be detected with time-resolved LIBS [11], [12], [13]. In this paper, we will present a new approach to analyze organic materials which consists in the study of the correlation between the molecular structure of a polymeric material and the time- and space-resolved emission from characteristic molecules or radials, such as C2 or CN, in the laser-induced plasma. We will demonstrate especially that the existence of such correlation allows potential applications for detection and identification of organic compounds. The technique of dual-wavelength differential spectroscopic imaging [14] is used in this study. The advantage of such technique for the detection of specific feature of molecular emission in the plasma will be therefore emphasized.

Section snippets

Experimental setup

The experimental setup is schematically presented in Fig. 1. Its exhaustive description together with that of the associated data treatment methods can be found elsewhere in Ref. [14]. The setup was an arrangement allowing differential spectroscopic images being taken for plasmas with the use of pairs of narrowband filters specifically chosen for studied species. A quadrupled Nd:YAG laser (266 nm) delivered pulses of 18.5 mJ with 5 ns pulse duration. A 5 cm focal length lens was used to focus the

Correlation between the molecular structure in the material and the spatial distribution of molecular fragments in the plasma

In order to study the correlation between spatial distribution of molecular species in the plasma and the molecular structure of the analyzed sample, we recorded emission images of CN and C2 molecules. Such correlation can provide molecular fingerprint of organic materials for their identification. Atomic nitrogen was also observed to have insight on the influence of the background gas. Fig. 4 shows emission images of CN, C2 and N for the 4 samples at a delay of 600 ns after the impact of laser

Conclusion

We have confirmed in this work that molecular emission from the plasma induced from an organic material is correlated to the molecular structure of the sample. Such correlation becomes much more obvious when temporally and spatially resolved observations are used to detect molecular emission from the plasma. In our work, differential spectroscopic imaging methodology was use to observe emission from CN and C2 form plasma induced on 4 different types of polymer PA, PE, PS and POM. Our results

Acknowledgements

One of us (S. G.) thanks ANRT (Association Nationale de la Recherche et de la Technologie) for their financial support.

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