FULLERENE ANALYSIS BY LASER MICROPROBE AND MASS-SPECTROMETRY - SEMIQUANTITATIVE STUDIES AND MECHANISTIC ASPECT

In this study, we present a semi-quantitative analysis of fullerenes by laser microprobe and mass spectrometry (LAMMA 500). First, we discuss the behavior during laser ablation of some aromatic compounds: graphite; coronene; anthracene; and 9, 10-bisphenylethinylanthracene (C30H18). In positive mode and at high energy (10(8) W/cm2), this leads to the detection of C(f)+ peaks (f > 32). The conditions of formation of these ions in the mass spectrometer are characterized by the simultaneous presence of low mass clusters C(n)H(m)+ (n < 15, m < 3) and polycyclic aromatic hydrocarbons (PAH). Under laser desorption conditions (low energy), only the molecular peaks are detected for PAH. This indicates that the analysis of carbonaceous materials in positive mode detection must be carried out under desorption conditions, in order to minimize the ion/molecule reactions in the plasma. In contrast, in negative mode, the C(f)- ion peaks are never detected during the LAMMA 500 analysis of aromatic compounds, whatever the analytical conditions, even though fullerenes (closed spherical molecules) may be characterized by the detection of C(f)- peaks. Consequently, the simultaneous detection of both C(f)+ and C(f)- peaks in the mass spectra reveals the presence of fullerenes in raw material (soot or deposits resulting from laser ablation of carbonaceous materials). Secondly, a methodology of semi-quantitative in situ analysis of fullerenes is proposed. The addition of an aromatic internal standard (coronene) to pure fullerenes has been tested but can not provide cannot acceptable quantitative results as the presence of PAH disrupts the measurements; the intensities of the Cf+ peaks and their relative distribution are disturbed. Nevertheless, when the LAMMA analysis is performed on pure fullerenes, the relative proportions of fullerenes (C60/C70 ratio, for example) may be determined with a similar precision and at an increased rate in comparison with the analysis by chromatographic and spectroscopic techniques. Furthermore, the LAMMA technique is able to detect trace levels (giant fullerenes; f > 100).