Femtosecond time-resolved laser-induced breakdown spectroscopy for detection and identification of bacteria : a comparison to the nanosecond regime

Abstract : Bacterial samples (Escherichia coli and Bacillus subtilis) have been analyzed by laser-induced breakdown spectroscopy (LIBS) using femtosecond pulses. We compare the obtained spectra with those resulting from the classical nanosecond LIBS. Specific features of femtosecond LIBS have been demonstrated, very attractive for analyzing biological sample: (i) a lower plasma temperature leading to negligible nitrogen and oxygen emissions from excited ambient air and a better contrast in detection of trace mineral species; and (ii) a specific ablation regime that favors intramolecular bonds emission with respect to atomic emission. A precise kinetic study of molecular band head intensities allows distinguishing the contribution of native CN bonds released by the sample from that due to carbon recombination with atmospheric nitrogen. Furthermore a sensitive detection of trace mineral elements provide specific spectral signature of different bacteria. An example is given for the Gram test provided by different magnesium emissions from Escherichia coli and Bacillus subtilis. An entire spectrum consists of hundred resolved lines belonging to 13 atomic or molecular species, which provides an ensemble of valuable data to identify different bacteria.