We demonstrate in this paper that laser ablation allows efficient analysis of organic and biological materials. Such analysis is based on laser-induced breakdown spectroscopy (LIBS) which consists in the detection of the optical emission from the plasma induced by a high intensity laser pulse focused on the sample surface. The optimization of the ablation regime in terms of laser parameters (pulse duration, wavelength, fluence) is important to generate a plasma suitable for the analysis. We first present the results of a study of laser ablation of organic samples with different laser parameters using time-resolved shadowgraph. We correlate the early stage expansion of the plasma to its optical emission properties, which allows us to choose suitable laser parameters for an efficient analysis of organic or biological samples by LIBS. As an illustration of the analytical ability of LIBS for biological materials, we show that the emission from CN molecules can be used to distinguish between biological and inorganic samples. Native CN molecular fragment directly ablated from a biological sample are identified using time-resolved LIBS. Those due to recombination with nitrogen contained in atmospheric air can be distinguished with their specific time evolution behavior.
Laser-Induced Breakdown Spectroscopy (LIBS) has been used since 40 years on typical samples such as metals, alloys,
rocks. Detection of organic hazards or analysis of biological compounds under atmospheric pressure with LIBS
represents a new challenge. For this purpose, we need better understandings of the physico-chemical properties of the
plasma in atmosphere and their influences on the LIBS signal.
As a model sample of organic materials, Nylon 6-6 has been studied under nanosecond ablation at different
wavelengths (1064 nm and 266 nm) and energies (from 1 to 5 mJ) in order to observe the influence of these parameters.
Shadowgraph technique is used to image the plasma at its early stage of expansion (0 to 40 ns). Time-resolved LIBS
signal is recorded for longer times (50 ns to 5 μs).
In the infrared regime, the expansion of the plume is faster along the laser axis, perpendicular to the sample
surface. On the contrary, for UV ablation, the expansion of the plume is quite isotropic. We can also observe different
regimes of expansion due to Laser-Supported Detonation Waves (LSDW) above 3 mJ in the UV regime.
In particular, these observations provide us ideas to understand the kinetics of the CN emission in the LIBS
signal. In the IR regime, a formation of CN due to carbon present in the sample and nitrogen in the air via the
reaction 2C + <i>N</i><sub>2</sub> → 2CN can be observed. In the UV regime, the direct ablation of CN bonds is clearly seen but other
effects like screening and recombination due to LSDW have also been observed.