Utilization of Calibration Free Femtosecond Laser--Induced Breakdown Spectroscopy (CF-Libs) for Monitoring of the Elemental Composition in Poplar Tree Leaves

摘要

Calibration free laser-induced breakdown spectroscopy (CF-LIBS) technique was applied to analyze the plasma formed by focusing a Ti:sapphire chirped pulse amplification laser system operated at around 800 nm at a repetition rate of 10 Hzand the pulse width of the laser beam was approximately 100 fs on the surface of poplar tree leaves in air at atmospheric pressure. The application of the this technique is to determine the elemental composition of poplar tree leaves without need of reference samples(internal standard). The technique includes the followingsteps: i) The electron density was obtained using stark broadening of well-isolatedMn I(280.106 nm) line in the plasma. ii) The Boltzmann's method was used as aguess estimation ofthe plasma temperature and this plasma temperature was refinedby Saha-Boltzmann's method in order to obtain more reliable and accurate resultunder the assumption of local thermodynamic equelibrium (LTE), optically thinplasma, and stoichiometric concervations. It has been found that the Saha-Boltzmann's plot is more reliable and accurate than Boltzmann's plot fordetermination of plasma temperature. iii) Partition function of each species of theemitted radiation was obtained at the average value of the plasma temperature. iv)The composition of the elements in poplar tree leaves was analyzed by eliminatingthe experimental factor though normalization. The same metallic elements Ca, Fe,Mn, Ti, Na, Ni, Al and non-metallic elements P, N, O were identified in all of theleaves collected from different sites in Changchun city. The obtained result indicatethat the concentrations of Ca, Fe, Mn, Ti, Na, Ni, Al and P have been found to increase from less polluted to high polluted area in units of parts per million (ppm).Moreover, the effect of change in concentration of the elements on plasmaparameters (plasma temperature and electron density) has been discussed.