Variables affecting LIBS performance

In developing an LIBS method, one should tune the equipment to be used in such a way as to obtain optimal values for those variables affecting performance — and hence the final results — to the greatest extent. 

Aragon et al. [138] studied the spatial distribution of Cu, Mg and Fe in plasmas produced on copper, stainless steel and alumina matrices as a function of laser power. They found the logarithmic emission intensity for the alumina sample to increase linearly with increasing laser power density over the range 80-900 GW/cm-. By contrast, emission intensity in the metallic samples remained constant up to 700 GW/cm- and then increased sharply. By observing emission and ratioing it to copper emission, the formation of a plasma in air was found to shield the metal targets at intermediate power values and the analysis of metallic samples in air to require power densities in excess of 700 GW/cm . 

The laser repetition rate is a key variable in the LIBS technique. Laser breakdown produces a persistent mass of aerosol above the sample, the production rate increasing with increasing repetition rate and yielding a higher steady-state aerosol concentration above the sample. 

Laser wavelength is also of paramount significance in LIB spectroscopy. Excitation temperatures examined as a function of laser wavelength revealed plasmas induced by UV wavelengths to provide higher temperatures than those induced by IR radiation, which was ascribed to the stronger shock caused by UV light. 

Each lens produces a different energy density distribution in space that results in different plasma characteristics. A laser beam can be focused to a minimum size d given by the diffraction limit relation [139], d = 2.44/A/D, where A is the laser wavelength. 

---