Investigation of helium addition for laser-induced plasma spectroscopy of pure gas phase systems: Analyte interactions and signal enhancement

The role of helium addition on the analyte signal enhancement in laser-induced breakdown spectroscopy for analysis of pure gaseous systems was examined using carbon and hydrogen atomic emission lines. Increased analyte response, as measured by peak-to-base and signal-to-noise ratios, was observed with increasing helium addition, with maximum enhancement approaching a factor of 7. Additional measurements revealed a significant decrease in plasma electron density with increasing helium addition. To explore the mechanisms of analyte signal enhancement, the helium emission lines were also examined and found to be effectively quenched with nitrogen addition. In consideration of the data, it is concluded that the role of metastable helium is not as important as the overall changes in plasma properties, namely electron density and laser-plasma coupling. Helium addition is concluded to affect the electron density via Penning ionization, as well as to play a role in the initial plasma breakdown processes.