Spectrochemical analysis of liquids using laser-induced plasma emissions: Effects of laser wavelength on plasma properties

We spectroscopically determined the temperature and electron density of the plasma plumes produced by pulsed-laser ablation of aqueous solutions containing sodium, lithium, and rubidium. With the use of a Nd:YAG laser at 532 nm and fluence of 3 J/cm², the plasma produced was hot (low eV range) and extensively ionized, with electron density in the 10¹⁸ cm^-3 range. Analyte line signals were initially masked by intense plasma continuum emissions and would only emerge briefly above the background when the plume temperature dropped below 1 eV during the course of its very rapid cooling. Since ionization was thermally induced, the intense plasma flash was inevitable. In contrast, 193-nm laser ablation at similar fluence generated plasmas of much lower (<1 eV) temperature but comparable electron density. Plasma continuum emissions were rel-atively weak, and the signal-to-background ratio was a thousand times better. Consequently, this "cold" plasma was ideal for sampling biologically important elements such as sodium, potassium, and calcium.