ArF laser-induced plasma spectroscopy for part-per-billion analysis of metal ions in aqueous solutions

Aqueous samples containing trace amounts of metal ions in 0.8 M HCl were ablated with an ArF laser. Plasma emissions were monitored for elemental analysis. The signal-to-noise ratio was optimized when the laser fluence was about 10 J cm^-2, while the detector gate delay and width were 1-2 μs and 3-4 μs, respectively. During that time, the temperature and electron density of the induced plasma were also measured spectroscopically. The temperature dropped from about 0.5 to about 0.3 eV, while the density remained fairly constant at about 3 × 10¹⁶ cm^-3. Background-free spectrochemical analysis was therefore possible. The detection limits for Na, Ca, Ba, and Pb were 0.4, 3, 7, and 300 ppb, respectively. These are 20 to 1000 times better than the best achieved by non-193-nm laser-induced breakdown spectroscopy.