Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas

Abstract

The effects of adding nitrogen to the central gas flow (Ar + He) of an Ar plasma in laser ablation inductively coupled plasma mass spectrometry are presented. The optimum central gas flow rate was found to be negatively correlated with the N₂ gas flow rate. The addition of 5–10 ml min⁻¹nitrogen to the central channel gas in LA-ICP-MS increases the sensitivity for most of the 65 investigated elements by a factor of 2–3. The degree of enhancement depends, to some extent, on the 1st ionization energy. Another important advantage of N₂ mixed gas plasma for LA-ICP-MS is that the oxide ratios (ThO⁺/Th⁺) are significantly reduced (one order of magnitude). The hydride ratio (ArH⁺/Ar⁺) is also reduced up to a factor of 3, whereas the doubly charged ion ratio (Ca²⁺/Ca⁺) is increased. The background signals at masses 29, 31, 42, 51, 52 and 55 are significantly increased due to the nitrogen based polyatomic interferences. Compared to the spatial profiles of the ion distributions in the normal mode (without nitrogen), the addition of 5 ml min⁻¹nitrogen leads to significant wider axial profiles and more uniform distribution of ions with different physical and chemical properties. Our results also show that the makeup gas flow (central channel gas) rate has a significant effect on the ion distribution of elements with different physical and chemical properties. A very consistent increase of argon signal by the addition of nitrogen (5 ml min⁻¹) corroborates better energy transfer effect of nitrogen in the plasma.