Ion formation mechanism in laser desorption ionization of individual nanoparticles.

Covariance mapping is used to study ion formation mechanisms in laser desorption ionization of individual 50 or 220 nm diameter particles having compositions similar to ambient aerosol. Single particle mass spectra are found to vary substantially from particle to particle. This variation is systematic--the energetically preferred ions (e.g., lowest ionization energy, highest electron affinity) are positively correlated with each other and negatively correlated with less preferred ions. For the compositions studied, the average positive ion yield is two to five times greater than the negative ion yield, indicating that free electrons are the main negatively charged species. For many particles, typically 20% to 40% of those analyzed, only positive ions are detected. Smaller particles give fewer negative ions, presumably because the plume is less dense and electron capture is less likely. The results suggest that ion formation occurs by a two stage process. In the first stage, photoionization of laser desorbed neutrals gives cations and free electrons. In the second stage, collisions in the plume cause electron capture and competitive charge transfer. When the particle ablates in a manner giving a dense plume with many collisions, the energetically preferred positive and negative ions are dominant. When the particle ablates in a manner giving a less dense plume with fewer collisions, the less preferred ions are able to survive and the energetically preferred ions constitute a lower fraction of the total ion signal. Systematic particle to particle variations of relative signal intensities can complicate ambient particle classification efforts by spreading a single particle composition over several classes.