Ion formation mechanism in laser desorption ionization of individual nanoparticles

Abstract

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. . In our laboratory, RSMS uses a single 193 nm laser pulse to ablate and ionize material from the particle [4]. Intense ion signals can be obtained from most particle compositions: refractory and semivolatile, organic and inorganic. A common feature of this and other laser desorption ionization sequences is that the ionization mechanism is not fully understood [2].The LDI mass spectra from single particle mass spectrometers are similar to those obtained with laser microprobe mass spectrometry (LAMMS), a decades old method capable of analyzing particles and bulk materials deposited on a substrate [5,6]. Similar to LAMMS, single particle mass spectrometers also ablate and ionize particles with high-energy laser pulses, but particles are suspended in the center of the source region and analyzed less than a millisecond after entering the vacuum [7].Some effort has been made to elucidate the mechanisms of LDI in single particle mass spectrometry. Measurements of the kinetic energy distribution of ions formed by LDI has provided some information regarding the physical processes that occur during ionization [8,9]. Computational models have characterized the disintegration of particles irradiated with a short laser pulse [10 -12]. Most studies of the ion formation mechanism in LDI, though, have been performed on solids and...

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