Britannica Sources used in this answer

How are clusters ionized and sorted by mass spectrometry?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

To study clusters, they are first ionized and then sorted using mass spectrometry to determine their size and composition.1

Ionization

Clusters are typically ionized in the gas phase using one of two primary methods:
* Electron impact: Clusters are bombarded with electrons, which can knock electrons out of the cluster to create ions.123
* Ultraviolet radiation: High-energy light, usually from a laser, is used to ionize the clusters.1

Additionally, resonant two-photon ionization can be employed for selective ionization, where a first laser excites the cluster and a second laser provides the energy required to reach the ionization potential.1

Sorting

Once ionized, the clusters are accelerated by an electric field and sorted based on their mass-to-charge ratios. This is generally achieved by:12
* Deflection: Charged clusters are passed through electric or magnetic fields that deflect their paths; smaller masses experience greater deflection, allowing for the selection of specific cluster sizes.124
* Time-of-flight: Ions are accelerated to the same kinetic energy, and the time they take to travel through a flight tube is measured. Since velocity depends on mass, this method effectively sorts the clusters by size.124