ICP-MS--Creating the Ions

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Last updated August 2008

The inductively-coupled plasma is a very aggressive ion source. Because the source operates at temperatures of 7000 K, virtually all molecules in a sample are broken up into their component atoms. In ICP, a radio frequency (RF) signal is fed into a tightly wound, water-cooled coil where it generates an intense magnetic field. In the center of this coil is a specially made glass or quartz plasma torch where the plasma is formed. The plasma is generated in the argon gas by "seeding" the argon with a spark from a Tesla unit (similar to that used on a car spark plug). When the spark passes through the argon gas, some of the argon atoms are ionized and the resultant cations and electrons are accelerated toward the magnetic field of the RF coil. Through a series of inelastic collisions between the charged particles (Ar⁺ and electrons) and neutral argon atoms, a stable high temperature plasma is generated. The concentrations of electrons, Ar⁺ and neutral species in the plasma very quickly reach equilibrium, after which the plasma will remain 'lit' as long as the RF field is maintained and there is a constant supply of argon gas into the plasma.
The plasma torch is designed in such a way as to allow a sample to be injected directly into the heart of the plasma. The sample consists of a fine aerosol, which can come from any number of sources including, but not limited to, nebulized liquids and ablated solids. As the sample aerosol passes through the plasma, it collides with free electrons, argon cations and neutral argon atoms.	Plasma in the VG Axiom at MURR
The result is that any molecules initially present in the aerosol are quickly and completely broken down to charged atoms. These are usually in the M⁺ state although a few M⁺⁺ are also formed. Some of these charged atoms will recombine with other species in the plasma to create both stable and meta-stable molecular species (e.g. MAr⁺, M₂⁺, MO⁺, etc.). Many of these molecular species will be positively charged and will also be transmitted into the mass analyzer along with the charged atoms (M⁺ and M⁺⁺).
to "ICP-MS--Sampling the Ions"