Page 81 - Inorganic Mass Spectrometry : Fundamentals and Applications

Inductively Coupled Plasma Mass Spectrometry

A 1- to 2-kW radio frequency power supply, either free-running or crystal-
controlled, drives current through a water- or air-cooled copper tube that acts as
the induction coil (often called a load coil). The oscillating current through the
load coil produces an oscillating electromagnetic field.
A tesla coil or high-voltage spark is used to seed the argon gas with electrons
in order to start the discharge. Once the plasma discharge has been initiated,
electrons in the plasma are accelerated by the oscillating magnetic field. Collisions
between electrons and argon atoms lead to heating of the argon gas. Collisions
between electrons and analyte atoms are thought to be the main mechanism for
analyte ionization although charge exchange may also be important. Typically,
less than 1% of the Ar (15.8-eV ionization potential) is ionized. The Ar plasma is
about 14 mm wide (in a torch with an 18-m-inner-diameter outer tube) and 30 to
40 m long. Gas temperatures of 3000 to 7000 K and electron temperatures of
4000 to 8000 K are produced [6].
Argon is the most commonly used plasma gas because it is generally in-
expensive (except in some parts of the world), inert (although Ar+ ions are
reactive), and monoatomic and produces a relatively simple background spec-
trum. Mixed gas plasmas (Arm,) have been used to reduce signals from some
molecular ions. Helium plasmas may more effectively ionize elements with high
ionization energies, such as As, Se, and halogens [7]. Elements that suffer from
interferences with polyatornic argide ions are better detected in He plasmas [7].
However, practical problems with helium plasmas have limited their use [7].
Helium plasmas have lower gas temperatures and are more severely affected by
solvent loading as sample aerosol is introduced into the plasma. The potential
necessary to sustain a He plasma is higher than for an argon plasma so problems
with discharges between the plasma and the mass spectrometer tend to be more
severe and difficult to control than for argon plasmas.
Gas flows into the plasma through the three tubes of the torch. The sample
aerosol is carried into the cooler, center channel of plasma through the center tube
by the Ar gas used for the nebulizer (0.5 to 1.0 L/min). The Ar gas flowing
between the outer tube and the intermediate tube (10 to 20 L/rnin), often called
the ~Zas~~ or outer gas, cools the outer tube between the plasma and the load coil
as well as providing plasma gas. A third Ar gas flow (0 to 1 .O Urnin) between the
intermediate tube and the center tube, often called the ~uxiZi~~ gas, is used
mainly to push the plasma up above the of the inner tube of the torch to prevent
top
it from overheating. In some cases, small amounts of nitrogen, oxygen, or other
gases are added to the argon (this is discussed in more detail later),
The plasma has an annular (or doughnut) shape because most of the radio
frequency current is carried in a thin skin on the outside of the plasma. The plasma
structure is important for two reasons: First, the sample is more easily carried into
the center of the discharge and confined from flowing around the outside of the
the
plasma by the hot, rapidly expanding gas in outer ring of the plasma doughnut.