Function and setup of the PlasmaQuant MS
PlasmaQuant MS product family
20
3
Function and setup of the PlasmaQuant MS
3.1
Physical principle of ICP-MS
Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful technique for
elemental and isotopic analysis that combines the efficiency and ease of use of the
inductively coupled plasma (ICP) with the sensitivity and selectivity of mass
spectrometry (MS).
The inductively coupled plasma (ICP) used in analytical spectrometry is an
atmospheric-pressure plasma, generated in argon gas flowing through a specially
designed torch. Energy transfers into the plasma by inductive coupling of the plasma
electrons and ions within the magnetic field of a radio-frequency coil.
Inductively coupled plasma offers an advantage over chemical flames with its much
higher temperature. Samples introduced into the ICP heat to more than 6 000 K. This
high temperature is focused on a very small area of approx. 5 cm
3
. The sample is
introduced to the plasma in the form of aerosol (small droplets in a gas). The droplets
typically decompose, vaporize, atomize and ionize very quickly and effectively.
The inductively coupled plasma has been used commercially as an atomization and
excitation source for optical emission spectrometry for over 30 years. Many of the
most intense lines in ICP optical emission spectra are produced by singly-charged ions;
therefore the ICP makes an efficient ion source for mass spectrometry. In ICP nearly all
the metals are over 90 % ionized. Even elements with relatively high ionization
potentials (such as P, As, Hg and I) are over 20 % ionized.
The ICP ion source operates at atmospheric pressure (ca. 1 bar), while the mass
spectrometer operates at a very low pressure (ca. 7 x 10
-5
mbar). Efficient transfer of
ions from the ICP to the mass spectrometer is therefore a major challenge,
complicated by the fact that the plasma is at very high temperatures and by the high
power radio frequency currents present in the plasma.
In the PlasmaQuant MS an ionized sample is extracted from the plasma through two
interface cones into the mass spectrometer. The pressure is reduced gradually in three
stages by differential pumping. The PlasmaQuant MS uses the ReflexION, a 3D
focusing, 90-degree ion mirror, to focus ions into the mass analyzer. The ion optics
allows elimination of neutrals, photons, droplets and solids that would otherwise
produce high background signals and would contaminate the mass analyzer.
The quadrupole mass spectrometer separates ions of a specific mass to charge ratio. In
the PlasmaQuant MS patented self-cleaning curved fringe rods are mounted in front of
the mass analyzer. These pre-filters cause the ions to follow a curved trajectory
shielding the quadrupole from excited neutrals. The all-digital (AD) Detector, a
Discrete Dynode Electron Multiplier with eleven orders of dynamic range, records ions
after they exit the mass spectrometer.