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Ionization
Potentials
THE PLASMA
Inductively Coupled Plasma Mass
Spectrometry (ICP-MS)
•  What is a Plasma?
-  The magnetic field created by a RF (radio frequency) coil
produces a current within a stream of Argon (Ar) gas,
which is ‘seeded’ with energetic electrons
-  A ‘spark’ is passed through the Argon in the presence of
the RF field of the coil to initiate the plasma
-  A steady-state plasma is produced when the rate at which
electrons are released by ionizing collisions equals the rate
at which they are lost by recombination.
-  A bluish-white light is characteristic of Ar ICP plasmas
Glass (quartz) torch
Plasma
Sample
(aerosol)
Argon gas
RF coil (copper)
Plasma
-  Why Argon (Ar)?
-  It is an ‘inert’ or ‘noble’ gas, thus not
explosive when subjected to an RF magnetic
field or spark
-  It is relatively cheap to manufacture since the
Ar is extracted directly from the atmosphere
-  The stream of Ar gas is usually between 8 to
20 Litres per minute
Advantages of a Plasma source Mass
Spectrometer
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high temperature – ‘good’ for ionization
reduction of interferences
stability
low background
low detection limits
inherently multi-element
wide calibration range
ELEMENT2 High ResolutionICP-MS
INSTRUMENT OVERVIEW
Plasma – Gases (Ar) & Gate Valve
Plasma - Gases
•  All gas flows are controlled by
Mass Flow Controllers
Plasma
•  RF (radio frequency) generator
–  RF power directed to the load coil from the RF
generator creates an oscillating current in the
coil
-  RF coil operates at a frequency of 27 to 40
MHz (mega-hertz)
-  Creates magnetic field in the region between
the coils
Plasma
•  It’s desirable to have the RF generator be able
to deliver its maximum power to the coil
•  This is possible if the impedance of the load (the
coil) is the same as the impedance of the source
(the RF generator)
•  The matching network tries to ensure they are
properly matched
Plasma
•  Matching network
–  if the coil and RF generator are not matched,
then a “standing wave” will be generated
along the line
–  The RF voltage and current will vary along the
line and if the mismatch is great enough, the
current/voltage can exceed the rated capacity
of the line
Plasma
•  750-2000 Watts to sustain plasma
•  All ICPs will have a readout of the “forward” and
“reflected” power
•  Forward power is ~ the power generated by the RF
generator
•  Reflected power is power not used if the load generated
by the RF generator is not “matched” at the coil
•  In ICP’s, a high reflected power value indicates that the
matching network is not functioning properly and that
excess power may be present in the line
Plasma
•  Spark initially ionizes some Ar atoms
•  Free electrons are accelerated by the magnetic
field
•  This process of energy addition through the use
of the RF energy supplied by the coil is the
induction part of ICP
•  High-energy electrons collide with other atoms
causing further ionization
•  Results in a chain reaction
Plasma
Components of a Plasma
•  Induction region (IR) - region where most of the energy
from the coils is coupled to the plasma. Hottest region ~
10,000K
•  Pre-heating zone (PHZ) - Region where cool aerosol
from nebulizer punches a hole in plasma. Coolest region
~ 5000-7000K
•  Initial radiation zone (IRZ) - Region where excitation
and ionization take place. ~ 7000K
•  Normal analytical zone (NAZ)
Components of a Plasma
Ion source & interface region
BEYOND THE PLASMA
ION PATH
•  Ions move from high pressure to progressively lower
pressures through small orifices
•  1. Plasma = 1000 mbar (atmospheric pressure)
•  2. Interface region (expansion stage) ~ 2 mbar during
pumping of the interface pump (mechanical rotary pump)
•  3. Intermediate stage (lenses) ~ 2x10-4 mbar
•  4. Analyzer stage (MS) ~ 2x10-7 mbar
Element 2 – High Resolution-ICP-MS
Vacuum readings – Element 2
•  Stage 1: mechanical pumping of expansion chamber
•  Stage 2: mechanical roughing pump (fore pump) for Turbo Pump A
- evacuates first part of lens stack
•  Stage 3: Turbo Pump B - evacuates second part of lens stack
•  Stage 4: Turbo Pump C - evacuates flight tube
•  Stage 5: Turbo Pump D – evacuates electrostatic analyzer
ION OPTICS –
acceleration, focusing
•  the plasma is a hostile environment
- hot, atmospheric pressure, lots of ions
•  ICP-MS: how do we get the desirable ions
to the MS?
ION OPTICS –
acceleration, focusing, detection
•  After being generated in the plasma in front of the sample cone, the
ions pass through the small orifices of the cones (sampler and
skimmer).
•  In the analyzer housing, the ions get attracted and accelerated by
the potential of the Extraction lens and then follow the Transfer
optics, which shape and focus the ion beam to the Entrance Slit.
•  The ion beam passes the Magnetic sector for mass separation,
and the Electrostatic sector (ESA) for energy separation and
energy focusing.
•  Ion detection behind the Exit Slit is realized by a Conversion
Dynode and an ‘off-axis’ Secondary Electron Multiplier (SEM).
Interface – Sampler & Skimmer cones
•  Cones are made of nickel (Ni)
most of the time, but they could
also be made of Pt, Cu or Al.
•  The metal must be characterized
by high thermal conductivity,
otherwise it will melt! A high
melting point is therefore
important and it should also be
as hard as possible.
•  However thermal conductivity
seems to be the best criteria with
regards to performance/price
ratio
Sampler & Skimmer Cones
• 
Material
Thermal
Conductivity
(Wm-1K-1)
Melting Point
(degrees C)
Hardness
Al
237
660
soft
Cu
401
1083
soft
Ni
90.9
1453
hard
Pt
71.6
1772
hard