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

RF coil (copper) Argon gas

Glass (quartz) torch

Sample (aerosol)

Plasma

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

•  high temperature – ‘good’ for ionization •  reduction of interferences •  stability •  low background •  low detection limits •  inherently multi-element •  wide calibration range

ELEMENT2 High Resolution-ICP-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