Plasma Interface
Interface (backside) & Extraction Lens
Extraction Lens (-2000 volts)
ION OPTICS Tip of the sampler cone is positioned to be in the region of maximum ionization Ions no longer under control Adiabatic expansion decrease in gas density and temperature enthalpy converted to directional flow = speed increases forms a free supersonic jet between sampler and skimmer cones
ION OPTICS The position of the mach disk relative to the skimmer cone is extremely important Mach disk represents a region where the supersonic jet impacts background gas Within Mach disk, jet becomes subsonic, ions can be lost due to scattering and collisions Thus, we should sample the jet BEFORE the mach disk
ION OPTICS Position of the Mach disk (relative to sampler cone) X m = 0.67D 0 (P 0 /P 1 ) 1/2 D 0 = diameter of sampler orifice P 0 = pressure in ICP P 1 = pressure in expansion chamber
ION OPTICS turns out the ideal position of the skimmer cone is ~ 2/3 of X m
Sampler cone - plasma interactions Boundary layer and Sheath (remember - sampler cone is water cooled) Boundary layer = layer of gas at intermediate temperature between that of the cone and the plasma Sheath - electrical interaction (as opposed to the physical interaction that forms the boundary layer)
ION OPTICS Why do we care about the Boundary Layer and Sheath? the Boundary Layer is a cool region that is a significant source of oxide formation design of the interface and Z axis position critical
ION OPTICS Implications of the sheath In the case of a grounded sampler cone the sheath causes the plasma to have a positive potential sheath acts as couple between the plasma and cone due to impedance differences between the sheathplasma couple and the load coil- plasma couple, the plasma acquires an even greater positive (+) potential
ION OPTICS IF this positive potential is great enough, a discharge from the plasma into the orifice is observed called secondary discharge ( ARCing ) host of bad effects including - orifice (cone) erosion - multiply charged ions - variance in the kinetic energy of the ions
ION OPTICS The question you should be asking... how representative is the distribution of ions entering the high vacuum region of the ions in the plasma?
ION OPTICS Factors that could affect the representativeness of the sampled ion stream - cooling of ion stream - cool boundary layer reactions - ion recombination - ion collisions with background gas
ION OPTICS Evidence for a representative ion stream: Short duration of extraction (~ 3 us) Lack of substantial collisions between ions in stream Electron density does not change greatly from plasma to supersonic jet
ION OPTICS Gas flow rate through sampler ~ 10 21 atoms/sec 1% also make it through the skimmer ~ 10 19 atoms/sec majority (nearly 100%) of these atoms are from the centerline essentially all ion sampling occurs in zone of silence
Ion Focusing Purpose - deliver ions to mass analyzer focuses and directs ion beam
ION LENS (TRANSFER LENS) STACK
ION LENS (TRANSFER LENS) STACK The Transfer Lens system is used to: 1. extract the ions entering the analyzer section through the orifices of the cones with very high velocity; i.e. supersonic speed; 2. focus the divergent ion beam onto the target (Entrance Slit), 3. correct the direction of the beam to the target (Entrance Slit) 4. accelerate the ions to their full speed by applying 8KV 5. shape the ion beam into a flat shape so as to make it through the Entrance slit. The ions are subsequently travelling with the desired velocity through the focus point (Entrance Slit) and start to diverge again slightly. In order to control the ion beam during the travel path through the Magnetic Field and the Electric Field, the system controls the rotation of the beam and focuses again to the next focus point.
The proportion of ions sampled from the plasma that make it through to the detector is actually extremely small (1 part in 10 6 to 10 8 ), therefore, a high ion gain detection system is required if low detection limits are to be achieved
Ion Focusing General Assumptions 1. All ions are free particles with positive charge 2. Density of ion beam is not great enough to induce repulsion ( space charge effects) 3. Presence of ions does not change electrostatic fields 4. Vacuum conditions are adequate to give ions necessary mean free path 5. Ions originate with a constant kinetic energy
IONS IN ARGON FLOW SAMPLER SKIMMER ICP SHOCK WAVES
IONS ENTRAINED IN Ar FLOW ACCELERATED TO SAME VELOCITY AS Ar AVG. KE OF Ar = AVG. KE IN ICP = 2.5 kt gas = 0.5 m Ar v Ar 2 ALL IONS (i) ACHIEVE SAME VELOCITY v i = v Ar KE i = 0.5 m i v i 2 IONS OF DIFFERENT MASS HAVE DIFFERENT KINETIC ENERGIES
Basics of Ion Transfer Positive ion of charge (z) forms in a region of potential V initial (plasma) This positve ion has a potential energy of z*v initial (z= charge) The ion will travel through any region with a potential LESS than V initial
Ion Optics Potential energy of ion is converted to kinetic energy of ion = z(v initial -V 1 ); in a region of potential V 1 The velocity of ion (v) = [2z(V initial V 1 )/m] 1/2 while in a region of constant V 1, the ion will experience a constant speed and direction once the ion nears the exit of the lens, it enters a new field region, which changes the kinetic energy (velocity) and potentially direction
Ion Optics In an ideal world... All ions leaving the plasma would have the same kinetic energy regardless of m/z ratio This would result in uniform ion transmission through the lens stack
Ion Optics In the real world... All ions are essentially accelerated to the supersonic velocity of Ar in the supersonic jet This results in a range of kinetic energy that is a function of mass
ION ENERGY vs m/z m/z MAX. ION KE (ev)
Ion Optics Sources of initial kinetic energy: - Supersonic expansion - Plasma potential (remember the sheath!) - Amount of initial kinetic energy is related to m/z ratio of the ion
Ion Optics What does this mean? A single set of ion lens settings will not be appropriate for all elements Therefore, must COMPROMISE (unless looking at a narrow mass range)
Space charge effects The mutual repulsion of ions of like (similar) charge limits the total number of ions that can be compressed into a beam of given size
Space Charge Effects Plasma - ion flux is balanced by the electron flux - essentially neutral Supersonic jet - ion flux is balanced by the electron flux - essentially neutral Lens stack - electrons are repelled by negative potential - ion beam gains positive charge
Space Charge Effects Repulsion of like charges limits the ion density of an ion beam ICP-MS beam current: ~ 1mA Beam currents of ~ 1 ua should be enough to have substantial space charge effects
EINZEL LENS EFFECT OF SPACE CHARGE BEAM CURRENT 0 1 A
Space Charge Effects Remember our assumptions #3 - density of ion beam not great enough to induce space charge effects ASSUMPTION NOT MET -further reason for non-ideal behavior and different response across the mass range
Question: what elements are most likely to be effected by space charge effects?
Space Charge Effects Space charge effect is most strongly felt by lighter mass elements The space charge force (positive-positive repulsion) acts on all ions equally
Space Charge Effects Recall most of the ions present are Ar = mass 40 Elements lighter than mass 40 are going to undergo space charge effects even if there is no other matrix element!
Question: why are the low mass elements effected by space charge effects to a greater degree?
Space Charge Effects Heavy elements are effected less than light elements Heavy matrices cause more problems than light matrices Best case scenario = analysis of uranium in water Worst case scenario = analysis of Li in U-rich solution
ESA Detector ELEMENT SCANNING HIGH RES ICP-MS DEVICE Entrance slit Quad lenses Magnet & flight tube Extraction lenses Skimmer Sampler ICP Neb & Spray chamber
MAGNET
MAGNETIC SECTOR MASS ANALYZER + ION MOVING THRU MAGNETIC FIELD STRENGTH B v B F m v F m F m F m = MAGNETIC FORCE ALWAYS ACTS PERPENDICULAR TO DIR. OF MOTION v
EXAMPLE B = 10 3 Gauss m = 100 z = +1 m/z = 100 V = 2000 volts r m = 64 cm m z = 2 B r 2 V 2 m SCAN m/z BY SCANNING EITHER: B (vary mag. field) V (vary acc. voltage) AND / OR r m (array detector)
Electrostatic Analyzers (ESA)