ICP-MS

• Types of Atomic Spectroscopy:

  Source ---> sample ---> detected particle --->spectrometric technique

  Start will the spectrochemical source:

  • Energy of a Photon
  • Electron
  • Heat
  • Ions

  Detected Particles:

  • Photons
  • X-Ray Fluorescence
  • Optical Spectrometry
  • Absorption
  • Emission
  • Fluorescence
  • Ions
  • Mass Spectrometry (our focus)
  • Electrons
  • Electron Spectrometry
• Generation of a Particle I:

  Measure the photon after atomic excitation

  Optical Spectroscopy

  • Absorption of Energy by an Electron, moves electron away from ground state(AAS)
  • Emission of Energy by an Electron, moves electron towards ground state (AES)
  • Light Energy Aimed at the Electron in the Ground State, gives of photon energy and moves electron away from ground state (AFS)
  • Primary X-radiation hits a Ground State Electron, this electron is kicked out of all the shells, outer shell electrons take it's place as well as the emission of X-ray fluorescence radiation (XRF)
• Generation of a Particle II:

  Measuring the mass of a charged species after ionisation.

  • ionising radiation hits electron
  • emits this electron
  • outer shell electron is excited and moves in towards the ground state, taking the place of the emitted electron, as well as giving off UV light at the same time
  • Mass Spec (AFS and XRF)
• Generation of a Particle III:

  Measuring the electronic kinetic energy after electron emission.

  Electron Spectroscopy

  • Ground state electrons move all the way by gaining kinetic energy (XPS or AES)
  • Middle state electrons can either bind and move towards the ground state or move towards the outer shell with the kinetic energy (AES)
  • Excited state electrons don't really move any further (UPS)
• Common Atomic Spectroscopy Techniques:

  • Flame Atomic Absorption Spectrometry
  • Graphite Furnace Atomic Absorption Spectrometry
  • Inductively Coupled Plasma Atomic Emission Spectrometry
  • Inductively Couple Plasma Mass Spectrometry
• Why ICP-MS:

  Gold Standard

  • good detection limit (ppt)
  • linear dynamic range
  • precise-ish
  • lots of elements can be tested
  • can do isotopes
  • sample throughput is less than 1 second

  Disadvantages:

  • cannot total dissolve solids (GF-AAS is better)
  • can only do low sample volumes (GF-AAS is better)
  • operating costs (F-AAS is cheaper)
  • capital costs (F-AAS is cheaper)
• Things that need to be Considered:

  Sensitivity:

  • degree of ionisation
  • relative isotopic abundance
  • transmission efficiency

  Background Noise (relative to counts)

  • source flicker noise (during sample introduction)
  • counting statistics (detector noise)
• Mass Filters that are used:

  • Quadrupole (selective)
  • Magnetic Sector Field (high resolution)
  • Time of Flight (considered the best choice)
• Inside the ICP-MS:

  Sample Introduction --> ICP --> Ion Extraction --> Collision Reaction Cell --> Quadrupole --> Detector
• Sample Introduction:

  Sample in --> nebulised using argon gas (becomes a mist) --> into the spray chamber --> large particles go down into the drain and small particles go up to the torch/spray chamber

  Nebulisation --> Desolvation --> Vaportisation --> Atomisation --> Ionisation --> Mass Analysis
• ICP Source:

  Physical Setup:

  • plasma gas, nebuliser gas, and coolant gas goes into a Quartz torch made up of concentric tubes.
  • Surrounded by RF-loaded coils that cause the spark for the reaction with the argon plasma, RF voltage induced rapid oscillation motion of the argon ions which causes heat
  • sample aerosol is carried through the centre into the plasma
• ICP-MS

  • Inductively - conductive material is heated using a gas with electromagnetic coils
  • Coupled - combine with
  • Plasma - one of the four fundamental states of matter, it is the presence of a significant portion of charged particles in any combination of ions or electrons.
• Ionisation Process:

  uses argon.

  • electron impact (molecular ion and 2 electrons)
  • charge transfer (molecular ion and argon)
  • Penning ionisation (molecular ion, argon and 1 electron) (argon impact causes the excited state that emits an electron)
• Ion Extraction:

  Samples goes through:

  Sample cone --> Skimmer cone --> ion beam is focused and guided using electrostatic lenses.

  
  

  it is all done under vacuum once the beam has passed through the Skimmer cone so that there is reduced particle collision and reduction of noise in the detector
• Types of Interface:

  Dual Cone System:

  • sample cone
  • Skimmer cone

  more distance that is travelled the lower the pressure gets due to the beam divergence. This can be very messy for the cell and the spectrometer

  
  

  Triple Cone System:

  • sample cone
  • Skimmer cone
  • hyper-Skimmer cone

  more distance the beam travels the pressure drops gradually in three steps, this reduces the beam divergence and keeps the cell and spectrometer clean
• Photon/Shadow Stop

  • ion beam is tightly focused by the extraction lenses
  • beam travels through the shadow stop
  • photons and neutrals are not deflected so they remain in the beam and are removed
  • ions of all masses are deflected off of the axis and go to the collision cell

  alternative

  Quadrupole Ion Deflector

  • ions, photon and neutrals travel in the ion beam
  • the surrounding negative and positive magnetic fields cause the light ions and heavy ions to be deflected at 90 degrees and the photons and neutrals carry on in the ion beam
• Non-Spectroscopic Interferences

  Matrix Effects:

  • introduction effects - issues with the aerosol transport efficiency
  • space-charge effects - high mass remain more focused on the beam whereas lower mass could have the possibility of drifting off
  • plasma effects - thermal ionisation equilibrium

  Instrument Drift

  • dissolved solids deposit left on the nebuliser cone and causes blockage and therefore signal suppression
  • temperature of ICP components impact ionisation efficiency
• Non-Spectroscopic Interferences:

  • dilution - reduces matrix and concentration and reduces signal
  • internal standard - element spiking to normalise matrix effects, assumes standard and analyte are the same
  • chemical purification - enhances analyte matrix ratio, time consuming and complex
• Spectroscopic Interferences:

  • isotopes/isobaric - confuse other analytes with the same m/z
  • polyatomic - sample and matrix ions recombine after ICP and give same m/z as the analyte
• Collision Reaction Cell

  Kinetic Energy Discrimination (the removal of polyatomics) depends on:

  • collision cross section (the differential)
  • number of collisions
  • cell pressure and length
• Collision Reaction Cell

  Dynamic Reaction Cell (remove isobaric and polyatomics)

  • polyatomics and analyte react with ammonia reaction gas - the rate of reaction with iron is slower than argon oxide
  • then goes to mass filter
• Quadrupole Mass Filter

  • use scan mode or sim mode

  tandem MS

  • on mass approach - reaction only with the anlyte of interest
  • mass shift approach - reaction with the analytes that are NOT of interest - keeping the analyte of interest going the the second filter (Q2)
• Detectors

  • faraday cups - metal hollow collectors, open one end and closed the other and electric current is measured to determine the number of ions collected in a specific time period
  • dynode electron multiplier - electrical pulse for every ion that strikes the inner surface with the sufficient kinetic energy and pulses are counted

  Signal

  • pulse - pulses corresponding to individual particle detection
  • analogue - signal has a continuous voltage and every intensity of a particle hitting it is detected
• Types of Samples:

  • soft tissue - break down using perchloric acid and then heated
  • environmental
  • cells
  • fluids - alkaline dilution or acid digestion
• Configurations/Applications

  • Single Cell ICP-MS - example cisplatin uptake and resistence
  • HPLC ICP-MS - example arsenic species in toxicology
  • Laser Ablation ICP-MS - example iron localisation in Alzheimer's disease brain