2 Introduction:Electrons of an atom absorb energy and jump to higher energy levelsWhen they return to normal states, they emit characteristic photons of energy.By isolating these photon wavelengths, we can determine the types andconcentrations of the elements present.
3 Atomic spectrometry Atomic Absorption Atomic Emission Light of specific characteristic wavelength is absorbed by promoting an electron to a higher energy level (excitation)Light absorption is proportional toelemental concentration-Light of specific wavelengthfrom Hollow Cathode Lamp (HCL)Atomic EmissionHigh energy (light and heat) promotes an electron to a higher energy level (excitation). Electron falls back and emits light at characteristic wavelengthLight emission is proportional toelemental concentration-Light and heat energy from high intensity source (flame or plasma)--Mass Spectrometry-High energy (light and heat) ejects electron from shell (ionization). Result is free electron and atom with positive charge (Ion)Ions are extracted and measured directly in mass spectrometerLight and heat energy from high intensity source (plasma)
4 Crucial steps in atomic spectroscopies and other methods Laser ablation ,etc.M+ X-MX(g)NebulisationSolid/liquid sampleSolutionMolecules in gas phaseDesolvationSamplepreparationVaporisationSputtering, etc.M(g) + X(g)Atomisation=DissociationAtoms in gas phaseIonisationExcitationM+IonsExcited Atoms ICP-MS and other MS methods AAS and AES, X-ray methodsAdapted from (Gary Hieftje)
5 Atomisation / Ionisation In plasma, sample moves through several zonesPreheating zone (PHZ): temp = 8000 K: Desolvation/evaporationInitial radiation zone (IRZ): K: Vaporisation, AtomisationNormal analytical zone (NAZ): K: IonisationStart gas flowSwitch on RF powerIonisation of argon gasAfter leaving injector, sample moves at high velocity Punches hole in centre of plasmaPlasma generated
6 Viewing position of plasma The plasma generated in an ICP can be viewed by the spectrometer,side-on or end-on. These viewing positions are called radial and axial viewing, respectivelyRadial: light emitted from analyte has to pass only a short distance in plasmai.e. less chance of self absorption and better for concentrated samples.Axial: direct view into plasma, lower sensitivity, shifts detection range lower.
7 Nebulizer & Spray Chamber Diagram of a Pneumatic Concentric Nebulizer.Diagram of a Pneumatic Babington Nebulizer.Diagram of a Pneumatic Cross-Flow Nebulizer.Typical spray chambers used with ICP-OES. A - Scott double pass type, B - conical single-pass type with impact bead.AB
8 Overview of a Basic Inductively Coupled Plasma Atomic Emission Spectrometry Demountable ICP Torch.
9 Types Of Spectrometers / ICP OES There are several devices available:Monochromators(only isolate one line at any given instant)PolychromatorsEchelle spectrometer(Interrogate several different lines simultaneously)monochromatorsRowland circle (polychromators)ICP sourceDiffraction gratingLensDetectorsExit slitsEchelle cross disperser (polychromator)
10 layout of a photomultiplier tube Spectrometerslayout of a photomultiplier tube(PMT)Diffraction GratingsDiffraction Grating is a mirrored surface that has closely spaced lines ruled or etched onto its surface.The continuum light that hits the grating will be diffracted at an angle that is dependent upon the wavelength. nλ = 2dsinθ Bragg's LawMost spectrometers use diffraction gratings to achieve dispersion.Photocathode, dynode and anode
11 Sample preparationSalt Fusions – typically lithium metaborate LiBO2 and sodium peroxide Na2O2.Sample is mixed with lithium metaborate in a 1:9 ratio.Mix is melted at 900C and dissolved in a nitric acid solution.Acid Digestions – nitric HNO3, hydrochloric HCl, perchloric HClO4 and hydrofluoric HF, acid.Certain materials require digestion in conc. HF.Br2 or H2O2 can be added to conc. acids to give a more oxidising medium and increase solubility.Sample is allowed to dissolve in an acid mix.Sample is typically heated to speed dissolution.Microwave Digestion – basically acid digestion in controlled temperature and pressure vessels.Sample is allowed under controlled temperature and pressure conditions in a pressure vessel.Graphite crucible with lithium metaborate in furnaceAcid digestion in a Pt dishRotor
12 Calculations%E =[(Result ppm x dilution) / Sample ppm] x 100
14 Elements by ICP-OESDifferent elements have different emission intensities.e.g. Alkalis (Na, K, Rb, Cs) are weakly emitting.Alkaline Earths (Be, Mg, Ca, Sr, Ba ) are strongly emitting.
15 Detection Limit / ICP-OES (ppb - μgr/Lit) Common Problems in ICP-OESSampling and Sample PreparationSpectral InterferenceMatrix EffectsInstrumental Drift
16 Schematic diagram of an ICP-MS instrument Mass analyzerLens opticsICP torchDetectorRF generatorSampler coneNebulizer and spray chamberSkimmer coneTurbomolecular pumpMechanical pump
17 ICP-MS - Ionization + Analyte present as M+ ions Aerosol is driedParticles are decomposed and dissociatedAtomized and then ionizedAnalyte present as M+ ionsHighest M+ population should correspond to lowest polyatomic populationHottest part of plasma is ~8000KSample channel is ~6700K
18 Inductively Coupled Plasma Mass Spectrometry Mass spectrometry method:Detects ions distinguished by theirmass-to-charge ratio (m/z value)Based on ions moving under influence of electrical or magnetic fieldMass analysers generally require operation under vacuum, to avoid ions colliding with other particles
20 ICP-MS – Interface (Ion Focusing) Focusing lens:+ve charged metallic cylinder which repels ions and refocus the ion beam. It Requires optimization.Grounded Shadow Stop:Traps photons and unionized materials from the plasma
21 ICP-MS – Mass analyzer (quadrupole) Four short, parallel metal rods are arranged symmetrically around the ion beam.DC and AC electrical potential is applied to the rods with opposite rods having a net negative or positive potential.The combined field causes the ions to oscillate about their central axis.Only those isotopes with certain mass to charge ratio can pass through the array without being removed.+ d.c. -+ a.c. -m/e = H2 r2 / 2v
22 ICP-MS – detector Electron Multiplier (EM) Dynode ElectronsIonM+e-Fast analogue detectionPulse counting detectionIon striking the 1st dynode causes the release of e- from the dynode surface. These e- are attracted to the 2nd dynode causing further release of e- and so on down the multiple detector dynodes.
23 Analysis using ICP-MS – interferences Isobaric Interference“Isobaric overlaps” produced by different isotopes of other elements in the sample that create spectral interferences at the same mass as the analyte.Matrix effectThe signal of the analyte suppressed by the matrix component. There are basically two types of matrix induced interferences.AnalyteInterference39K+38Ar1H+40Ca+40Ar+51V+35Cl16O+52Cr+40Ar12C+56Fe+40Ar16O+63Cu+23Na40Ar+75As+40Ar35Cl+80Se+40Ar2+Sample transport effectImpact on ionization temperature of the plasmaPolyatomic interferenceInterferences arising from the component of the plasma and the sample matrix. For example, Cl- in sample matrix interferes with 75As by 40Ar35Cl+
24 Principles of Dynamic Reaction Cell (DRC) samplerskimmerlensreaction cellmass analyzerventPolyatomic interference can be eliminated by DRC method.DRC is a quadrupole placed inside an enclosed reaction chamber. This enclosed quadrupole is positioned between the ion optics and the analyzer quadrupole. A reaction gas such as NH3, CH4, H2 and He2 is used to pressurize the reaction chamber to eliminate the interference by either to:convert interfering ions into new polyatomic species which no longer interfere; orconvert the analyte ion to a new polyatomic species at a new m/z ratio which is not interfered
25 Ion-molecule reactions and collisions Example of ICP- DRC –MSReactive Fill Gas Inlet (NH3)Quadrupole IonGuide ControlAr++++50Ar16O+++++++56Fe+Quadrupole++56Fe+++=IsobaricIon-molecule reactions and collisionsNH3+OArO+ + NH3O + Ar + NH3+
26 Principles of Octopole Reaction System (ORS) The stainless steel ORS cell, which can be pressurized with a gas, typically hydrogen or helium, is positioned between the ion lens assembly and the quadrupole mass filter. As analyte ions enter the cell, they interact with the gas, resulting in the reduction of the molecular interference .Hydrogen mode:Charge transfer: Ar+ + H2 H2+ + ArProton transfer: ArH+ + H2 H3+ + ArHelium mode:CID: When the collision energy between the He atom and polyatomic ion is significantly above the dissociation energy of the polyatomic ion, fragmentation occurs.ED: the larger polyatomic species collide more frequently with the cell gas, so they lose more energy than the smaller analyte species. The cell acts as a molecular filter by resolving low energy (polyatomic) and higher energy (analyte) ions from each other in the ion beam.
27 ICP-MS System with Collision Reaction Cell (CRC) Multi-element interference removal by on-axis octopole reaction cellFast simultaneous dual mode detectorReaction Gas InletHigh temperature 27MHz plasma generatorPlasmaOctopoleOff-axis LensHigh frequency hyperbolic quadrupoleLow flow sample introduction system
29 Price/Performance – How does ICP-MS Compare With Other Inorganic Techniques ? 200Inductively Coupled Plasma Mass Spectrometry (Quadrupole)Typically 1ppt to 100ppm150Typical price range US$kInductively Coupled Plasma Optical Emission SpectroscopyTypically 1ppb to >1000ppm (Simultaneous)100Graphite Furnace Atomic Absorption Typically 10ppt to 100ppb50Flame Atomic AbsorptionTypically 50ppb to 500ppm1ppt10ppt100ppt1ppb10ppb100ppb1ppm10ppm100ppm1000ppmTypical measurement range
30 ICP-OES Advantages Disadvantages multielement, fast flexible element selectionwell documented methodsvery good tolerance to dissolved solidsgood linear dynamic rangeDisadvantagesrelatively poor detection limitsmany spectral interferencessample consumption high (1 to 5 mL/min)
31 ICP-MS Disadvantages Advantages excellent detection limits for most elements (ppb - ppt)most elements in Periodic Table availablegood sample throughputmuch simpler spectra than optical techniqueslow sample volume consumptionmass spec - so isotopic information availableDisadvantagesdissolved solids/matrix effects - need to dilute samples more than other techniquescapital cost highrequires knowledgeable operator