Nima yadollahi Postgraduate Student (MSc) Materials Science Engineering Corrosion & Protection of materials ICP OES/MS ICP OES/MS Inductively Coupled Plasma Optical Emission Spectrometry Optical Emission Spectrometry& Mass Spectrometry
Introduction: Electrons of an atom absorb energy and jump to higher energy levels When they return to normal states, they emit characteristic photons of energy. By isolating these photon wavelengths, we can determine the types and concentrations of the elements present.
Atomic Absorption Mass Spectrometry Atomic Emission Light of specific characteristic wavelength is absorbed by promoting an electron to a higher energy level (excitation) Light absorption is proportional to elemental concentration Light of specific wavelength from Hollow Cathode Lamp (HCL) Light and heat energy from high intensity source (flame or plasma) Light and heat energy from high intensity source (plasma) High energy (light and heat) promotes an electron to a higher energy level (excitation). Electron falls back and emits light at characteristic wavelength Light emission is proportional to elemental concentration 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 spectrometer - -
Adapted from (Gary Hieftje) Solid/liquid sample Solution Molecules in gas phase Sample preparation Nebulisation Atomisation=Dissociation Vaporisation Desolvation Atoms in gas phase Ions Excited Atoms Laser ablation,etc. Sputtering, etc. ICP-MS and other MS methods AAS and AES, X-ray methods Ionisation Excitation M + X - MX(g) M(g) + X(g) M+M+
Atomisation / Ionisation In plasma, sample moves through several zones In plasma, sample moves through several zones Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Initial radiation zone (IRZ): K: Vaporisation, Atomisation Initial radiation zone (IRZ): K: Vaporisation, Atomisation Normal analytical zone (NAZ): K: Ionisation Normal analytical zone (NAZ): K: Ionisation Start gas flowSwitch on RF power Plasma generated After leaving injector, sample moves at high velocity Punches hole in centre of plasma Ionisation of argon gas
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, respectively Axial: direct view into plasma, lower sensitivity, shifts detection range lower. Radial: light emitted from analyte has to pass only a short distance in plasma i.e. less chance of self absorption and better for concentrated samples.
Typical spray chambers used with ICP-OES. A - Scott double pass type, B - conical single-pass type with impact bead. Nebulizer & Spray Chamber A B Diagram of a Pneumatic Concentric Nebulizer. Diagram of a Pneumatic Babington Nebulizer. Diagram of a Pneumatic Cross-Flow Nebulizer.
Demountable ICP Torch. Overview of a Basic Inductively Coupled Plasma Atomic Emission Spectrometry
ICP source Lens Detectors Diffraction grating monochromators Rowland circle (polychromators) Echelle cross disperser (polychromator) Types Of Spectrometers / ICP OES Exit slits There are several devices available: - Monochromators (only isolate one line at any given instant) - Polychromators - Echelle spectrometer (Interrogate several different lines simultaneously)
Diffraction 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. n λ = 2dsin θ Bragg's Law Most spectrometers use diffraction gratings to achieve dispersion. Spectrometers Diffraction Gratings Photocathode, dynode and anode layout of a photomultiplier tube (PMT)
Salt Fusions – typically lithium metaborate LiBO 2 and sodium peroxide Na 2 O 2. 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 HNO 3, hydrochloric HCl, perchloric HClO 4 and hydrofluoric HF, acid. Certain materials require digestion in conc. HF. Br 2 or H 2 O 2 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. Acid digestion in a Pt dish Graphite crucible with lithium metaborate in furnace Rotor Sample preparation
Calculations %E =[(Result ppm x dilution) / Sample ppm] x 100
Elements by ICP-OES Different 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.
Detection Limit / ICP-OES (ppb - μ gr/Lit) Common Problems in ICP-OES Sampling and Sample Preparation Spectral Interference Matrix Effects Instrumental Drift
Schematic diagram of an ICP-MS instrument ICP torch Nebulizer and spray chamber Sampler cone Skimmer cone Lens optics Mass analyzer Detector Mechanical pump Turbomolecular pump RF generator
+ Aerosol is dried Particles are decomposed and dissociated Atomized and then ionized Analyte present as M + ions Highest M + population should correspond to lowest polyatomic population ICP-MS - Ionization Hottest part of plasma is ~8000K Sample channel is ~6700K
Mass spectrometry method: Detects ions distinguished by their mass-to-charge ratio (m/z value) Based on ions moving under influence of electrical or magnetic field Mass analysers generally require operation under vacuum, to avoid ions colliding with other particles
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
+ a.c. - + d.c. - 1.Four short, parallel metal rods are arranged symmetrically around the ion beam. 2. DC and AC electrical potential is applied to the rods with opposite rods having a net negative or positive potential. 3.The combined field causes the ions to oscillate about their central axis. 4.Only those isotopes with certain mass to charge ratio can pass through the array without being removed. m/e = H 2 r 2 / 2v
Electron Multiplier (EM) Dynode Electrons Ion M + e - e - M + Fast analogue detection Pulse counting detection Ion 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.
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 effect The signal of the analyte suppressed by the matrix component. There are basically two types of matrix induced interferences. a.Sample transport effect b.Impact on ionization temperature of the plasma Polyatomic interference Interferences arising from the component of the plasma and the sample matrix. For example, Cl - in sample matrix interferes with 75 As by 40 Ar 35 Cl + AnalyteInterference 39 K +38 Ar 1 H + 40 Ca + 40 Ar + 51 V +35 Cl 16 O + 52 Cr + 40 Ar 12 C + 56 Fe + 40 Ar 16 O + 63 Cu + 23 Na 40 Ar + 75 As + 40 Ar 35 Cl + 80 Se + 40 Ar 2 +
sampler skimmer lens reaction cell mass analyzer vent Polyatomic 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 NH 3, CH 4, H 2 and He 2 is used to pressurize the reaction chamber to eliminate the interference by either to: (i)convert interfering ions into new polyatomic species which no longer interfere; or (ii)convert the analyte ion to a new polyatomic species at a new m/z ratio which is not interfered
56 Fe + Reactive Fill Gas Inlet (NH 3 ) Quadrupole + + Quadrupole Ion Guide Control = Isobaric 50 Ar 16 O + 56 Fe + Ar NH 3 + O Ion-molecule reactions and collisions ArO + + NH 3 O + Ar + NH 3 +
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 + + H 2 H Ar Proton transfer: ArH + + H 2 H Ar Helium 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.
Low flow sample introduction system High temperature 27MHz plasma generator Multi-element interference removal by on-axis octopole reaction cell High frequency hyperbolic quadrupole Fast simultaneous dual mode detector Off-axis Lens Reaction Gas Inlet Octopole Plasma ICP-MS System with Collision Reaction Cell (CRC)
ppb1000ppm10ppb100ppb1ppm100ppm Typical price range US$k Typical measurement range Flame Atomic Absorption Typically 50ppb to 500ppm Inductively Coupled Plasma Optical Emission Spectroscopy Typically 1ppb to >1000ppm (Simultaneous) Inductively Coupled Plasma Mass Spectrometry (Quadrupole) Typically 1ppt to 100ppm Graphite Furnace Atomic Absorption Typically 10ppt to 100ppb 10ppm100ppt10ppt1ppt
Advantages multielement, fast flexible element selection well documented methods very good tolerance to dissolved solids good linear dynamic range Disadvantages relatively poor detection limits many spectral interferences sample consumption high (1 to 5 mL/min)
Advantages excellent detection limits for most elements (ppb - ppt) most elements in Periodic Table available good sample throughput much simpler spectra than optical techniques low sample volume consumption mass spec - so isotopic information available Disadvantages dissolved solids/matrix effects - need to dilute samples more than other techniques capital cost high requires knowledgeable operator
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