Inductively Coupled Plasma ICP OES/MS Inductively Coupled Plasma Optical Emission Spectrometry & Mass Spectrometry Nima yadollahi Postgraduate Student (MSc) Materials Science Engineering Corrosion & Protection of materials

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 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 to elemental concentration - Light of specific wavelength from Hollow Cathode Lamp (HCL) Atomic Emission 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 - 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 spectrometer Light and heat energy from high intensity source (plasma)

Crucial steps in atomic spectroscopies and other methods Laser ablation ,etc. M+ X- MX(g) Nebulisation Solid/liquid sample Solution Molecules in gas phase Desolvation Sample preparation Vaporisation Sputtering, etc. M(g) + X(g) Atomisation=Dissociation Atoms in gas phase Ionisation Excitation M+ Ions Excited Atoms  ICP-MS and other MS methods  AAS and AES, X-ray methods Adapted from www.spectroscopynow.com (Gary Hieftje)

Atomisation / Ionisation In plasma, sample moves through several zones Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Initial radiation zone (IRZ): 6500-7500 K: Vaporisation, Atomisation Normal analytical zone (NAZ): 6000-6500 K: Ionisation Start gas flow Switch on RF power Ionisation of argon gas After leaving injector, sample moves at high velocity Punches hole in centre of plasma Plasma generated

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, respectively 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. Axial: direct view into plasma, lower sensitivity, shifts detection range lower.

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. A B

Overview of a Basic Inductively Coupled Plasma Atomic Emission Spectrometry Demountable ICP Torch.

Types Of Spectrometers / ICP OES There are several devices available: Monochromators (only isolate one line at any given instant) Polychromators Echelle spectrometer (Interrogate several different lines simultaneously) monochromators Rowland circle (polychromators) ICP source Diffraction grating Lens Detectors Exit slits Echelle cross disperser (polychromator)

layout of a photomultiplier tube Spectrometers layout of a photomultiplier tube (PMT) Diffraction Gratings 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. nλ = 2dsinθ Bragg's Law Most spectrometers use diffraction gratings to achieve dispersion. Photocathode, dynode and anode

Sample preparation Salt 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 furnace Acid digestion in a Pt dish Rotor

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 - μgr/Lit) Common Problems in ICP-OES Sampling and Sample Preparation Spectral Interference Matrix Effects Instrumental Drift

Schematic diagram of an ICP-MS instrument Mass analyzer Lens optics ICP torch Detector RF generator Sampler cone Nebulizer and spray chamber Skimmer cone Turbomolecular pump Mechanical pump

ICP-MS - Ionization + Analyte present as M+ ions 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 Hottest part of plasma is ~8000K Sample channel is ~6700K

Inductively Coupled Plasma Mass Spectrometry 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

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

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

ICP-MS – detector Electron Multiplier (EM) Dynode Electrons Ion M + e - 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. Analyte Interference 39K+ 38Ar1H+ 40Ca+ 40Ar+ 51V+ 35Cl16O+ 52Cr+ 40Ar12C+ 56Fe+ 40Ar16O+ 63Cu+ 23Na40Ar+ 75As+ 40Ar35Cl+ 80Se+ 40Ar2+ Sample transport effect 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 75As by 40Ar35Cl+

Principles of Dynamic Reaction Cell (DRC) 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 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; or convert the analyte ion to a new polyatomic species at a new m/z ratio which is not interfered

Ion-molecule reactions and collisions Example of ICP- DRC –MS Reactive Fill Gas Inlet (NH3) Quadrupole Ion Guide Control Ar + + + + 50Ar16O+ + + + + + + 56Fe+ Quadrupole + + 56Fe+ + + = Isobaric Ion-molecule reactions and collisions NH3+ O ArO+ + NH3 O + Ar + NH3+

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+ + Ar Proton transfer: ArH+ + H2  H3+ + 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.

ICP-MS System with Collision Reaction Cell (CRC) Multi-element interference removal by on-axis octopole reaction cell Fast simultaneous dual mode detector Reaction Gas Inlet High temperature 27MHz plasma generator Plasma Octopole Off-axis Lens High frequency hyperbolic quadrupole Low flow sample introduction system

Typical detection limits of ICP-MS instrument

Price/Performance – How does ICP-MS Compare With Other Inorganic Techniques ? 200 Inductively Coupled Plasma Mass Spectrometry (Quadrupole) Typically 1ppt to 100ppm 150 Typical price range US$k Inductively Coupled Plasma Optical Emission Spectroscopy Typically 1ppb to >1000ppm (Simultaneous) 100 Graphite Furnace Atomic Absorption Typically 10ppt to 100ppb 50 Flame Atomic Absorption Typically 50ppb to 500ppm 1ppt 10ppt 100ppt 1ppb 10ppb 100ppb 1ppm 10ppm 100ppm 1000ppm Typical measurement range

ICP-OES Advantages Disadvantages 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)

ICP-MS Disadvantages 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

Some websites for ICP-AES and ICP-MS: http://www.uni-wuerzburg.de/mineralogie/links/tools/icp-aes.html http://www.thespectroscopynet.com/ http://icp-oes.com/ http://www.epa.gov/SW-846/pdfs/6020a.pdf and http://www.meritlabs.com/Methods/6020a.metals.pdf http://www.ce.vt.edu/program_areas/environmental/teach/smprimer/icp/icp.html http://www.ce.vt.edu/program_areas/environmental/teach/smprimer/icpms/icpms.htm http://cp.chem.agilent.com/scripts/LiteraturePDF.asp?iPubNo=5988-9689EN http://www.ivstandards.com/tech/icp-ops/part07.asp http://www.chem.agilent.com/Scripts/Generic.ASP?lPage=31571&indcol=N&prodcol=Y

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