Nima yadollahi Postgraduate Student (MSc)

Materials Science EngineeringCorrosion & Protection of materials

ICP OES/MS

Inductively Coupled Plasma Optical Emission Spectrometry

&Mass Spectrometry

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.

Atomic spectrometry

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 toelemental concentration

Light of specific wavelengthfrom 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 wavelengthLight emission is proportional toelemental 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

-

-

Crucial steps in atomic spectroscopies and other methods

Adapted from www.spectroscopynow.com (Gary Hieftje)

Solid/liquid sample

Solution Molecules in gas phaseSample

preparation

Nebulisation

Atomisation=Dissociation

VaporisationDesolvation

Atoms in gas phase

IonsExcited 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+

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

Plasma generated

After leaving injector, sample moves at high velocity Punches hole in centre of plasma

Ionisation of argon gas

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

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. nλ = 2dsinθ 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 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.

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 - μgr/Lit)

Common Problems in ICP-OES

Sampling and Sample PreparationSpectral Interference

Matrix EffectsInstrumental 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+

ionsHighest M+ population should correspond to

lowest polyatomic population

ICP-MS - Ionization

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 fieldMass analysers generally require operation under vacuum, to avoid ions colliding with other particles

I CP-MS – I nterface (Sampler and Skimmer Cones)

I ons extracted f rom atmospheric pressure through a cooled metal sampler cone with an orifi ce (< 1 mm diameter) to a zone of silence at ~2 Torr. Then the ions extracted through a second cone called a skimmer (with an orifi ce diameter < 0.9 mm) and go to a region of ion optics at 1 Torr.

The role of the interf ace is to extract representative analyte ions f rom I CP, which are at atmospheric pressure and transf er them effi ciently to detector regions

(10-5 Torr). I CP-MS interf ace consists of several step-down vacuum stages.

Sample cone

Plasma torch

RF coil

Skimmer cone

Ion optics

~2 Torr vacuum

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)

+ 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 = H2 r2 / 2v

ICP-MS – detector

Electron Multiplier (EM)

Dynode

ElectronsIonM+

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 effectThe signal of the analyte suppressed by the matrix component. There are basically two types of matrix induced interferences.a. Sample transport effectb. 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+

Analyte Interference 39K+ 38Ar1H+

40Ca+ 40Ar+

51V+ 35Cl16O+

52Cr+ 40Ar12C+

56Fe+ 40Ar16O+

63Cu+ 23Na40Ar+

75As+ 40Ar35Cl+

80Se+ 40Ar2+

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:

(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

Example of ICP- DRC –MS

56Fe+

Reactive Fill Gas Inlet (NH3)

Quadrupole

+

+

Quadrupole Ion Guide Control

+ + + + ++

+ +

+

+

++

+

++

++

++ =Isobaric

50Ar16O+

56Fe+

Ar

NH3+

OIon-molecule

reactions and collisions

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

+ + ArProton 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.

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

OctopolePlasma

ICP-MS System with Collision Reaction Cell (CRC)

Typical detection limits of ICP-MS instrument

Price/Performance – How does ICP-MS Compare With Other Inorganic Techniques ?

100

200

50

150

1ppb 1000ppm10ppb 100ppb 1ppm 100ppm

Typ

ical p

rice r

an

ge U

S$

k

Typical measurement range

Flame Atomic AbsorptionTypically 50ppb to 500ppm

Inductively Coupled Plasma Optical Emission SpectroscopyTypically 1ppb to >1000ppm (Simultaneous)

Inductively Coupled Plasma Mass Spectrometry (Quadrupole)Typically 1ppt to 100ppm

Graphite Furnace Atomic Absorption Typically 10ppt to 100ppb

10ppm100ppt10ppt1ppt

ICP-OES

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)

ICP-MS 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.htmlhttp://www.thespectroscopynet.com/http://icp-oes.com/http://www.epa.gov/SW-846/pdfs/6020a.pdf and http://www.meritlabs.com/Methods/6020a.metals.pdfhttp://www.ce.vt.edu/program_areas/environmental/teach/smprimer/icp/icp.htmlhttp://www.ce.vt.edu/program_areas/environmental/teach/smprimer/icpms/icpms.htmhttp://cp.chem.agilent.com/scripts/LiteraturePDF.asp?iPubNo=5988-9689ENhttp://www.ivstandards.com/tech/icp-ops/part07.asphttp://www.chem.agilent.com/Scripts/Generic.ASP?lPage=31571&indcol=N&prodcol=Y

the end !!