BCH 4028

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BCH 4028 Instrumental Analytical
Chemistry ICP-AES and ICP-MS
Look in the Library at ICP Softbook by Cognitive
Solutions Ltd., nps(cf) QD96.P62 I37 1996 Also,
for ICP-AES, look at slide 16 and the following
slides in this presentation, and for ICP-MS look
at the following slides 4 - 28 in this
presentation.
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Atomic spectrometry
Atomic Absorption
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)
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Computer
Schematic diagram of ICP-AES instrument
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Sample introduction

• Liquid sample introduction
• Peristaltic pump
• Nebulizers
• Cross-Flow Nebulizers
• Concentric Nebulizers
• Babington Nebulizers
• Ultrasonic Nebulizers
• Spray chamber

• Solid sample introduction
• Direct solid sample insertion
• Electrothermal vaporization
• Laser ablation
• Slurry nebulization

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Liquid sample introduction peristaltic pump
To Nebulizers
Peristaltic pump head
From sample vessel

• Function
• The sample uptake rate of a nebulizer is
  dependent on the physical characteristics of the
  sample, such as viscosity and surface tension.
• Therefore, the delivery system commonly consists
  of a peristaltic pump and capillary tube to
  deliver a constant flow of analyte solution into
  the nebulizer.

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Liquid sample introduction Nebulizers

• Function
• To transform the stream of liquid sample into an
  aerosol.

Cross-Flow Nebulizer

• Two adjustable capillary tubes set at right
  angles to each other. The positions of the
  capillary tips can be adjusted to achieve
  maximum performance.
• Contact with the high speed gas and the liquid
  stream cause the liquid to break up into
  aerosol.
• It is tolerant to solutions with a high salt
  content, but is susceptible to blockage by
  stray matter as well as tip blockage due to
  salting-out.

Argon gas
Liquid sample
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Liquid sample introduction Nebulizers

• Concentric Nebulizer

• The solution is introduced through a capillary
  tube surrounded by a highvelocity argon gas
  stream parallel to the capillary axis. The
  constriction atthe end of the nebulizer causes
  an increase in flow rate of argon gas, creating
  a vacuum within the capillary
  tube, thus drawing solution out of the
  samplevessel.
• It provides excellent stability, but the small
  orifices can be blocked if the salt content is
  too high

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Liquid sample introduction Nebulizers

• Babington Nebulizer

Since the hole through which liquid emerges is
relatively large, so it is not prone to be
blocked by particulates or solutions with high
(gt10) salt content.
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Liquid sample introduction Nebulizers

• Ultrasonic Nebuliser

• Sample is pumped onto the surface of a
  piezoelectric transducer operating at a
  frequency of up to 10 MHz. The ultrasonic
  waves shatter the film of liquid and produce
  extremely fine droplets. The nebular is then
  transported to plasma by a flow of argon gas.
• If water loading of the analyte is too high,
  this would extinguish the plasma. So a
  desolvation device is usually necessary to
  evaporate the droplets and condense the
  resulting solvent vapor. Thus, the dried
  aerosol particles may then enter the plasma.
• It often suffers from memory effects so
  requires a long wash out period.

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Liquid sample introduction spray chamber
Smaller droplets go to ICP
Sample solution
Ar carrier gas
Larger droplets emerge from the tube by gravity
and exit via drain tube
Schematic of a Scott double-pass spray chamber

• Function
• It acts as a droplet size filter to discriminate
  against larger drops and pass these to the waste,
  while allowing the smallest ones to be
  transported to the plasma.

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Solid sample introduction - Direct solid
sample insertion

• Sample is placed on a probe that is usually
  made of graphite. The probe is then inserted
  up the injector of the torch and positioned so
  that the sample vaporizes into the plasma.

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Solid sample introduction - Electrothermal
vaporization
Output
Low voltage high current electrodes

• After introducing a small amount of sample onto
  the graphite rod, drying, charring and
  vaporization are achieved by slowly heating the
  graphite tube.
• The sample material is vaporized into a flowing
  stream of carrier gas, which passes through the
  furnace during the heating cycle.
• The analyte vapor re-condenses in the carrier gas
  and then swept into the plasma.

Input
Graphite rod
Ar gas supply
Water-cooled support blocks
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Solid sample introduction - Laser ablation
Both conducting and non-conducting solid and
powdered materials can be sampled by laser
ablation.
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Solid sample introduction - Slurry
nebulization
Solid sample can be nebulized into the plasma
suspended in a slurry. This technique is useful
for the determination of volatile elements which
may be lost during sample digestion procedure.
However, precision is adversely affected by the
problems of passing a slurry through a nebulizer.
So this depends critically on particle size,
slurry concentration and calibration methods
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Plasma Torches

• The plasma torch consists of three concentric
  quartz tubes through which the gas (normally
  argon) flows
• The outer tube contains the coolant gas flow,
  which spirally flows tangentially through the
  torch at a high velocity. This assists in
  cooling the torch and hence prevents damage.
• The middle tube contains the auxiliary gas flow
  to keep the plasma discharge away from the
  auxiliary and nebulizer tubes
• The innermost tube has the nebulizer gas flow
  which carries the sample aerosol to the
  plasma.

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Plasma Torches Fassel torches
This type of torch uses lower gas flows than
other types
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Plasma Torches Greenfield torches
Small opening of the tube causes the aerosol to
travel at a high velocity to punch a hole through
the plasma skin thus allowing analyte to undergo
excitation
Coolant gas can quench the plasma to some extent,
So Greenfield plasma are operated at higher
power than other types.
Ar gas flow
Analyte aerosol
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Plasma Torches Demountable torches
Demountable torches where the injector may be
removed and replaced with another of different
internal diameter (e.g. high salt solutions are
to be analyzed) or different materials (e.g.
ceramic or alumina injector tubes, which are more
robust and resistant to HF).
However, poor reproducibility of the annular
cooling gap between the inner and outer tubes.
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Nature of the Plasma

• Plasma is a substantially ionized gas, the
  properties of which depend significantly on the
  ionization. The energy required to maintain
  ionization is supplied by an external power
  source.
• Higher energy content and temperature (6000
  10000 K) than the chemical flame, i.e. greater
  excitation capability
• Overall, the plasma is neutral in terms of
  charge.
• (no. of ve ions no. of free electrons)

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Formation of the Plasma
Spark is then applied, electrons are stripped
from some of the Argon atoms
Connected to the RF generator
RF power is applied, an a.c. is set up which
oscillates at frequency governed by the generator
? oscillating current generates magnetic field.
Electrons are accelerated by the magnetic field
and collide with other argon atoms causing
further electrons to be ejected
Plasma formed consisting of neutral argon atoms,
ionized argon and electrons.
With the result that the K.E. of the charged
particles is distributed uniformly throughout,
the gas cloud has high energy.
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Formation of the plasma Saha Distribution

• For a plasma in thermodynamic equilibrium, the
  population distribution for the first ionization
  is given by the Saha equation
• ni ne/na (1/h3) 2Zi(T)/Za(2pmkT)3/2
  exp(-Eip/kT)

ni, ne and na the number of densities of the
first ions, electrons and atoms
respectively. m the electron mass k the
Boltzmann constant h Plancks constant Eip
the ionization potential Zi and Za the ionic
and atomic partition functions which vary slowly
with temperature if T lt 10000K
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Properties of the Plasma

• The nebulizer gas flow is placed at the
  centre of the torch assembly so that the gas
  punches a hole through the centre of the
  plasma. Thus the plasma is toroidal in shape.
  It is through this hole that sample is
  introduced.
• The diagram shows the approximate temperatures
  found in each region. These high temperatures
  (5000-6000 K in the tail flame) cause the
  analytes introduced to the plasma to become
  excited and emit radiation.

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Properties of the Plasma

• Spectral observations are normally made at a
  height of 15-20 mm above the induction
  region, where the background radiation is free
  from argon lines the normal analytical zone
  (NAZ), which contains all the analyte atoms and
  ions in their excited states.

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Plasma optimization
Viewing Height
Viewing Height
Viewing Height
Emission intensity is dependent on the
observation height, the flow rate of the injector
gas and the radio-frequency power levels supplied
to the plasma. Optimization the response for
ICP-AES/ ICP-MS usually means finding the
combination of conditions which maximizes that
intensity.
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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 better collection efficiency of emitted
light i.e. improved sensitivity
Radial light emitted from analyte has to pass
only a short distance in plasma i.e. less chance
of self absorption
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Spectrometers

• Function
• To isolate specific spectral lines emitted from
  the analyte(s) from all of the other emissions
  produced by other atoms, ions and molecules
  within the plasma.
• There are several devices available
• Monochromators
• Polychromators
• Echelle spectrometer

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Spectrometers - 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.
• Most spectrometers use diffraction gratings to
  achieve dispersion.

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Spectrometers - monochromators
(a) Light enters through the entrance slit
(b) It is reflected off a collimating mirror on
to diffraction grating
Lens
Source
Mirror
(c) As the grating rotates, the light separates
into its different spectral components, and
passes the desired wavelength to a second
collimating mirror.
Grating (rotates)
Mirror
Detector
(d) The monochromatic light focuses onto an exit
slit positioned in front of the detector
Schematic of a Czerny Turner mounted
Monochromator
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Spectrometers - polychromators
Diffraction grating
ICP source
Lens
Exit slits
Detectors
Polychromator consists of a lens, an entrance
slit, a diffraction grating and exit slits. The
slits and the centre of the grating are located
on a circle, known as the Rowland circle. The
radius of the Rowland circle equals the focal
length of the grating. Since the grating does not
rotate in this case, the detectors are positioned
at the focal point for each spectral line.
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Spectrometers Echelle spectrometer
Highly resolved plasma spectra are directed via
an aperture plate to an array of detectors.
It achieves high resolution by using a coarse
grating in combination with a second dispersive
element to separate diffraction orders. The
emission beam from plasma is separated by a
diffraction grating and a prism, set
perpendicular to each other.
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Detector Photomultipier tubes (PMT)
PMT are commonly used as detector in atomic
spectroscopy to convert optical radiation
(photons), into an electrical signal (electrons).
These vacuum tube devices are very sensitive and
cover a large wavelength range. It consists of a
vacuum photocell with an anode, photocathode and
a number of dynodes which have an increasingly
positive potential with respect to the
photocathode.
Incoming radiation ejects electrons from
photocathode and these electrons are accelerated
down a dynode chain.
Look at some components of ICP-AES http//www.ele
mentalscientific.com/products/new.asp
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Analysis using ICP Sample preparation

• For many applications, the sample analyzed by
  ICP-AES will not be in suitable form . In order
  to transform solid samples into suitable form
  for introduction, sample pre-treatment is
  required.
• The pre-treatment method used will be depend on
  the nature of the sample and the element
  which are to be determined.
• Those methods commonly used are
• Dry-ash,
• Wet-ashing,
• Fusion,
• Solubilization
• Microwave digestion.

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Analysis using ICP Sample preparation
Dry Ashing

• Dry at 105 -100 ?
• Ash at 200-800 ?
• Dissolve the ashed sample in acids, usually HCl,
  H2SO4, HNO3 and HCl/ HNO3

Preparation is simple and widely applicable but
sample losses through volatilization and retention
Kjeldahl flask
Wet Ashing
Use of strongly oxidizing mineral acid, such as
HNO3, HF, H2SO4 and HClO4 to oxidize the
resistant components, with gentle heat. It can be
carried out in closed or open reaction systems.
Closed systems minimize the losses in
volatilization.
Isomantle
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Analysis using ICP Sample preparation
Fusion

• Sample is mixed in a platinum crucible with a
  flux which attacks all the major rock-forming
  silicates.
• Fused in a furnace.
• Cooled to room temperature.
• Dissolved in HNO3.

Fusion methods are commonly used with geological
samples.
Solubilization
Biological samples can be solubilized by
quaternary ammonium hydroxides. These have an
intensive hydrolytic action and are capable of
transforming animal tissue into a solution-like
form. However, strong sample dilution is required
to achieved efficient nebulization. So
determinand is present at lower concentration.
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Analysis using ICP Sample preparation
Microwave digestion
Microwave sample preparation uses microwave power
to heat several samples at once, which can speed
up a digestion processes. Advantages improved
detection limits, low acid concentration and a
reduced need for dry ashing or fusion.
Reaction pressure, temperature and time are
computer controlled
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Analysis using ICP Sample preparation
Microwave digestion
USEPA method 3051 MICROWAVE ASSISTED ACID
DIGESTION OF SEDIMENTS, SLUDGES, SOILS, AND
OILS
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Analysis using ICP Example applications
There is a wide range of applications in
different field of analysis Water Heavy metals
and pollutants in fresh-water, sea-water and
waste water samples . Agriculture and Food
Samples often requiring multi-element analysis,
such as soil, plant-materials, fertilizer, foods
and animal feedstuffs, animal tissues and body
fluids. Geological applications Determination
of rare-earth elements, marine geochemistry and
biogeochemical prospecting. Biological
applications Determination of essential trace
elements, nonessential trace elements used
therapeutically, and toxic trace elements.
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• Introduction to ICP-MS

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Mass analyzer
Lens optics
ICP torch
Detector
RF generator
Sampler cone
Nebulizer and spray chamber
Skimmer cone
Turbomolecular pump
Mechanical pump
Schematic diagram of an ICP-MS instrument
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ICP-MS - Ionization
Hottest part of plasma is 8000K
Sample channel is 6700K
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(No Transcript)
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ICP-MS Interface (Ion Focusing)
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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. -
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ICP-MS detector
Electron Multiplier (EM)
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.
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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. Example Vanadium has two isotopes
at 50 and 51 amu. 50V is only practical isotope
to use in the present at a chloride matrix,
because of the large contribution from the
16O35Cl interference at mass 51. However, the
isotope 50V (0.25) coincides with isotopes of
50Ti (5.4) and 50Cr (4.34).
Mathematical Correction
Example 114Sn in sample matrix interfered on 114
Cd Intensity 114Cd Intensity114
Abundance(114Sn/118Sn) x Intensity118Sn
Intensity114 0.65/24.23 x Intensity118Sn
This correction is not valid if the abundance of
the interfering element is many times greater
than that of the analyte, since the error of
measurement will be too large.
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Analysis using ICP-MS interferences
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
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Principles of Dynamic Reaction Cell (DRC)

• 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
• DRC can be controlled to filter out ions with a
  m/z ratio below and above the target analyte m/z
  ratio. This is known as band-pass.

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Example of ICP- DRC MS
Reactive Fill Gas Inlet (NH3)
Quadrupole Ion Guide Control
Ar


50Ar16O
56Fe
Quadrupole


56Fe

Ion-molecule reactions and collisions
NH3
O
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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 ORS has two modes of
interference removal processes, Hydrogen mode
eliminate interfering ions by charge transfer and
proton transfer Helium mode Collisional Induced
Dissociation (CID) and Energy Discrimination (ED)
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Principles of Octopole Reaction System (ORS)
Hydrogen mode
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.
There are 2 videos from Agilent for you to watch
or download at http//www.chem.agilent.com/script
s/generic.asp?lpage11690indcolNprodcolY
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Advantage of ICP-MS over ICP-AES and Flame AAS

• Detection limits up to 3 order of magnitude
  better than ICP-AES.
• Multi-element analysis over a wide range of
  elements.
• Dynamic range of up to 10 orders of magnitude.
• Fast analysis times.
• Relatively simple spectra.
• Isotope ratio measurement possible allowing the
  application of isotope dilution techniques

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• Some websites for ICP-AES and ICP-MS
• http//www.uni-wuerzburg.de/mineralogie/links/tool
  s/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/t
  each/smprimer/icp/icp.html
• http//www.ce.vt.edu/program_areas/environmental/t
  each/smprimer/icpms/icpms.htm
• http//cp.chem.agilent.com/scripts/LiteraturePDF.a
  sp?iPubNo5988-9689EN
• http//www.ivstandards.com/tech/icp-ops/part07.asp
  
• http//www.chem.agilent.com/Scripts/Generic.ASP?lP
  age31571indcolNprodcolY
• FEP and AAS
• http//www.resonancepub.com/atomicspec.htm

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the end !!