Spectrochemical Analysis James D' Ingle

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Spectrochemical AnalysisJames D.
IngleStanley R. Crouch

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Introduction

• These methods deal with the absorption and
  emission of radiation by atoms.
• The methods deal with free atoms line spectra
  are observed.

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• Specific spectral lines can be used for
• elemental analysis - both quantitative and
• qualitative.
• The sample introduction step is extremely
  important in atomic spectrochemical methods.

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• Introduction
• Based on the breakdown of a sample into atoms,
  followed by the measurement of the atoms
  absorption or emission of light.
• deals with absorbance fluorescence or emission
  (luminescence) of atoms or elemental ions rather
  then molecules

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• ii- atomization process of converting sample to
  gaseous atoms or elementary ions.
• iii. Provides information on elemental
  composition of sample or compound

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• iv- UV/Vis, IR, Raman gives molecular functional
  group information, but no elemental
  information.
• v. Basic process the same as in UV/Vis,
  fluorescence etc. for molecules

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Atomization devices The process of forming free
atoms by applying heat to a sample is known as
atomization , and devices that carry out the
atomization process are called atomizers.
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These devices can be continuous or pulsed (non
continuous) atomizers.Continuous atomizers , the
atomization conditions (e. g. temperature) are
constant with time.
Such as flames or plasma
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• with a non continuous atomizer these conditions
  vary With time.
• Such as furnaces, spark

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Energy Level Diagrams
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Energy Level Diagrams energy level diagram for
the outer electrons of an element describes
atomic spectroscopy process. i. every
element has a unique set of atomic orbitals
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• ii. p, d, f split by spin-orbit coupling
• iii. Spin (s) and orbital (l) motion create
  magnetic fields that perturb each other (couple)

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Na
Mg
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-parallel ? higher energy antiparallel ? lower
energy
Similar pattern between atoms but
different spacing Spectrum of ion different
to atom
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• Separations measured in electron volts (eV)
• 1eV 1.602x10-19 J
• 96. 484 kJ mol-1

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As number of electrons increases, number
of levels increases Emission spectra
become more complex Li 30
lines, Cs 645 lines, Cr 2277 lines
As number of electrons increases, number
of levels increases Emission spectra
become more complex Li 30
lines, Cs 645 lines, Cr 2277 lines
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• GENERAL SELECTION RULES - TRANSITION
• ALLOWED IF
• Transition stays within same ionization stage
• atom atom ion ion ion ? atom
• state state
• 2. DL 1 any J
• 0 except J 0 to J 0
  is forbidden

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• P S or D P allowed (fast,
  favorable)
• S0 S0 or D S forbidden (slow,
  unfavorable)
• Also DS 0 singlet singlet
  triplet triplet allowed
• singlet
  doublet forbidden

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• STRONGEST LINES FROM GIVEN ELEMENT?
• DOMINANT IONIZATION STAGE (neutral atom or
  1 ion)
• 2. RESONANCE LINES (involve ground state)
  (esp. absorption!)

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• 3. UPPER LEVEL CORRESPONDS TO LOWEST
  ENERGY ALLOWED TRANSITION TO GROUND STATE
• Some elements only a few strong lines (Ca,
  Mg)
• Other elements many lines (Fe, U)

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BROADENING
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• SELECTIVITY, MEAS. LINEWIDTH DETERMINED BY
• 1. RESOLUTION OF SPECTROMETER
• 2. INHERENT WIDTH OF LINES
• ATOMIC LINEWIDTHS - DOPPLER BROADENING
• MOTION OF ATOMS IN SOURCE BROADENS LINES
• LINEWIDTHS DEP. ON VELOCITIES OF ATOMS (T)

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Peak line-width is defined as width in
wavelength at half the signal intensity
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• MAXWELL-BOLTZMANN DISTRIBUTION
• f (vx) dx fraction of atoms moving along
  x direction
• velocities between vx and vx dvx
• m/2pkT3/2 exp (-mvx2/2kT) dvx

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GAUSSIAN DIST. OF VELOCITIES GAUSSIAN
DIST. OF FREQS. ABOUT LINE CENTER RANDOM
VELOCITIES SYMMETRICAL LINE nmax CORR.
TO LINE CENTER Eji/h
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Desire narrow lines for accurate
identification Broadened by i. uncertainty
principle ii. pressure broadening iii. Doppler
effect iv. (electric and magnetic fields)
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• Doppler effect
• emitted or absorbed wavelength changes as a
  result of atom movement relative to detector
• wavelength decrease if motion toward receiver
• - wavelength increases if motion away from
  receiver

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DOPPLER BROADENING GAUSSIAN PROFILE SnD
2(ln 2)1/2/DnD p1/2 exp - (4 ln 2) (n -
nm)2 / (DnD)2 (SnD)m 2(ln 2)1/2/DnD p1/2
at n nm
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DOPPLER BROADENING GAUSSIAN PROFILE DnD
2 2 (ln 2)kT/m1/2 (nm/c) 7.16 x 10-7 nm
(T/M)1/2 DlD (FWHM) 7.16 x 10-7 lm
(T/M)1/2 T in K M in
g/mole
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SHARPER LINES? LOWER T LARGER
M LINEWIDTH NOT V. SENSITIVE TO EITHER T
OR M MEAS. T BY MEAS. DlD ?
T T1/2 4000 K 63 5000 71 6000 7
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EXAMPLES - FABRY - PEROT INTERFEROMETER
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Usage in measurement of velocity of galaxies, age
of universe and big bang theory
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DE h Aji / 2p h Dnl h DnN
broadening due to radiative lifetime of
exc. State ALSO CALLED NATURAL
BROADENING, always present
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ESTIMATE NATURAL BROADENING DnN Aji 108
s-1 tr 1/Aji 10-8 s n c / l
c l-1 Dn - c l-2 Dl (c
/ l2 ) Dl Na (I) 589 nm Dl DlN (l2
/ c ) Dn l2 Aji / 2pc

0.02 pm
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• ACTUAL LINES GENERALLY MUCH BROADER!
• NATURAL BROADENING MINOR CONTRIBUTION TO
  LINE PROFILE
• (UNLESS REMOVE OTHER CAUSES OF BROADENING).

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COLLISIONAL BROADENING
DE h / (2p Dt) Dt 1/kj
DE hkj / 2p h Dn Dn
kj / 2p FASTER COLLISION RATE
REDUCES LIFETIME OF UPPER STATE
LARGER kj BROADER LINE!
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COLLISIONAL BROADENING
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LORENTZIAN LINE SHAPE
SnL
LORENTZIAN FUNCTION Broader in wings More
sharply peaked in center than Gaussian
DnL
n
nm
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BOTH DOPPLER LORENTZ BROADENING USUALLY
OCCUR AT SAME TIME COMPOSITE LINE PROFILE?
RELATIVE CONTRIBUTIONS, LORENTZ vs. DOPPLER?
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DIST. OF VELOCITIES DOPPLER
BROADENING, GAUSSIAN COMPONENT
S
LOSS OF EXC. ATOMS BY COLLISIONS LIFETIME
BROADENING, LORENTZ COMPONENT AT
EACH SECTION OF GAUSSIAN
n
COMBINED LINE PROFILE CALLED SnV ASSUME NO
LINE SHIFT
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Doppler profile at peak max.
y dummy variable
nr REL. FREQUENCY a VOIGT PARAMETER
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VOIGT PARAMETER
a 0 Dn L ltlt DnD line mostly
Gaussian a D n L gtgt D nD line mostly
Lorentzian NOTE DnV ¹ DnD Dn L

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Pressure broadening Collisions with
atoms/molecules transfers small quantities of
vibrational energy (heat) - ill-defined ground
state energy Effect worse at high pressures
For high pressure Xe lamps (gt10,000 torr) turns
lines into continua!
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Effect of Temperature on Atomic Spectra -
temperature changes number of atoms in ground and
excited states - need good temperature control
Boltzmann equation
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• N1 and No are the number of atoms in excited
  and ground states
• k Boltzmann constant (1.28x10-23 J/K)
• T temperature
• DE energy difference between ground and excited
  states
• P1 and Po number of states having equal energy
  at each quantum level

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Na atoms at 2500 K, only 0.02 atoms in first
excited state! Less important in absorption
measurements - 99.98 atoms in ground state!
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Other causes of line broadening

• 1. Stark broadening
• 2.Radiation or power broadening
• 3.Saturation broadening

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OTHER CAUSES OF BROADENING
STARK BROADENING E field induced by ions,
free electrons, dipoles, External E interacts
with bound electrons on absorber/emitter. Esp
. significant for H lines, method to meas.
density of free electrons in plasma.
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POWER BROADENING, RADIATION BROADENING
- include stimulated emission
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Sample Atomization
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Sample Atomization expose sample to flame or
high-temperature i. For atomic absorbance,
fluorescence or emission need to break sample up
into atom to observe atomic spectra ii. Basic
steps involved in atomization of solution
sample
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• nebulization solution sample, get into fine
  droplets by spraying thru thin nozzle or passing
  over vibrating crystal.
• b) desolvation - heat droplets to evaporate off
  solvent just leaving analyte and other matrix
  compounds

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d) dissociation break-up molecules in gas phase
into atoms. e) ionization cause the atoms to
become charged f) excitation with light,
heat, etc. for spectra measurement.
c) volatilization convert solid analyte/matrix
particles into gas phase d) dissociation
break-up molecules in gas phase into atoms. e)
ionization cause the atoms to become
charged f) excitation with light, heat, etc.
for spectra measurement.
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Sample Atomization expose sample to flame or
high-temperature i. For atomic absorbance,
fluorescence or emission need to break sample up
into atom to observe atomic spectra ii.
Basic steps involved in atomization of solution
sample
Sample Atomization expose sample to flame or
high-temperature i. For atomic absorbance,
fluorescence or emission need to break sample up
into atom to observe atomic spectra ii.
Basic steps involved in atomization of solution
sample
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Nebulizers
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Nebulizers

• 1) Pneumatic nebulaizers
• 2) Frit nebulizers
• 3) Ultrasonic nebulaizer
• 4) High solids nebulizers
• 5) Isolated droplet generator

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Nebulizer A device
called a nebulizer is used to convert the
solution sample into a fine spary of
droplets.
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• The nebulizing gas flows through an
• orifice that surrounds the sample-
• containing capillary concentrically.

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pneumatic nebulizer
Adjustment of position of inner capillary
Typical uptake rate 5 mL/min Typical delivery
efficiency 5
http//www.chemistry.nmsu.edu/Instrumentation
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pneumatic nebulizer
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A major disadvantage of conventional pneumatic
nebulizer is the wide range of droplet
diameters they produce.
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• In the angular or crossed - flow nebulizer ,
• the nebulizing gas flows over the sample
• capillary at right angles and causes
• aspiration and nebulization of the sample
• solution.

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Frit Nebulizers.The sample solution flow over
the surface of a fritted glass disk, while
nebulizing gas is passed through thd many
small holes in the disk.
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Ultrasonic NebulizersSolution is fed onto the
surface of the piezoelectric crystal by gravity
flow or by a pump.
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Vibration of the crystal (20 kHz to 5 MHZ)
cause the solution to break into small
droplets, which are transported by the carrier
gas to the flame or plasma.
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Advantage the nebulizer parameters (frequency
of vibration , power applied to the
transducer) are independent of any flame or
plasma gas flow rate so that separate
optimizations can be made.
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Disadvantagethe major limitation of ultrasonic
nebulizers is their poor efficiency with
viscous solutions and with solution that have
high particulate content.
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High Solids NebulizersThe solution is delivered
through a tube of much larger inside diameter
than those used with pneumatic nebulizers.
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Isolated Droplet Generator An alternating
voltage is applied to a piezoelectric
transducer.
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The resulting vibrations cause an attached
capillary, through which the sample is
pumped, to vibrate and result in a pressure
ware along the stream emerging from the
capillary.
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A stream of equally spaced and uniformly sized
droplets is produced at a frequency of 50 to
200 KHz.
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Free atom Formation after nebulization
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• Desolvation of the droplets is the first that
  must occur after nebulization.

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Volatilization

• The solid or molten particle remaining after
  desolvation must be vaporized to obtain free
  atoms.

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• CHANGE MATRIX OR ACID MAY PRODUCE
• SOLID PARTICLES OF DIFFERENT CHEM/PHYS
  PROPS.
• DRY VAPORIZE INTO DIFFERENT COMPOUNDS,
• atomize different time or place
• INTERFERENCES

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IONIZATION DEPLETES NEUTRAL ATOMS!
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• ne NOT CONSTANT W. T!
• ICP
• Ar Ar e-
• O O e-
• MOST ELECTRONS IN ICP
• COME FROM IONIZATION
• OF BACKGROUND ATOMS

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Spectral line intensities
j
Aji
hnji
i
F
V vol. observed
M
F radiant power (W) µ
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INTENSITY OF EMISSION LINE?

• Ej rel to ground state,
• even if state i is not ground state

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GENERAL EQN. FOR LINE OR CONTINUUM
SOURCE BEER-LAMBERT LAW IN ATOMIC ABSORPTION?
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Over region observed, k (l) kmax aL abs.
factor for line source 1 - exp(-kmaxl) AL
- log (1 - aL) kmax l
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AL LINEAR WITH ni µ M in sample
AL HIGHEST WHEN fij LARGE (ALLOWED
TRANSITION) ni LARGE (GRAPHITE FURNACE)
LONG l (1ST STRONG RESONANCE
LINE)
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Flame and Plasma Atomic Emission Spectrometry
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Flame atomic emission source
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• Different mixes and flow rates give different
  temperature profile in flame
• - gives different degrees of excitation of
  compounds in path of light source

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ii. Types of Flame/Flame Structure selection of
right region in flame important for
optimal performance a)
primary combustion zone blue inner cone (blue
due to emission from C2, CH
other radicals) - not in thermal equilibrium
and not used
Primary region for spectroscopy
Not in thermal equilibrium and not used for
spectroscopy
Flame profile depends on type of fuel and
oxidant and mixture ration
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• b) interconal region
• - region of highest temperature (rich in free
  atoms)
• - often used in spectroscopy
• - can be narrower in some flames (hydrocarbon)
  tall in others (acetylene)
• c) outer cone
• - cooler region
• - rich in O2 (due to surrounding air)
• - gives metal oxide formation

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Flame structure and temperatures (example for
premix flame)
http//www.unlv.edu/faculty/czerwinski
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Analyte distribution in flame
Rann, C. S. Hambly, A. N. Distribution of atoms
in an atomic absorption flame. Anal. Chem.
(1965), 37(7), 879-84.
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• Flame Source
• - used mostly for alkali metals
• gt easily excited even at low temperatures
• - Na, K
• - need internal standard (Cs usually) to
  correct for variations flame

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Flame Source

• Advantages
• - cheap
• Disadvantage
• - not high enough temperature to extend to many
  other elements

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Plasma (inductively coupled plasma - ICP)

• - plasma electrically conducting gaseous
• mixture (cations electrons)
• - temperature much higher than flame
• - possibility of doing multiple element analysis
• gt 40-50 elements in 5 minutes

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ICP

• Advantages
• - uniform response
• - multi-element analysis, rapid
• - precision accuracy (0.3 3)
• - few inter-element interferences
• - can use with gas, liquid or solids sample

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Inductively Coupled Plasma (ICP) Emission
Spectroscopy

• - involves use of high temperature plasma for
  sample atomization/excitation

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• - higher fraction of atoms exist in the excited
  state, giving rise to an increase in emission
• signal and allowing more types of atoms to be
  detected

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Inductively coupled plasma (ICP)torch design
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ICP temperatures
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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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Demountable torches

• where the injector may be removed and
• replaced with another of different internal
• diameter (high salt solutions) or different
• materials (ceramic or alumina injector tubes,
• which are more robust and resistant to HF).

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Plasma Torches Demountable torches
However, poor reproducibility of the annular
cooling gap between the inner and outer tubes.
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Formation of the Plasma
RF power is applied, an a.c. is set up which
oscillates at frequency governed by the generator
? oscillating current generates magnetic field.
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Properties of the Plasma

• Spectral observations arenormally made at a
  height of
• 15-20 mm above the induction region, where
  the background
• radiation is free from argon lines.

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Properties of the Plasma

• 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.
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Arc Spark Emission Spectroscopy
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• - involves use of electrical discharge to give
  high temperature environment
• - higher fraction of atoms exist in the excited
  state, giving rise to an increase
• in emission signal and allowing more types of
  atoms to be detected

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• - can be used for solids, liquids or gas phase
  samples
• - types of discharge used
• DC arc high sensitivity, poor precision
• DC spark intermediate sensitivity and precision
• AC spark low sensitivity, high precision

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Arc Spark

• Because of difficulty in reproducing the
• arc/spark conditions, all elements of interest
• are measured simultaneously by use of
• appropriate detection scheme.

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Concave grating disperse frequencies,
photographic film records spectra
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Arc created by electrodes separated by a fewmm,
with an applied current of 1-30 A
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Comparison of ICP and Arc/Spark Emission
Spectroscopy

• - Arc/Spark first instrument used widely for
  analysis
• - all capable of multielement detection
  with appropriate instrument design (e.g. 40-50
  elements in 5 min for ICP

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• ICP tends to have better precision and stability
  than spark or arc methods
• - ICP have lower limits of detection than spark
  or arc methods
• - ICP instruments are more expensive than spark
  or arc instruments

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Atomic Absorption Spectroscopy (AAS)
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AAS

• commonly used for elemental analysis
• expose sample to flame or high-
  temperature
• characteristics of flame impact use of
  atomic absorption spectroscopy

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Flame AAS
simplest atomization of gas/solution/solid
laminar flow burner - stable "sheet" of flame
flame atomization best for reproducibility
(precision) (lt1) relatively insensitive -
incomplete volatilization, short time in flame
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Basic instrument design (Flame atomizer)
Single beam
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Basic instrument design (Flame atomizer)
Double beam
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• atomizer
• Laminar Flow Burner
• - adjust fuel/oxidant mixture for optimum
  excitation of desired
• compounds
• usually 11 fuel/oxidant mix but some metals
  forming oxides use increase fuel mix
• - different mixes give different temperatures.

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Laminar nonturbulent streamline flow
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• Laminar nonturbulent streamline flow
• sample, oxidant and fuel are mixed
• only finest solution droplets reach burner
• most of sample collects in waste
• provides quite flame and a long path length

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Electrothermal (Lvov or Graphite furnace) -
place sample drop on platform inside tube - heat
tube by applying current, resistance to current
creates heat - heat volatilizes sample,
atomizers, etc. inside tube - pass light through
to measure absorbance
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Place sample droplet on platform
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Interferences
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Interferences

• Corrections For Spectral Interferences Due to
  Matrix
• - molecular species may be present in flame

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Interferences

• - problem if absorbance spectra overlap since
  molecular spectrum is much broader with a
  greater net absorbance
• - need way of subtracting these factors out

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Zeeman Effect

• - placing gaseous atoms in magnetic field causes
  non-random orientation of atoms
• - not apparent for molecules
• - splitting of electronic energy levels occurs (
  0.01 nm)
• - sum of split absorbance lines ? original line
• - only absorb light with same orientation

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Zeeman Effect

• - can use Zeeman effect to remove background gt
  place flame polarized light through sample in
  magnetic field get
• absorbance (atommolecule) or absorbance
  (molecule) depending
• on how light is polarized

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Zeeman Effect
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Chemical Interference - more common than spectral
interference

• 1) Formation of Compounds of Low Volatility
• - Anions Cations ? Salt
• Ca2 SO42- ? CaSO4 (s)
• - Decreases the amount of analyte atomized ?
  decreases the absorbance signal

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Chemical Interference

• 2) Formation of Oxides/Hydroxides
• M O MO
• M 2OH M(OH)2
• - M is analyte
• 3) Ionization
• M M e-
• - M is analyte

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X-Ray Methods
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