Elemental Analysis Atomic Spectroscopy

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Elemental Analysis - Atomic Spectroscopy
A) Introduction Based on the breakdown of a
sample into atoms, followed by the measurement of
the atoms absorption or emission of
light. i. deals with absorbance fluorescence
or emission (luminescence) of atoms or
elemental ions rather then molecules -
atomization process of converting sample to
gaseous atoms or elementary ions ii.
Provides information on elemental composition of
sample or compound - UV/Vis, IR, Raman gives
molecular functional group information, but no
elemental information. iii. Basic process the
same as in UV/Vis, fluorescence etc. for
molecules
Absorbance
Fluorescence
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iv. Differences for Molecular Spectroscopy -
no vibration levels ? much sharper absorbance,
fluorescence, emission bands - position
of bands are well-defined and characteristic of a
given element - qualitative analysis is easy in
atomic spectroscopy (not easy in molecular
spectroscopy)
Examples carbon oxygen nitrogen
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B) 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 ii. p, d,
f split by spin-orbit coupling iii. Spin (s)
and orbital (l) motion create magnetic fields
that perturb each other (couple) - parallel ?
higher energy antiparallel ? lower energy
Similar pattern between atoms but
different spacing Spectrum of ion different
to atom Separations measured in
electronvolts (eV) 1eV 1.602x10-19 J
96.484 kJ mol-1 As number of
electrons increases, number of levels
increases emission spectra more complex
Li 30 lines Cs 645 lines Cr 2277 lines
Na
Mg
Note slight differences in energy due to
magnetic fields caused by spin
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C) Desire narrow lines for accurate
identification Broadened by i. uncertainty
principle
Uncertainty principal Dt . DE h Dt .
Dn 1 Dt minimum time for measurement Dn
minimal detectable frequency difference
Peak line-width is defined as width in wavelength
at half the signal intensity
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C) Desire narrow lines for accurate
identification Broadened by ii. Doppler effect
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
Usage in measurement of velocity of galaxies, age
of universe and big bang theory
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C) Desire narrow lines for accurate
identification Broadened by iii. Pressure
broadening
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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D) Effect of Temperature on Atomic Spectra
- temperature changes number of atoms in ground
and excited states - need good temperature
control
Boltzmann equation
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
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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E) Sample Atomization expose sample to flame or
high-temperature

• Need to break sample into atoms to observe atomic
  spectra
• ii. Basic steps
• a) 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
• 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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E) Sample Atomization expose sample to flame or
high-temperature
iii. Types of Nebulizers and Atomizers
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F) Atomic Absorption Spectroscopy (AAS)
commonly used for elemental analysis expose
sample to flame or high-temperature
characteristics of flame impact use of atomic
absorption spectroscopy
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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• 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 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
Temperature varies significantly across flame
need to focus on part of the flame
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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Most sensitive part of flame for AAS varies with
analyte
Consequences - Sensitivity varies with
element - must maximize burner position - makes
multi-element detection difficult
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iii. Basic instrument design (Flame atomizer)
Single beam
Double beam
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a) atomizer 1) 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.

• 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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2) 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
Po
P
Place sample droplet on platform
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3) Comparison of atomizers

• a) Electrothermal (Lvov or Graphite furnace)
• advantages
• - all sample used
• - longer time of sample in light beam
• lower limit of detection (LOD)
• can use less sample (0.5 10)
• disadvantage
• - slow (can be several minutes per element or
  sample)
• - not as precise as flame (5-10 vs. 1)
• - low dynamic range (lt 102, range of detectable
  signal intensity)
• use only when there is a need for better
  limit of detection or have less sample than
  Laminar flow can use

b) Laminar Flow Burner advantages - good b
(5-10 cm) - good reproducibility disadvantag
es - not sample efficient (90-99 sample loss
before flame) - small amount of time that
sample is in light path (10-4 s) - needs lots
of sample
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b) Light source - need light source with a
narrow bandwidth for light output - AA lines
are remarkably narrow (0.002 to 0.005 nm) -
separate light source and filter is used for each
element

• problem with using typical UV/Vis continuous
  light source
• - have right l, but also lots of others
  (non-monochromatic light)
• - hard to see decrease in signal when
  atoms absorb in a small bandwidth
• - only small decrease in total signal
  area
• - with large amount of elements ? bad
  sensitivity

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2) Solution is to use light source that has line
emission in range of interest - laser
but hard to match with element line of interest
- hollow cathode lamp (HCL) is common choice
Hollow Cathode Lamp
Coated with element to be analyzed
Process use element to detect element 1.
ionizes inert gas to high potential (300V) Ar ?
Ar e- 2. Ar go to - cathode hit
surfaces 3. As Ar ions hit cathode, some of
deposited element is excited and
dislodged into gas phase (sputtering) 4.
excited element relaxes to ground state and emits
characteristic radiation - advantage sharp
lines specific for element of interest -
disadvantage can be expensive, need to use
different lamp for each element tested.
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c) Source Modulation (spectral interference due
to flame) - problem with working with flame in
AA is that light from flame and light source
both reach detector - measure small signal
from large background - need to subtract out
flames to get only light source signal (P/Po) i.
done by chopping signal ii. or
modulating P from lamp
Flame P
Flame only
P
Flame P
Flame only
time
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d) Corrections For Spectral Interferences Due to
Matrix - molecular species may be present in
flame - 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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Methods for Correction 1) Two-line
method - monitor absorbance at two l close
together gt one line from sample one from light
source gt second l from impurity in HCL
cathode, Ne or Ar gas in HCL, etc - second l
must not be absorbed by analyte gt absorbed by
molecular species, since spectrum much broader -
A e are constant if two l close - comparing
Al1, Al2 allows correction for absorbance for
molecular species Al1 (atommolecule) Al2
(molecule) A (atom) Problem Difficult to get
useful second l with desired characteristics
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• 2) Continuous source method
• - alternatively place light from HCL or a
  continuous source D2 lamp thru flame
• - HCL ? absorbance of atoms molecules
• - D2 ? absorbance of molecules
• advantage

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3) 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 - 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
Background
z
z



BackgroundAbsorbance

z
z
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e) 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 - Avoid by gt increase
temperature of flame (increase atom
production) gt add releasing agents other
items that bind to interfering ions eg. For
Ca2 detection add Sr2 Sr2 SO42- ? SrSO4
(s) increases Ca atoms and Ca absorbance
gt add protecting agents bind to analyte but
are volatile eg. For Ca2 detection add
EDTA4- Ca2 EDTA4- ? CaEDTA2- ? Ca atoms
2) Formation of
Oxides/Hydroxides M O MO M 2OH
M(OH)2 - M is analyte - Avoid by gt
increase temperature of flame (increase atom
production) gt use less oxidant
non-volatile intense molecular absorbance
A
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3) Ionization M M e- - M is
analyte - Avoid by gt lower temperature gt
add ionization suppressor creates high
concentration of e- suppresses M by
shifting equilibrium.
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G) Atomic Emission Spectroscopy (AES) similar
to AA with flame now being used for
atomization and excitation of the sample for
light production 1) Atomic
Processes
heat
Degree of Excitation Depends on Boltzmann
Distribution
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
Increase Temperature ? increase in N1/No (more
excited atoms)
I (emission) N1, so signal increases with
increase in temperature
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Need good temperature control to get reproducible
signal eg. For Na, temperature difference of 10o
2500 ? 2510 results in a
4 change in N1/No
Temperature Dependence Comparison between AA and
AES - AA is relatively temperature
independent. Need heat only to get atoms,
not atoms in excited state. - AA looks at
99.98 of atoms - AES uses only small fraction
(0.02) of excited atoms
2) Comparison of AA and AES Applications
AES - emission from multiple species
simultaneously
Comparison of Detection Limit
Some better by AA others better by AES
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3) Instrumentation - Similar to AA, but no need
for external light source (HCL) or chopper gt
look at light from flame gt flame acts as
sample cell light source
Atomization Sources
Electrothermal usually not used too slow and
not as precise
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a) 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 Advantages - cheap Disadvantage -
not high enough temperature to extend to many
other elements
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b) 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 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 -
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
Ions forced to flow in closed path, Resistance
to flow causes heating
Temperature Regions in Plasma Torch
Magnetic field
Ar charges by Tesla coil (high voltages at high
frequency)
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Overall Design for ICP Emission Spectrometer
Rowland circle - curvature corresponds to
focal curve of the concave grating. -
frequencies are separated by grating and
focused onto slits/photomultiplier tubes
positioned around the Rowland circle - slits
are configures to transmit lines for a
specific element
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Arc Spark Emission Spectroscopy - 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 - 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 Because of
difficulty in reproducing the arc/spark
conditions, all elements of interest are measured
simultaneously by use of appropriate detection
scheme.
Arc created by electrodes separated by a few mm,
with an applied current of 1-30 A
Concave grating disperse frequencies,
photographic film records spectra
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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 - 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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Example 11 For Na atoms and Mg ions, compare
the ratios of the number of particles in the 3p
excited state to the number in the ground state
in a natural gas-air flame (2100K) and an ICP
source (6000K)