Atomic Spectroscopy and Atomization in Analytical Chemistry

 
Chem. 133 – 4/4 Lecture
 
Announcements
 
Second Homework Set – (additional problem –
slight error in first 2.1.1 key posted; since fixed)
Today’s Lecture
Atomic Spectroscopy (Chapter 20)
Theory (Boltzmann Distribution Problem)
Atomization
Flame
Electrothermal
ICP
Spectrometers
AA instruments
AE instruments
 
Atomic Spectroscopy
Theory
 
Example problem:
Calcium absorbs light at 422 nm.  Calculate the
ratio of Ca atoms in the excited state to the
ground state at 3200 K (temperature in N
2
O
fueled flame).  g*/g
0
 = 3 (3 5p orbitals to 1 4s
orbital).
Atomic Spectroscopy
Atomization
 
Flame Atomization
used for liquid samples
liquid pulled by action of
nebulizer
nebulizer produces spray
of sample liquid
droplets evaporate in spray
chamber leaving particles
fuel added and ignited in
flame
atomization of remaining
particles and spray
droplets occurs in flame
optical beam through
region of best atomization
 
sample in
 
fuel (HCCH)
 
oxidant (air or
N
2
O)
 
burner head
 
spray chamber
 
nebulizer
 
light
beam
 
nebulizer
 
air
 
liquid
Atomic Spectroscopy
Atomization
 
Atomization in flames – Processes
nebulization of liquid: MgCl
2
(aq) 
MgCl
2
(spray droplet)
evaporation of solvent: MgCl
2
(spray droplet)
→ MgCl
2
(s)
Volatilization in flame: MgCl
2
(s) → MgCl
2
(g)
Atomization (in hotter part of flame): MgCl
2
(g)
→ Mg(g) + Cl
2
(g)
 
Target species for absorption
measurement
Atomic Spectroscopy
Atomization
 
Complications/Losses
Ideally, every atom entering nebulizer ends
up as gaseous atom
In practice, at best only a few % of atoms
become atoms in flame
The nebulization process is not that efficient
(much of water hits walls and goes out drain)
Poor volatilization also occurs with less volatile
salts (e.g. many phosphates)
Atomic Spectroscopy
Atomization
 
Complications/Losses (continued)
Poor atomization also can occur due to secondary
processes such as:
Formation of oxides + hydroxides (e.g. 2Mg (g) + O
2
 (g) 
2MgO (g))
Ionization (Na (g) + Cl (g) → Na
+
 (g) + Cl
-
 (g))
If the atomization is affected by other compounds in
sample matrix (e.g. the presence of phosphates), this
is called a matrix effect (discussed more later)
Atomic Spectroscopy
Atomization
 
Electrothermal Atomization
Atomization occurs in a graphite furnace
Process is different in that a small sample is placed in
a graphite tube and atomization occurs rapidly but in
a discontinuous manner
Electrothermal atomization is more efficient; atoms
spend more time in the beam path, and less sample is
required resulting in much greater sensitivity
Concentration LODs are typically ~100 times lower (e.g. 100
ppt for EA vs. 10 ppb for flame)
Mass LODs are even lower (100 pg/mL*0.01 mL = 1 pg for
EA vs. 10 ng/mL*2 mL = 20 ng for flame)
Atomic Spectroscopy
Atomization
 
Electrothermal
Atomization
(Process)
Sample is placed
through hole onto
L
vov platform
Graphite tube is
heated by resistive
heating
This occurs in steps
(dry, char, atomize,
clean)
 
Graphite Tube in
Chamber (not shown)
 
L’vov Platform
 
Sample
in
 
T
 
time
 
dry
 
char
 
atomize
 
Clean +
cool down
 
Ar in chamber flow stops and
optical measurements made
Atomic Spectroscopy
Atomization
 
Inductively Coupled Plasma
(ICP)
A plasma is induced by radio
frequency currents in
surrounding coil
Once a spark occurs in Ar gas,
some electrons leave Ar
producing Ar
+
 + e
-
The sample is introduced by
nebulization in the Ar stream
The accelerations of Ar
+
 and e
-
induce further production of ions
and great heat production
Much higher temperatures are
created (6000 K to 10000 K vs.
flames)
 
ICP Torch
 
Quartz tube
 
Argon + Sample
 
RF Coil
 
Plasma
Atomic Spectroscopy
Atomization
 
Advantages of ICP Atomization
Greater atomization efficiency than in flame AA
(partly because better nebulizers are used than with
flames due to higher total instrument cost and partly
due to higher temperatures)
Fewer matrix effects because atomization is more
complete at higher temperatures
High temperature atomization allows much greater
emission flux + more ionization allowing coupling with
emission spectrophotometers and mass
spectrometers
Emission and MS allow faster multi-element analysis
Chapter 20 Questions
 
1.
Why would it be difficult to use a broadband light source
and monochromator to produce light used in AA
spectrometers?
2.
List three methods for atomizing elements.
3.
List two processes that can decrease atomization efficiency
in flame atomization.
4.
What is an advantage in using electrothermal atomization in
AAS?
5.
Which atomization method tends to result in the most
complete breakdown of elements to atoms in the gas
phase?
6.
Why is ICP better for emission measurements than flame?
Atomic Spectroscopy
Absorption Spectrometers
 
The lamp is a hollow cathode lamp containing the element(s) of
interest in cathode
The lamp is operated under relatively cool conditions at lower
pressures to reduce Doppler and pressure broadening of atomic
emission lines
A very narrow band of light emitted from hollow cathode lamps is
needed so that absorption by atoms in flame mostly follows Beer’s
law
The monochromator serves as a coarse filter to remove other
wavelength bands from light and light emitted from flames
 
Lamp source
 
Flame or
graphite tube
 
monochromator
 
Light detector
Atomic Spectroscopy
Absorption Spectrometers
 
A narrower emission
spectrum from hollow
cathode lamp (vs. flame
absorption) results in
better Beer’s law behavior
 
wavelength
 
Intensity or absorbance
 
hollow cathode
lamp emission
 
Atomic absorption
spectrum in flame
 
Additional broadening in
flame from temperature
(Doppler) or pressure
Atomic Spectroscopy
Interference in Absorption Measurements
 
Spectral Interference
Very few atom – atom interferences
Interference from flame (or graphite tube) emissions
are reduced by modulating lamp
no lamp: signal from flame vs. with lamp
then with lamp: signal from lamp + flame – absorption by
atoms
Interference from molecular species absorbing lamp
photons (mostly at shorter wavelengths and light
scattering in EA-AA)
This interference can be removed by periodically
using a deuterium lamp (broad band light source) or
using the Zeeman effect (magnetic splitting of
absorption bands)
Atomic Spectroscopy
Interference in Absorption Measurements
 
Chemical Interference
Arises from compounds in sample matrix or atomization
conditions that affects element atomization
Some examples of specific problems (mentioned previously) and
solutions:
Poor volatility due to PO
4
3-
 
 add Ca because it binds strongly to
PO
4
3-
 allowing analyte metal to volatilize better or use hotter flames
Formation of metal oxides and hydroxides 
 use fuel rich flame
Ionization of analyte atoms 
 add more readily ionizable metal (e.g
Cs)
Another approach is to use a standard addition calibration
procedure (this won
t improve atomization but it accounts for it
so that results are reliable)
Atomic Spectroscopy
Interference in Absorption Measurements
 
Standard Addition
Used when sample matrix
affects response to analytes
Commonly needed for AAS
with complicated samples
Standard is added to
sample (usually in multiple
increments)
Needed if slope is affected
by matrix
Concentration is determined
by extrapolation (= 
|
X-
intercept
|
)
 
Absorbance
 
Concentration
Added
 
Analyte
Concentration
 
standards in water
 
Sample
Atomic Spectroscopy
Emission Spectrometers
 
In emission measurements, the plasma (or flame) is the light source
Flame sources are generally limited to a few elements (only hot enough
for low E – visible light emissions)
A monochromator or polychromator is the means of wavelength
discrimination
Sensitive detectors are needed
ICP-AES is faster than AAS because switching monochromator settings
can be done faster than switching lamp plus flame conditions
 
Plasma (light
source + sample)
 
Monochromator or
Polychromator
 
Light detector or
detector array
 
Liquid sample, nebulizer, Ar source
Atomic Spectroscopy
Emission Spectrometers
 
Sequential vs. Simultaneous Instruments
Sequential Instruments use:
A standard monochromator
Select for elements by rotating the monochromator grating to
specific wavelengths
Simultaneous Instruments use:
A 1D or 2D polychromator (Harris Color Plate 24/25)
1D instruments typically use photomultiplier detectors behind
multiple exit slits
2D instrument shown in 4/1 lecture slide 13
Selected elements (1D instruments) or all elements can be
analyzed simultaneously resulting in faster analysis and less
sample consumption.
Atomic Spectroscopy
Interference in Emission Measurements
 
Interferences
Atom – atom interferences more
common than in atomic absorption
because monochromators offer
less selectivity than hollow
cathode lamps
Interference from molecular
emissions are reduced by scanning
to the sides of the atomic peaks
Chemical interferences are less
prevalent due to greater
atomization efficiency
 
Emission
Spectrum
 
Atomic
peak
 
background
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Explore the principles of atomic spectroscopy through examples and theories, focusing on topics such as the Boltzmann distribution problem and atomization processes using flames. Learn about the challenges and complications in atomization, including issues with nebulization efficiency and poor volatilization. Gain insights into matrix effects and secondary processes affecting atomization in analytical chemistry applications.

  • Atomic spectroscopy
  • Atomization
  • Analytical chemistry
  • Matrix effects
  • Nebulization

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  1. Chem. 133 4/4 Lecture

  2. Announcements Second Homework Set (additional problem slight error in first 2.1.1 key posted; since fixed) Today s Lecture Atomic Spectroscopy (Chapter 20) Theory (Boltzmann Distribution Problem) Atomization Flame Electrothermal ICP Spectrometers AA instruments AE instruments

  3. Atomic Spectroscopy Theory Example problem: Calcium absorbs light at 422 nm. Calculate the ratio of Ca atoms in the excited state to the ground state at 3200 K (temperature in N2O fueled flame). g*/g0= 3 (3 5p orbitals to 1 4s orbital).

  4. Atomic Spectroscopy Atomization air nebulizer Flame Atomization used for liquid samples liquid pulled by action of nebulizer nebulizer produces spray of sample liquid droplets evaporate in spray chamber leaving particles fuel added and ignited in flame atomization of remaining particles and spray droplets occurs in flame optical beam through region of best atomization liquid light beam burner head spray chamber oxidant (air or N2O) fuel (HCCH) nebulizer sample in

  5. Atomic Spectroscopy Atomization Atomization in flames Processes nebulization of liquid: MgCl2(aq) MgCl2(spray droplet) evaporation of solvent: MgCl2(spray droplet) MgCl2(s) Volatilization in flame: MgCl2(s) MgCl2(g) Atomization (in hotter part of flame): MgCl2(g) Mg(g) + Cl2(g) Target species for absorption measurement

  6. Atomic Spectroscopy Atomization Complications/Losses Ideally, every atom entering nebulizer ends up as gaseous atom In practice, at best only a few % of atoms become atoms in flame The nebulization process is not that efficient (much of water hits walls and goes out drain) Poor volatilization also occurs with less volatile salts (e.g. many phosphates)

  7. Atomic Spectroscopy Atomization Complications/Losses (continued) Poor atomization also can occur due to secondary processes such as: Formation of oxides + hydroxides (e.g. 2Mg (g) + O2 (g) 2MgO (g)) Ionization (Na (g) + Cl (g) Na+ (g) + Cl- (g)) If the atomization is affected by other compounds in sample matrix (e.g. the presence of phosphates), this is called a matrix effect (discussed more later)

  8. Atomic Spectroscopy Atomization Electrothermal Atomization Atomization occurs in a graphite furnace Process is different in that a small sample is placed in a graphite tube and atomization occurs rapidly but in a discontinuous manner Electrothermal atomization is more efficient; atoms spend more time in the beam path, and less sample is required resulting in much greater sensitivity Concentration LODs are typically ~100 times lower (e.g. 100 ppt for EA vs. 10 ppb for flame) Mass LODs are even lower (100 pg/mL*0.01 mL = 1 pg for EA vs. 10 ng/mL*2 mL = 20 ng for flame)

  9. Atomic Spectroscopy Atomization Graphite Tube in Chamber (not shown) Electrothermal Atomization (Process) Sample is placed through hole onto L vov platform Graphite tube is heated by resistive heating This occurs in steps (dry, char, atomize, clean) Sample in L vov Platform Ar in chamber flow stops and optical measurements made Clean + cool down T dry char atomize time

  10. Atomic Spectroscopy Atomization Inductively Coupled Plasma (ICP) A plasma is induced by radio frequency currents in surrounding coil Once a spark occurs in Ar gas, some electrons leave Ar producing Ar+ + e- The sample is introduced by nebulization in the Ar stream The accelerations of Ar+ and e- induce further production of ions and great heat production Much higher temperatures are created (6000 K to 10000 K vs. flames) ICP Torch Plasma RF Coil Quartz tube Argon + Sample

  11. Atomic Spectroscopy Atomization Advantages of ICP Atomization Greater atomization efficiency than in flame AA (partly because better nebulizers are used than with flames due to higher total instrument cost and partly due to higher temperatures) Fewer matrix effects because atomization is more complete at higher temperatures High temperature atomization allows much greater emission flux + more ionization allowing coupling with emission spectrophotometers and mass spectrometers Emission and MS allow faster multi-element analysis

  12. Chapter 20 Questions 1. Why would it be difficult to use a broadband light source and monochromator to produce light used in AA spectrometers? List three methods for atomizing elements. List two processes that can decrease atomization efficiency in flame atomization. What is an advantage in using electrothermal atomization in AAS? Which atomization method tends to result in the most complete breakdown of elements to atoms in the gas phase? Why is ICP better for emission measurements than flame? 2. 3. 4. 5. 6.

  13. Atomic Spectroscopy Absorption Spectrometers Flame or graphite tube Lamp source monochromator Light detector The lamp is a hollow cathode lamp containing the element(s) of interest in cathode The lamp is operated under relatively cool conditions at lower pressures to reduce Doppler and pressure broadening of atomic emission lines A very narrow band of light emitted from hollow cathode lamps is needed so that absorption by atoms in flame mostly follows Beer s law The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames

  14. Atomic Spectroscopy Absorption Spectrometers hollow cathode lamp emission A narrower emission spectrum from hollow cathode lamp (vs. flame absorption) results in better Beer s law behavior Atomic absorption spectrum in flame Intensity or absorbance Additional broadening in flame from temperature (Doppler) or pressure wavelength

  15. Atomic Spectroscopy Interference in Absorption Measurements Spectral Interference Very few atom atom interferences Interference from flame (or graphite tube) emissions are reduced by modulating lamp no lamp: signal from flame vs. with lamp then with lamp: signal from lamp + flame absorption by atoms Interference from molecular species absorbing lamp photons (mostly at shorter wavelengths and light scattering in EA-AA) This interference can be removed by periodically using a deuterium lamp (broad band light source) or using the Zeeman effect (magnetic splitting of absorption bands)

  16. Atomic Spectroscopy Interference in Absorption Measurements Chemical Interference Arises from compounds in sample matrix or atomization conditions that affects element atomization Some examples of specific problems (mentioned previously) and solutions: Poor volatility due to PO43- add Ca because it binds strongly to PO43- allowing analyte metal to volatilize better or use hotter flames Formation of metal oxides and hydroxides use fuel rich flame Ionization of analyte atoms add more readily ionizable metal (e.g Cs) Another approach is to use a standard addition calibration procedure (this won t improve atomization but it accounts for it so that results are reliable)

  17. Atomic Spectroscopy Interference in Absorption Measurements standards in water Standard Addition Used when sample matrix affects response to analytes Commonly needed for AAS with complicated samples Standard is added to sample (usually in multiple increments) Needed if slope is affected by matrix Concentration is determined by extrapolation (= |X- intercept|) Absorbance Sample Analyte Concentration Concentration Added = = + b = m 0 A X mX b /

  18. Atomic Spectroscopy Emission Spectrometers In emission measurements, the plasma (or flame) is the light source Flame sources are generally limited to a few elements (only hot enough for low E visible light emissions) A monochromator or polychromator is the means of wavelength discrimination Sensitive detectors are needed ICP-AES is faster than AAS because switching monochromator settings can be done faster than switching lamp plus flame conditions Plasma (light source + sample) Monochromator or Polychromator Light detector or detector array Liquid sample, nebulizer, Ar source

  19. Atomic Spectroscopy Emission Spectrometers Sequential vs. Simultaneous Instruments Sequential Instruments use: A standard monochromator Select for elements by rotating the monochromator grating to specific wavelengths Simultaneous Instruments use: A 1D or 2D polychromator (Harris Color Plate 24/25) 1D instruments typically use photomultiplier detectors behind multiple exit slits 2D instrument shown in 4/1 lecture slide 13 Selected elements (1D instruments) or all elements can be analyzed simultaneously resulting in faster analysis and less sample consumption.

  20. Atomic Spectroscopy Interference in Emission Measurements Interferences Atom atom interferences more common than in atomic absorption because monochromators offer less selectivity than hollow cathode lamps Interference from molecular emissions are reduced by scanning to the sides of the atomic peaks Chemical interferences are less prevalent due to greater atomization efficiency Emission Spectrum Atomic peak background

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